Influence of Velocity Encoding and Position of Image Plane in Patients with Aortic Valve Insufficiency Using 2D Phase Contrast MRI

Transcription

1 Influence of Velocity Encoding and Position of Image Plane in Patients with Aortic Valve Insufficiency Using 2D Phase Contrast MRI M. Sc. Thesis Frida Svensson Supervisors: Kerstin Lagerstrand Christian Polte Åse Johnsson Department of Radiation Physics University of Gothenburg Gothenburg, Sweden June 2014

2 Abstract Aortic valve insufficiency (AVI) is a condition causing left ventricular dysfunction which eventually might lead to heart failure. The regurgitant flow can be measured using through-plane 2D phase contrast magnetic resonance imaging (MRI) flow measurements in an image plane orthogonally to the blood flow in the ascending Aorta. The grade of the insufficiency is important for the determination of the progression of the disease. Though, the phase contrast technique is encumbered with effects that may influence on the accuracy and precision of the flow estimation, such as background effect and complex flow. This work is aimed to study the accuracy and precision in flow estimates in regard to position of image plane and velocity encoding in healthy volunteers and in patients with AVI. All subjects were scanned using a 1.5 T whole-body Philips MRI scanner equipped with a five channel cardiac coil. Automatic compensation for background effects was applied during scanning. Post procession of all images was performed using the dedicated work station of the MR system and the research tool Segment. An offline tool based on Matlab was also used for further analysis of velocity encoding effects. Flow estimates of regurgitation, i.e. volume and fraction, and velocity offsets were determined and compared statistically in regard to position of image plane and velocity encoding. Flow rate curves were also determined and compared visually in regard to different position of the image plane. The result showed significant differences in flow estimates and in variation of flow estimates in regard to position of the image plane. No significant differences in flow estimates or in variation of flow estimates were found in regard to velocity encoding. Velocity offsets in the background were present in all velocity images and differed significantly with the choice of velocity encoding, and with the distance from the isocenter of the magnet. No significant differences in velocity offsets were found in regard to position of the image plane. Decomposition of the estimated flow rate over the cardiac cycle in forward and backward flow rate revealed new interesting flow characteristics in patients with AVI. The characteristics of the flow rate were similar for all healthy volunteers showing low flow rate over the whole cardiac cycle and increased flow rate at end-systole, when the flow rate in the coronary arteries was high. In most patients, backward flow rate was measured during the whole cardiac cycle and two characteristic appearances of the flow rate curves were seen. In conclusion, we have shown that the accuracy and precision in flow estimates using 2D phase contrast MRI for the grading of AVI depends on the position of the image plane. From our results, we conclude that the most optimal position is closest to the Aortic valve plane. Flow estimates based on phase contrast flow measurements with effective compensation for background effects, as we estimated in this study, were not influenced by the choice of velocity encoding. Though, in images acquired with both high and low velocity encoding, there was a positive velocity bias that may have affected the grading of the insufficiency. The backward flow rate curves revealed that some patients may not have been accurately diagnosed with conventional regurgitation flow volume and fraction estimates. We have also shown that backward flow rate curves are promising for further analysis as it reveals characteristic flow pattern in patients with AVI. i

3 Acknowledgment It is a pleasure to thank those who made this thesis possible. I would like to express my gratitude to my supervisors Kerstin Lagerstrand, Christian Polte and Åse Johnsson for letting me be a part of this project. I owe my deepest gratitude to my main supervisor Kerstin, for her excellent encouragement, supervision and support during the past few months. It has been a pleasure to learn from you, to hear about your projects and to get an insight in your everyday life as an MR-physician. You are a true inspiration! Many thanks to Christian and Åse for their important clinically inputs. I would also like to thank all the MR-physicists at Sahlgrenska University Hospital for making this period of time enjoyable and for their good advice and helpfulness whenever needed. Thanks to my classmate Julia for being a devoted friend and for the giving discussions during our weekly Wednesday meetings. Lastly, sincere thanks to my family and friends whom have supported me and believed in me. All this would not have been without you! ii

4 Nomenclature AVI CINE CMR ECG FOV MR NSA RF RV SA SENSE TE TFE TR US 2CH 3CH 4CH Aortic Valve Insufficiency Cinema images of the motion of the heart over the cardiac cycle Cardiovascular Magnetic Resonance Electrocardiography Field of View Magnetic Resonance Number of Signal Averages Regurgitation Fraction Regurgitation Volume Short Axis Sensitivity Encoding Echo Time Turbo Field Echo Repetition Time Ultrasound 2-chamber 3-chamber 4-chamber iii

5 Table of contents Abstract... i Acknowledgment... ii Nomenclature... iii 1 Introduction Methods and material Imaging system and protocol Image analysis Statistical analysis Regurgitation volume and regurgitation fraction Flow rate Background Results Regurgitation volume and regurgitation fraction Flow rate Backward flow rate Background Discussion Regurgitant flow visualization Eddy current effect Influence of position of image plane Influence of velocity encoding Potential errors during image analysis Limitations about the study Future aspects Conclusions References Appendix I-Aortic valve insufficiency... 1 Anatomy... 1 Cardiac cycle... 1 Aortic valve insufficiency... 1 Appendix II-2D phase contrast MRI... 1 Basic theory... 1 Velocity encoding... 1 Angulation and position of the image plane... 2 Background effect... 2 Appendix III-Statistics... 1 iv

6 1 Introduction Aortic valve insufficiency (AVI) (Appendix I) is a condition initiated by a failure in the closure of the Aortic valve, which allows blood to leak back into the heart from the circulation system. This can cause left ventricular dysfunction, which eventually might lead to heart failure (1). The condition is treatable and is in advanced cases done surgically by repairing the Aortic valve or replacing the valve with a biological or mechanical implant. It is important that the change is done in the right time in relation to progression of the disease as valve replacement only can be performed a limited number of times. Also, the likelihood of developing irreversible myocardial dysfunction increases if the intervention is delayed. To grade the insufficiency, blood flow measurements with 2D phase contrast magnetic resonance imaging (MRI) (Appendix II) and Doppler echocardiography with ultrasound (US) technology is frequently used. These techniques are noninvasive and generate information about the cardiovascular blood flow. The need for surgical intervention partly relies on established guidelines of flow estimates, which are based on the two techniques. Unfortunately, the results are not always consistent (2) and there are also indications that the valve repair or replacement sometimes is done too early and other times too late. At Sahlgrenska University Hospital, research is conducted with purpose to improve the characterization of AVI so that the severity of the disease can be determined and the need for surgical intervention can be established. In regard to that project, it is of interest to investigate the 2D phase contrast MRI technique used in the study. MRI has been used since the 1980 s for quantitative flow measurements (3). In ideal conditions, the 2D phase contrast MRI technique is reliable (4) and accurate (5) and can be used to measure the cardiovascular blood flow through an image plane positioned orthogonally to the blood flow. The technique relies on detecting changes in the phase of the transverse magnetization of the blood (the phase shift in spins) as it moves along a magnetic field gradient. During phase contrast MRI, the flow is measured throughout several cardiac cycles in a predetermined number of time point, evenly distributed over each cycle. One measurement contains two acquisitions with different velocity encoding gradients. The acquisitions are subtracted to remove any velocity unrelated phase shifts. The measurement generates two series of CINE images, velocity and magnitude images. Each pixel in the velocity image displays the mean velocity by which the spins are moving and is proportional to the mean phase difference of the spins in that pixel (6). The velocity encoding is the constant that links phase difference with blood velocity. It is set by the user and specifies the maximum velocity that will be properly encoded by the sequence. When the velocity encoding is set, the strength of the velocity encoding gradients is adjusted so that the maximum velocity selected corresponds to a 180 phase shift. The magnitude image is used for anatomically orientation and contains information about the signal amplitude. Integration of the pixel values over the area of interest in the velocity image results in the instantaneous flow rate. With CINE acquisition, the flow rate can be measured as a function of time over the cardiac cycle and the grade of insufficiency can be estimated as the regurgitation volume (RV), which is found by integrating the flow rate over the diastolic part of the cardiac cycle. The grade of the insufficiency can also be estimated as the regurgitation fraction (RF), which is calculated as the relation between RV and the integrated flow volume during systole. There are challenges related to the phase contrast technique, which may affect the accuracy and precision of the flow estimates, e.g. random variation of spin phases, background effect, accelerating 1

7 flow, coronary flow, Aortic compliance, and movement of the Aortic root (Appendix II). These effects will vary in extent (1, 7, 8), depending on the position in the Aorta, how the image plane is angulated, and how well the velocity encoding matches the true velocities. Other has shown that the accuracy is highest closest to the Aortic valve (1) but it has not been reported how the repeatability is affected by the position of the image plane. Another parameter that has been reported to affect the accuracy and precision significantly is the choice of velocity encoding (9). The use of variable velocity encoding has even been suggested to better adjust the velocity encoding to the velocities during the cardiac cycle (9). We hypnotize that the velocity encoding should not affect the repeatability of the average flow estimate as long as the signal-to-noise-ratio (SNR) is high enough. However, there could be a bias in the flow determination, which in turn could affect the flow estimates. The accuracy could be affected if no, or lacking, background correction is applied. The aim of the present work was to study the influence of position of image plane and velocity encoding in flow measurements using 2D phase contrast MRI in healthy volunteers and in patients with AVI 1) to optimize both accuracy and precision in the estimation of the regurgitant flow and 2) to look for new ways to better characterize the regurgitation. Specifically, we aimed to: compare accuracy and precision for RV obtained with different velocity encoding in volunteers and patients compare accuracy and precision for RV and RF for different positions of the image plane in volunteers and patients compare flow rate curves for different positions of the image plane in volunteers and patients compare velocity offsets for different positions of the image plane and for different velocity encoding in volunteers and patients 2

8 2 Methods and material The study was performed on a cohort of 27 healthy volunteers (24-58 years) and 37 patients with AVI (27-83 years) with given informed consent. Before cardiovascular MRI (CMRI), all volunteers were scanned for the presence of shunt and for the present of contraindications following standard safety procedures. The volunteers had no evidence of valve regurgitation or any other cardiac disease. 2.1 Imaging system and protocol All examinations were performed on a 1.5 T whole-body Philips MRI scanner equipped with the five channel cardiac coil (Achieva, Philips Healthcare, Best, Holland). The complete scan protocol included scout imaging, morphological and functional studies of the heart, and phase contrast flow measurements. The scan protocol was completed within 45 minutes and without the use of any form of sedation. Multi planar thoracic scout imaging in sagittal, coronal and transaxial views with the heart at the center of the magnet were performed. CINE scout imaging leading to double oblique horizontal long axis (semi 4-chamber view) of the heart was then performed to line up the heart for preceding morphological and functional CMRI. For highest accuracy, all scouts were acquired during breath-hold at end-expiration. Morphological and functional measurements of the heart, used for guidance of the flow measurements, was visualized during breath-hold at end-expiration using retrospectively electrocardiography (ECG) gating CINE CMRI with a steady state free-precession sequence (FOV=320*320 mm, voxel size=167*167 mm, TR/TE=3.4/1.69 ms, bandwidth= Hz/pixel, flip angel=60, time frames per cardiac cycle=30, SENSE acc factor=2, TFE factor=14, TFE shots=7, NSA=1). To give clear views of all four cardiac valves and their inflow/outflow tracts, scans in short axis (SA), 2-chamber (2CH), 3- chamber (3CH), 4-chamber (4CH), left ventricle outflow tract, and right ventricle outflow tract views were performed. For direct estimation of AVI, the flow volume over the cardiac cycle was quantified using ECG-gated 2D through-plane phase contrast flow measurement (FOV= 320*260 mm, voxel size=2.5*2.5 mm, TR/TE=4.8/2.9 ms, bandwidth=477.8 Hz/pixel, flip angel=12, time frames per cardiac cycle=40, SENSE acc factor=2, TFE factor=4, TFE shots=13, NSA=1). The AVI was determined at two different image planes (image plane 1, image plane 2) that allowed quantification at four regions in the Aorta, i.e. two regions in the ascending Aorta (image plane 1: Ao1, image plane 2: Ao2) and two regions in the descending Aorta (image plane 1: DA1, image plane 2: DA2). Image plane 1 was positioned at the sinotubular junction and image plane 2 was positioned approximately 10 mm from image plane 1, in the cranial direction (Figure 1). All phase contrast flow measurements were carefully planned with the image plane orthogonal to the direction of flow in the ascending Aorta using the signal void of the regurgitant jet on the CINE images in the left ventricle outflow tract view as input information. Visualization of the regurgitant jet also enabled an insight to the severity of the insufficiency and to the velocity of the blood at the position of the phase contrast flow measurements. The velocity encoding was chosen high enough to avoid aliasing of velocities of the examined blood flow. If the maximum velocity in the examined flow was more than 20 % lower than the chosen velocity encoding, a new flow measurement was performed with adjusted velocity encoding. Examination of diastolic flows of lower velocity was performed at image plane 1. In the diastolic examination, the velocity encoding was set 10 cm/s higher than the maximum diastolic velocity accepting velocity aliasing in the systolic phase of the cardiac cycle. Measurements with velocity encoding adjusted to the flows in systole are hereafter referred to as high venc (high venc at Ao1: 3

9 AoH) and velocity encoding adjusted to flows in diastole are referred to as low venc (low venc at Ao1: AoL) (Figure 1). All measurements were repeated once, in order to generate information about the repeatability. In all CMRI measurements, the scan time was reduced by choosing the phase encoding axis of the field-of-view (FOV) in the narrowest anatomic direction and by minimizing the FOV as much as possible, ensuring that no wraparound artifacts affected the interpretation of the images. Special care was also taken to improve the temporal resolution. In all measurements, a correct heart rate was used and all slices were carefully planned in the end-diastole. Ao2 Image plane 2 Ao2 DA2 DA2 Image plane 2 Ao1 AoH AoL DA1 Image plane 1 Figure 1. Positions of the image planes with their given names (left) and resulting images of the Aorta with definitions of regions (right). The image planes are positioned orthogonal to the blood flow in the ascending Aorta. Image plane 1 is positioned at the sinotubular junction (ascending Aorta: Ao1(black), descending Aorta: DA1 (red)) and image plane 2 is positioned approximately 10 mm from image plane 1, in the cranial direction (ascending Aorta: Ao2 (blue), descending Aorta: DA2 (green)). Measurements with image plane 1 is done with two different velocity encodings, one adjusted to the flows in systole, high venc (AoH), and the other adjusted to flows in diastole, low venc (AoL). 2.2 Image analysis Conventional post procession of all images was performed using the dedicated work station of the MRI system (Easy Vision, Philips Healthcare, Best, Holland). The research tool Segment v1.9 R2046 (10) was also used for post processing to enable quantification of forward and backward flow rate. Both these analysis tools were used for flow quantification using a semiautomatic algorithm to segment vessels. A region of interest (ROI) was manually drawn around the ascending (ROI 1) and descending Aorta (ROI 2) in the image with highest contrast (Figure 2). The ROI was automatically adjusted to the vessel contour and the tools then propagated the ROI over all images in the cardiac cycle. If the tools failed to segment the vessel and follow the vessel contour in all images, the ROI was manually corrected. 4

10 Flow measurements affected by velocity aliasing artifact and measurements with low image quality for semiautomatic segmentation were excluded from the analysis. Measurements with large image plane angulation were also excluded. Adaption of the image plane orthogonal to the flow in the ascending Aorta resulted, for some patients, in oblique flow in the descending Aorta. To reduce the influence of image plane angulation in the study, flow measurements performed with image plane angulated > 15 (11) relative the descending Aorta was excluded from further analysis. To get an idea whether there was any bias in the flow measurements the velocity offset was studied in stationary tissue where no flow was expected. The software tool of Philips was used to quantify the velocity offset in the velocity image. Four ROIs (ROI 3, ROI 4, ROI 5 and ROI 6) with approximately 100 pixels were drawn in stationary muscle tissue, one in each corner of the image (Figure 2). The location and size were constant throughout the cardiac cycle. ROI 3 ROI 4 ROI 1 ROI 2 ROI 6 ROI 5 Figure 2. A magnitude image (right) and a velocity image (left) with ROIs positioned around the ascending (ROI 1) and descending Aorta (ROI 2) and in stationary muscle tissue (ROI 3, ROI 4, ROI 5 and ROI 6). 2.3 Statistical analysis Statistical analysis was performed for RV, RF, and total, forward and backward flow rate, estimated from ROI 1 and ROI 2, and for velocity offsets, estimated from ROI 3 - ROI 6. The RV, the RF, the total flow rate and the velocity offsets were quantified using the software tool of Philips. The forward and backward flow rate was quantified using the research tool Segment. The analysis was performed for repeated measurements and included calculations of mean, standard deviation of the mean, and coefficient of variation (cv), and tests for significance using Wilcoxon signed-rank test (Appendix III) Regurgitation volume and regurgitation fraction Mean values and variations of mean values were calculated for the RV and the RF in the volunteer and patient cohort for region Ao1, Ao2, DA2 and DA1 (Table 1). To get an idea of the influence of velocity encoding on the grading of the insufficiency mean values and variations of mean values were also calculated for the RV obtained with high and low venc at Ao1 (Table 1). The RV was then calculated using an offline software tool based on Matlab (Version 7.13 and software release R2011b, The Math Works Inc., Natick, MA). Analyzed data at region Ao1 were pushed forward to the Matlab 5

11 tool. The tool calculated, from the total flow rate, the flow volume over the systolic and diastolic phase of the cardiac cycle. End-systole was defined as the last positive flow rate value. The diastolic flow volume was defined as the RV. Subjects were excluded from the analysis if the peak velocity in the diastolic phase was higher than the velocity encoding. Table 1. Identification number of subjects included in the analysis of measurements at region Ao1, Ao2, DA2 and DA1 and with high (AoH) and low venc (AoL) at Ao1. The regions in the Aorta are presented in Figure 1. Volunteers (number of volunteers) Patients (number of patients) Ao1 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 1, 2, 4, 6, 7, 8, 9, 10, 12, 13, 14, 15, 16, 17, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27 (24) 29, 30, 31, 32, 33, 34, 37 (34) AoH 2, 9, 10, 13, 14, 19, 25, 26 (8) 3, 13,14, 16, 20, 26, 27, 28, 29, 30 (10) AoL 2, 9, 10, 13, 14, 19, 25, 26 (8) 3, 13,14, 16, 20, 26, 27, 28, 29, 30 (10) Ao2 2, 9, 24, 25, 26, 27 (6) 1, 2, 3, 4, 5, 6, 7, 8, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 37 (33) 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, DA2 2, 9, 26, 27 (4) 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 37 (35) DA1 1, 2, 4, 6, 7, 8, 9, 10, 12, 13, 14, 15, 16, 17, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 15, 16, 18, 19, 18, 19, 20, 21, 22, 23, 24, 26, 27 (23) 20, 21, 23, 26, 27, 28, 29, 30, 32, 33, 37 (26) To test for significance, Wilcoxon signed-rank test was performed for mean and cv for all combinations of region Ao1, Ao2, DA2 and DA1 and for high and low venc at Ao1. Significant differences in mean and cv was determined at a significance level of p<0.05. Table 2. Identification number of subjects included in Wilcoxon signed-rank test for combinations of region Ao1, Ao2, DA2 and DA1 and for high (AoH) and low venc (AoL) at Ao1. The regions in the Aorta are presented in Figure 1. Volunteers (number of volunteers) Patients (number of patients) Ao1-Ao2 2, 9, 24, 25, 26, 27 (6) 2, 3, 4, 5, 6, 7, 8, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 37 (32) AoH-AoL 2, 9, 10, 13, 14, 19, 25, 26 (8) 3, 13,14, 16, 20, 26, 27, 28, 29, 30 (10) Ao1-DA2 2, 9, 26, 27 (4) 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 37 (34) Ao1-DA1 1, 2, 4, 6, 7, 8, 9, 10, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 26, 27 (23) Ao2-DA2 2, 9, 26, 27 (4) Ao2-DA1 2, 9, 24, 26, 27 (5) DA1-DA2 2, 9, 26, 27 (4) 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 15, 16, 18, 19, 20, 21, 23, 26, 27, 28, 29, 30, 32, 33, 37 (25) 1, 2, 3, 4, 5, 6, 7, 8, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 37 (33) 1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 15, 16, 18, 19, 21, 23, 26, 27, 28, 29, 30, 32, 33, 37 (24) 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 15, 16, 18, 19, 20, 21, 23, 26, 27, 28, 29, 30, 32, 33, 37 (26) Flow rate To better characterize the regurgitant flow the total flow rate throughout the cardiac cycle was decomposed into forward and backward flow rate. The appearance of the flow rate curves in the 6

12 volunteer and patient cohort for region Ao1, Ao2, DA2 and DA1 (Table 1) were analyzed visually and compared for the different regions at the Aorta and between the volunteer and patient cohort Background Mean values and variations of mean values were calculated for the velocity offset in a cohort of both volunteers and patients with repeated measurements at the different image planes and velocity encodings. Identification number of volunteers included in the analysis were 2, 9, 25, 26 and 27, and identification number of patients included in the analysis were 1, 2, 3, 6, 7, 8, 9, 11, 12, 13, 14, 16, 17, 19, 20, 21, 22, 24, 26, 27, 28, 29, 30, 31, 32, 33, 34 and 37. The total number of subjects analyzed was 33. To test for significance in difference of velocity offset between images acquired at image plan 1 and 2 and with high and low venc, and between ROIs, Wilcoxon signed-rank test was performed for values of mean and cv. Significant differences in mean and cv was determined at a significance level of p<

13 3 Results In this study, a large cohort of patients with AVI was successfully examined to study the influence of position of image plane and velocity encoding on regurgitation estimates. Also, a small cohort of volunteers was successfully examined but with a limited scan protocol. The result showed significant differences in flow estimates and in variation of flow estimates in regard to the position of the image plane. No significant differences in flow estimates or in variation of flow estimates were found in regard to velocity encoding. Velocity offsets in the background were present in all velocity images and differed significantly with the choice of velocity encoding, and with the distance from the isocenter of the magnet. No significant differences in velocity offsets were found in regard to position of the image plane. Decomposition of the estimated flow rate over the cardiac cycle in forward and backward flow rate revealed new interesting flow characteristics in patients with AVI. The characteristics of the flow rate were similar for all healthy volunteers showing low flow rate over the whole cardiac cycle and increased flow rate at end-systole, when the flow rate in the coronary arteries was high. In most patients, backward flow rate was measured during the whole cardiac cycle and two characteristic appearances of the flow rate curves were seen. 3.1 Regurgitation volume and regurgitation fraction Low values of RV and RF were estimated in the cohort of healthy volunteers, and no major differences were found between the individuals. No significant differences were found for RV, cv of RV, RF or cv of RF in regard to position of image plane and velocity encoding (p>0.06). RV with high venc: 2.1±0.5 ml and low venc: 1.9±0.2 ml; cv of RV with high venc: 50.7±44.0 % and low venc: 45.5±49.3 %; RV at Ao1: 0.9±0.1 ml, Ao2: 0.6±0.1 ml, DA2: 0.8±0.1 ml and DA1: 0.8±0.1 ml; cv of RV at Ao1: 24.8±26.8 %, Ao2: 30.5±71.7 %, DA2: 19.1±60.1 % and DA1: 31.5±50.6 %; RF at Ao1: 1.0±0.1 %, Ao2: 0.6±0.2 %, DA2: 1.1±0.1 %, DA1: 1.2±0.1 %; cv of RF: Ao1: 25.9±28.3 %, Ao2: 42.4±93.6 %, DA2: 15.6±48.6 % and DA1: 28.7±43.7 %. High values of RV and RF were estimated in the cohort of AVI patients, and large differences were seen between individuals (Figure 3a,b). The RV decreased in general with increased distance from the Aortic valve (Figure 3a) and the RF was decreased in the descending Aorta relative the ascending Aorta (Figure 3b). The cv of RV and the cv of RF were also different in regard to position of image plane. No general differences were seen regarding choice of velocity encoding (Figure 3a). 8

14 Regurgitation fraction (%) Regurgitation volume (ml) a 0 Ao1; AoH; AoL Ao2 DA2 DA Ao1 Ao2 DA2 DA1 b Figure 3. a) Regurgitation volume (RV) and b) regurgitation fraction (RF) determined in the patient cohort. Grey dots visualize the mean of repeated measurements of each individual patient and black dots visualize the mean of all patients with assumed increased distance from the valve plane, i.e. region Ao1, Ao2, DA2 and DA1 (black ) as well as with high (AoH (red )) and low venc (AoL (blue )) at image plane 1 in the ascending Aorta (Ao1).The standard deviation of the mean (propagated from repeated measurements in individual patients) is smaller than the extension of the symbols in the Figures. The analyzed regions in the Aorta are presented in Figure 1. The RV significantly decreased in the patient cohort when the distance from the Aortic valve increased, except in the descending Aorta between region DA1 and DA2 (Table 3). No significant difference in RV was found between high and low venc at region Ao1 (Table 3). It was found that cv of RV was significantly higher at Ao1 compared with DA1 and that no other differences for positions and choice of velocity encodings were significant (Table 3). The cv of RV were at Ao1: 6.3±5.6 %, Ao2: 10.3±13.0 %, DA2: 10.3±9.6 % and DA1: 17.7±27.0 % and with high venc: 22.2±20.1 % and low venc: 16.0±21.3 %. The RF in the patient cohort was significantly lower in the descending Aorta compared with the ascending Aorta (Table 3). There were no significant differences between region Ao1 and Ao2 in the ascending Aorta or between region DA2 and DA1 in the descending Aorta (Table 3). It was found that cv of RF was significantly higher at Ao1 compared with Ao2 and DA1 and that no other differences were significant (Table 3). The cv of RF was at Ao1: 7.4±6.3 %, Ao2: 12.8±14.3 %, DA2: 11.0±9.9 % and DA1: 19.1±28.2 %. 9

15 Table 3. Significant differences in regurgitation volume (RV), cv of RV (cv(rv)), regurgitation fraction (RF) and cv of RF (cv(rf)) displayed as p-value for Wilcoxon signed-rank test between different regions in the Aorta (Ao1, Ao2, DA2 and DA1) and between high (AoH) and low venc (AoL) at Ao1in the ascending Aorta. The analyzed regions in the Aorta are presented in Figure 1. Number of compared patients RV cv(rv) RF cv(rf) Ao1-Ao2 32 <0.0001* * <0.4744* * AoH-AoL 10 <0.4922* * - - Ao1-DA1 34 <0.0001* * <0.0001* * Ao1-DA2 25 <0.0001* * <0.0001* * Ao2-DA1 33 <0.0001* * <0.0001* * Ao2-DA2 24 <0.0001* * <0.0001* * DA1-DA2 26 <0.7509* * <0.3602* * * Symbols significance 3.2 Flow rate The magnitude of the estimated total, forward and backward flow rate were consistently higher in the patient (Figure 4b,d) than in the volunteer cohort (Figure 4a,c). In both cohorts, the magnitude of total, forward and backward flow rate were consistently higher at Ao1 and Ao2 in the ascending Aorta than at DA2 and DA1 in the descending Aorta. The appearance of the curves corresponding to measurements at Ao1 and Ao2 was similar as well as the curves for DA2 and DA1. It was also seen that the curves for DA2 and DA1 were shifted in time relative the curves for Ao1 and Ao2. In both cohorts, the forward flow rate was generally higher than zero during diastole. In the volunteer cohort, a dip was seen in the total flow rate at Ao1 and Ao2 in end-systole, when the flow rate in the coronary arteries was high (Figure 4a). Decomposition of the total flow rate in forward and backward flow rate exposed the dip more clearly (Figure 4c). In the patient cohort, regurgitant flow was clearly visualized at Ao1, Ao2, DA2 and DA1 as a strong reduction of the total flow rate during diastole (Figure 4b). Decomposition of the total flow rate in forward and backward flow rate exposed the regurgitant flow more clearly (Figure 4d). Backward flow at Ao1 and Ao2 was found during the whole cardiac cycle and the appearance of the backward flow rate curves was oscillating. 10

16 Flow rate (ml/s) Flow rate (ml/s) Flow rate (ml/s) Flow rate (ml/s) a -250 Cardiac cycle b -250 Cardiac cycle c -250 Cardiac cycle Figure 4. Total flow rate curves for the a) volunteer and b) patient cohorts, and forward and backward flow rate curves for the c) volunteer and d) patient cohorts in the ascending Aorta at Ao1 (black ) and Ao2 (blue ), and in the descending Aorta at DA2 (green Δ) and DA1 (red ). The analyzed regions in the Aorta are presented in Figure Backward flow rate There was a clear difference between the size of the backward flow rate in the volunteer (Figure 5a-d) and patient cohorts (Figure 5e-i). The backward flow rate was low in all volunteers (Figure 5a-d). During end-systole, the backward flow rate showed a peak in both the ascending (Figure 5a,b) and the descending Aorta (Figure 5c,d). The backward flow rate was in general much larger in the patients than in the volunteers. The backward flow rate was high in most patients (Figure 5e-i). Regurgitant flow was found in both the ascending (Figure 5e,f) and the descending Aorta (Figure 5g,h). Two different appearances of the backward flow rate curves were found in the ascending Aorta (Figure 5e,f). Figure 5i attempts to visualize these different appearances. The red curves represent a oscillating appearance with an increased backward flow rate with start in early systole and the blue curves represent a less oscillating appearance with an increased backward flow rate more located towards diastole. For some patient, the appearance of the curve differed between Ao1 and Ao2. d -250 Cardiac cycle 11

17 Flow rate (ml/s) Flow rate (ml/s) Flow rate (ml/s) Flow rate (ml/s) Flow rate (ml/s) Flow rate (ml/s) Flow rate (ml/s) Flow rate (ml/s) Flow rate (ml/s) Cardiac cycle Cardiac cycle Cardiac cycle Cardiac cycle a b c d Cardiac cycle Cardiac cycle Cardiac cycle Cardiac cycle e f g h i Cardiac cycle Figure 5. Backward flow rate curves in individual a-d) volunteers and e-h) patients in the ascending Aorta at Ao1 and Ao2, and in the descending Aorta at DA2 and DA1. The figures in the two upper rows, from left to right, represent the quantified regions at the Aorta with an assumed increased distance from the valve plane. i) Enlargement of individual backward flow rate curves in the patient cohort at Ao1. The red curves represent a oscillating appearance with an increased backward flow rate in early systole and the blue curves represent an increased backward flow more located to diastole. The y-axis is located approximately between the systolic and diastolic phase of the cardiac cycle. The analyzed regions in the Aorta are presented in Figure Background The velocity offset in the images was in general lower when a low venc was used and when quantification was performed close to the isocenter of the magnet (Figure 6a). In general, the highest velocity offsets were obtained in ROI 4 and lowest velocity offsets was obtained in ROI 6. The position of the image plane did not have large influence on the velocity offset. No major difference in cv of the velocity offset between image plane 1 and 2 or between high and low venc was found (Figure 6b). 12

18 Velocity offsets (cm/s) cv (%) High venc; 1 High venc; 2 Low venc; High venc; 1 High venc; 2 Low venc; 1 a Figure 6. a) Velocity offset and b) cv of the velocity offset in ROI 3 (black ), ROI 4 (blue ), ROI 5 (green Δ) and ROI 6 (red ). The positions of the ROIs are presented in Figure 2. The error bars in a) represent the error propagated standard deviation from repeated measurements of individual subjects end the error bars in b) represent the standard deviation among all subjects. High venc; 1 represents high venc at image plane 1, High venc;2 represents high venc at image plane 2 and Low venc; 1 represents low venc at image plane 1. When high venc was used at image plane 1, the velocity offset in ROI 4 was significantly higher compared with all other ROIs (ROI 3: p<0.001, ROI 5: p<0.006, ROI 6: p<0.001). When high venc was used at image plane 2, the velocity offset in ROI 3 and ROI 4 were significantly higher compared with ROI 5 (p=0.006, p<0.04) and the velocity offset in ROI 3 was significantly higher compared with ROI 6 (p<0.03). When low venc was used at image plane 1, the velocity offset in ROI 3 and ROI 4 was significantly higher compared with ROI 6 (p<0.002, p<0.004). Statistical comparison in velocity offsets between image plane 1 and 2 and high and low venc showed that the velocity offset in all ROIs where significantly lower when low venc was used compared with when high venc was used (p<0.02), and that the velocity offset in ROI 4 in image plane 2 was significantly lower compared with image plane 1 (p<0.005). Statistical comparison in cv of the velocity offset showed that the cv for ROI 5 was significantly higher than for ROI 3 when high venc was used at image plane 2 (p<0.05). No other significant differences for velocity offset were found (p>0.05). b 13

19 4 Discussion This work describes if, and to some extent how, accuracy and precision in estimation of regurgitant flow in healthy volunteers and patients with AVI are affected by the position of the image plane and the choice of velocity encoding in flow measurements using 2D phase contrast MRI. In volunteers, no significant differences in the mean or in the variation of the mean of regurgitant flow estimates were seen in regard to choice of velocity encoding or position of the image plane. In patients though, we found significant differences in the mean and in the variation of the mean of regurgitant flow estimates in regard to position of the image plane. 4.1 Regurgitant flow visualization An interesting finding was obtained from observations of flow rate curves in the patients. When the total flow rate was decomposed in forward and backward flow rate, the regurgitant flow was visualized throughout the whole cardiac cycle in the ascending Aorta. Aside from the expected backward flow in diastole, backward flow was also measured during systole. Whether the observation is true, that blood actually passes the Aortic valve during systole, or caused by complex flow adjacent to the image plane is not possible to determine from the results of this study. The finding is, however, interesting as it adds more knowledge to the diagnosis of the patient and to the confidence of the diagnosis. The grading of the AVI by RV and RF highly depends on the time dependence of the flow during the cardiac cycle. The RV is conventionally estimated by integrating the total flow rate curve over the diastolic phase of the cardiac cycle and the RF is conventionally calculated as the relation between RV and the integrated flow volume during systole. If the observation is true, the RV may be underestimated as regurgitant flow during systole then is excluded. The excluded volume is included in the resulting forward volume during systole, and will together with the underestimated RV affect the estimated RF. If the observation is false, the RV may be correctly estimated as the false regurgitation volume is excluded from the calculation. The RF may be overestimated as the forward volume then includes the false regurgitation volume and becomes underestimated. The size of the error depends on the appearance of the backward flow rate curves. In this study, two characteristic appearances of the curves on an individual level were visualized. The appearance were in general either oscillating with an increased backward flow rate with start in early systole or less oscillating with an increased backward flow rate more located towards diastole. The error in the grading of the AVI may then in higher extent affect patients with the first mentioned appearance. 4.2 Eddy current effect Phase contrast measurements are sensitive to eddy currents due to difference in gradient waveforms in the subtracted acquisitions (9, 12). Eddy currents affect the images by adding a velocity offset to the pixel values (12). The scan protocol was set up to minimize the eddy current effect in the images by for example application of retrospective gating (9) and local phase correction filter (LPC-filter). Even so, detectable velocity offsets were present in all images. A velocity offset of 0.6 cm/s was representative for our measurements. As even a small velocity offset errors can cause large errors in measurements of blood flow volume (13), a rough calculation was performed to get an idea of the size of the error. The velocity offset 0.6 cm/s was integrated over the cardiac cycle and an area corresponding to the average area of the ascending Aorta. The flow volume obtained was compared with the estimated flow volume in systole. The resulting error was < 5 %. Hence, we do not believe that it should cause any significant errors in the calculation of regurgitation volume. Obviously, this is only a rough estimation and we cannot guarantee that larger errors can occur. The estimated velocity offsets were consistently positive, indicating that a positive bias was present in the images. This was 14

20 seen in the forward flow rate. That is, during diastole the forward flow rate was higher than zero in both the volunteer and patient cohort. In general, the velocity offsets were higher in ROI 4, which anatomically was positioned further away from the isocenter of the magnet. In the image, the distance certainly seems to be about the same for all ROIs, but in reality, ROI 4 is anatomically further away. This is probably due to the scaling of the eddy current effect with the distance from the isocenter in combination with better correction of the effects closer to the isocenter. An even higher value was seen in ROI 4 at image plane 1. This might be because blood and lymphatic flow, not visible due to low spatial resolution and SNR, have been included in the quantification. 4.3 Influence of position of image plane The size of the eddy current effect may depend on the position and angulation of the image plane as the gradient waveforms then changes. This was not seen in our study. The position and angulation of the two image planes were probably too similar to cause any difference in velocity offsets caused by the eddy current effect. The choice of the position of the image plane is, though, of great importance to quantify the AVI accurately (1). Our measurements in the ascending Aorta most likely better describe what happens at the Aortic valve than the measurements in the descending Aorta. This assumption is based on the larger distance from the actual regurgitating valve. 2D phase contrast flow measurements register the through plane flow at the position of the image plane and do not register the blood volume between the image plane and the Aortic valve that does not pass through the image plane during the cardiac cycle. In the descending Aorta, the blood volume between the image plane and the Aortic valve is larger than in the ascending Aorta, which prevents a larger volume of regurgitating blood from being registered by the image plane. Also, the blood volume to the arteries that branch from the Aortic arch is not registered by the image plane in the descending Aorta. Measurements of regurgitation flow should therefore be performed in the ascending Aorta as close as possible to the regurgitating valve. Further discussion will be focused on measurements that were performed closer to the valve, i.e. measurements in the ascending Aorta. In this study, the image planes were positioned at and above the sinotubular junction to reduce the influence of convergent, accelerating diastolic flow in the vicinity of the regurgitant orifice (9). Complex flow cause considerably velocity errors (14) and misregistration of the exact location of the measurement (14, 15), but should not be a problem in neither of our image planes. As both image planes used in this study were positioned above the coronary ostia, all flow measurements will include the coronary flow. This effect was seen both in the total and backward flow rate curves in volunteers as a peak value at end-systole. The coronary flow is in general 2 to 4 ml/beat (1) which is proximately 5 % of the cardiac output (13). The contribution from coronary flow will cause an overestimation of the true regurgitant flow volume and may influence the grading of mild AVI (1). Results of others (1) indicates that the coronary flow effect may be lower further downstream the coronary ostia. In regard to this, the effect should be less in image plane 2 than in image plane 1. In the same study, it was reported that the effect of Aortic compliance caused an underestimation of the true regurgitant flow volume. The errors caused by the Aortic compliance effect were reported to be considerable at the sinotubular junction and even higher beyond the sinotubular junction (1). In regard to this, the effect of Aortic compliance should be less in image plane 1 than in image plane 2. When the regurgitant jet flows back into the left ventricle the Aortic root will move in the opposite direction, typically 6-12 mm (9). This will results in a volume change below the image plane. The volume corresponding to the 15

21 volume change, minus the coronary flow volume will be registered as forward flow and the true regurgitant flow volume will be underestimated (9). The volume change and the underestimation are probably in the same order when using image plan 1 and 2. We believe that the total effect of coronary flow, Aortic compliance and movement of the Aortic root will cause an underestimation of the true regurgitant flow volume. Therefore, we think that the highest RV and RF in our measurements should be the most accurate. The RV decreased as the distance from the Aortic valve increased and the RF was higher in the ascending Aorta than in the descending Aorta. Image plan 1 should then provide more accurate measurements than image plane 2. Our results also show that the precision of RV and RF is highest at the position closest to the Aortic valve, which is in line with the results of others (1). Therefore, when estimating the valve insufficiency in the ascending Aorta the image plane should be positioned at the sinotubular junction to obtained highest accuracy and precision of the regurgitant flow. Holodiastolic flow, defined as presence of regurgitated flow in the descending Aorta during the whole diastolic cycle, is indicative of severe AVI. It has been reported that it can be used in combination with regurgitant flow estimates to stratify the severity of AVI (16). To enable quantification of holodiastolic flow in the descending Aorta simultaneously with the quantification of RV and RF in the ascending Aorta, the flow measurements should be performed in image plane 2 that is an image plane positioned approximately 10 mm above the sinotubular junction. Using such image plane position, the measurement will be less hampered by oblique flow. In our study, approximately 25 % of all measurements performed at image plan 1 had to be excluded from the analysis due to too large image plane angulation in the descending Aorta. 4.4 Influence of velocity encoding The choice of velocity encoding has been identified as an important factor in the estimation of total, forward and backward flow measurements in the mid-ascending Aorta (17). It has been reported that lower velocity encoding in diastole significantly improves the accuracy and precision of blood flow volume measurements in the ascending Aorta (7) and that a too large velocity encoding systematically underestimates the flow during diastole (7, 18). Our results partly coincide with these reports. The mean velocity offset, studied in muscle tissue, was significantly reduced with lower velocity encoding. This was expected due to less eddy current effects with lower amplitude of the gradient waveforms. The cv of the mean velocity offset and the estimated RV, however, did not show a significance dependence on the choice of velocity encoding. This is probably due to an effective reduction of the velocity offset by the LPC-filter in the MRI scanner. 4.5 Potential errors during image analysis The image analysis may have influenced the accuracy of the flow quantification. For example, noise and edge pixels in the vessel contours can result in erroneous mean velocity information and hence inaccurate mean flow measurements (19). As the software tool of Philips had no tool for analysis of the variable velocity encoding measurements and no tool for visualization of the forward and backward flow rate, two other analysis softwares were also used for the image analysis. We believe that the use of different analysis softwares may not have affected the conclusion of this study as they were based on comparisons of results extracted from only one analysis software at a time. 4.6 Limitations about the study Wilcoxon signed-rank test requires six pairs of data to test for significance. With a significance level of p<0.05, less than six pairs will result in a p-value higher than 0.05 even though all differences have 16

22 the same sign. The statistical patient data base was sufficiently large to allow for validation of accuracy and precision of flow measurements in regard to position of image planes. In the volunteer cohort, however, we could not test for significance for the regions Ao2 and DA2 as we lacked sufficient numbers of measurements at those regions. 4.7 Future aspects The visualized backward flow rate during systole may have significantly affected the grading of AVI and requires further analysis. Firstly, it is important to verify whether the observation is true or not and, if true, to determine how the flow estimates are affected. Secondly, it would be interesting to correlate the different appearances on the backward flow versus time curves with functional estimates and to correlate the appearance with patient status and progression of the disease with purpose to improve the characterization of the AVI. In the analysis of variable velocity encoding, many measurements were excluded as the peak velocity in diastole exceeded the velocity encoding. The analysis still contained enough subjects to test for significance, but no differences was found in contrary to what has been reported by others (7). It would be interesting to improve the statistics by collecting a larger material and to see if we will achieve significant differences for different velocity encodings. 17