ECG Gated CT Aorta in Transcatheter Aortic Valve Implantation

Transcription

1 ECG Gated CT Aorta in Transcatheter Aortic Valve Implantation Poster No.: C-2014 Congress: ECR 2014 Type: Educational Exhibit Authors: M. A. Ottesen; Oslo/NO Keywords: Cardiac, Arteries / Aorta, CT, CT-Angiography, Diagnostic procedure, Arteriosclerosis DOI: /ecr2014/C-2014 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. Page 1 of 13

2 Learning objectives To acquire knowledge about ECG gated CT aorta and the background for performing this examination. This poster reviews the CT protocol for ECG gated aorta and the contribution of Multidetector CT Angiography to make measurements of the aortic valve annulus possible, and to evaluate the arteries to plan a Transcatheter Aortic Valve Implantation (TAVI) procedure preoperatively. The main focus will be on CT scanning and reformatting protocols. The aim is to share information about how to perform a gated CT Aorta which in turn provides the surgeons and cardiologists with reliable information about the vascular anatomy and as accurate measurements of the aortic valve as possible thus ensuring a safer and faster TAVI-procedure for the patient. Background Aortic stenosis is a common disease that affects 5% of people over 75 years of age [1]. The disorder consists of pathological changes in the aortic valve, causing restriction of valve motion, restricting blood flow from the left ventricle to aorta, and causing increased pressure in the left ventricle. If the condition is left untreated the prognosis is poor. Transcatheter Aortic Valve Implementation procedure has been developed as an alternative to open heart surgery. The method is less invasive as a mechanical valve is inserted through the blood vessels. The aortic annulus is a complex structure. CT provides a detailed anatomical map of the aorta including the femoral arteries, and retrospective ECG gating of the aortic root renders motion free images of the aortic annulus. The CT examination facilitates accurate measurement of the annulus for correct sizing of the artificial valve. In addition, it enables evaluation of the vascular route for accessing the valve during an interventional procedure. Accurate measurement of the aortic valve is necessary as artificial valves are designed in specific sizes. Open heart surgery enables direct access to determine the size of the valve opening, but when the TAVI-technique is used, medical imaging is crucial for determining valve size. In addition to CT, transthoracic and transesophageal echocardiography (TTE and TEE) are used to measure the annulus. Page 2 of 13

3 This treatment is to insert a new, functioning valve inside the opening of the native valve. If the new valve is not positioned correctly there is a risk of severe complications. To visualize both a motion free aortic root and the entire aorta and iliofemoral arteries with MDCT there are traditionally two possible approaches. Either one scan with ECG-gating covering the entire aorta and iliofemoral arteries may be performed, resulting in a high radiation exposure, or alternatively, the examination may be performed in two parts, first a scan covering the thoracic aorta with retrospective ECG-gating, followed by a helical, non-gated scan of the abdominal aorta and the iliofemoral arteries. This option requires either a double or very long contrast agent bolus. Variable Helical Pitch (vhp) available on the 320-row scanner, facilitates a combination of gated and non-gated acquisitions within one scan thereby reducing both radiation dose and contrast media load compared to traditional protocols. First the thoracic aorta from above the arch to below the aortic root is scanned with ECG-gating and a low pitch, then the scan mode is switched to a higher pitch without ECG-gating covering the remaining aorta and the iliofemoral arteries. Images for this section: Page 3 of 13

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5 Fig. 1: Edwards Sapien XT valve used in a TAVI procedure. [2] Fig. 2: Medtronic CoreValve used in a TAVI procedure. [2] Page 5 of 13

6 Findings and procedure details Preparation: The patient is positioned supine with the feet first entering the scanner. An 18G peripheral venous catheter is placed in the right arm for contrast media administration, as injection in the right arm reduces artifacts from incoming contrast media. The patient is instructed to maintain a shallow inspiration during the topograms, and the scan itself. Front and lateral topograms from over the apex of the lung to the bottom of the pelvis are acquired. ECG electrodes are connected to the patient, and it is important to check if the pulse is stable, as retrospective gating seldom is successful in patients with arrhythmia; in these case an alternative scan protocol is preferred. Breath hold: A breath exercise needs to be performed before scanning. The pulse often falls during breath hold, and measurement of this effect is used to automatically adjust the pitch if needed. Shallow inspiration is preferable Pitch: A variable helical pitch is used during the scan. A low pitch is used over the thoracic area with ECG gating to gather data of the heart in all phases of contraction. The length of the low pitch area can be adjusted for each exam. It is important to ensure that the aortic annulus is covered by the low pitch area. A higher pitch is used over the abdomen with no gating. Contrast medium injection parametres: A two phase injection protocol is employed. The first contrast bolus is adjusted to patient weight, 0,8 ml/kg Iomeron 350 mgi/ml, and is followed by the simultaneous injection of contrast media and saline, 50/50%. To ensure sufficient attenuation throughout the entire scan the injection duration must be relatively long. The injection rate is adjusted to achieve an injection duration of 16 seconds for the first bolus while the volume of the mixed bolus is adjusted to achieve a total injection duration that is 10 seconds longer than the planned acquisition. Simultaneous injection of contrast and saline serves to maintain attenuation while minimising artefacts from dense contrast media on the way to the heart. The injection parameters have been adjusted over time with increasing experience resulting in a robust protocol. Scan timing: When the injection commences a dynamic measurement of contrast density in a region of interest (ROI) in the aorta is performed, just below carina. The scan is triggered when contrast density in the descending aorta reaches 180 Houndsfield Units. Page 6 of 13

7 Aquisition parameters Toshiba Aquillion One: kv ma Collimation Slice Rotation Pitch thickness time Scan SFOV direction (AEC) 80 Cranio- 0,5 0,35 vhp Iterative reconstruction Medium AIDR 3D caudal The kv can be adjusted for large or slim patients. A successful procedure is defined as images of the aortic valve without motion artifacts, and images of the entire aorta and connecting vessels with a contrast density of 300 HU or higher. Post processing: Multiple phases of the heart's contraction are reconstructed retrospectively. Reconstructions include images of the aortic valve and root in phases 25-35% (5% interval) and 75% of the R-R interval. These images are sent to a workstation. The radiologist selects the phase best suited for measurements. Double oblique axial images of the aortic root are reconstructed and used to measure area-derived, maximal and minimal diameter of the annulus. The distance from the aortic annulus to the coronary ostias are measured as well as the smallest luminal diameter of the femoral arteries on both sides to evaluate ileofemoral access. Images for this section: Page 7 of 13

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9 Fig. 3: Illustration of the anatomical area covered by different pitch. The low pitch area is ECG gated. Fig. 4: Measurements of the area-derived, maximal and minimal diameter of the annulus. Page 9 of 13

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11 Fig. 5: Measurements of the smallest luminal diameter of the right femoral arteries, to evaluate ileofemoral access during a TAVI procedure. Page 11 of 13

12 Conclusion Variable helical pitch with ECG gating on a 320 row scanner facilitates the acquisition of motion free images of the aortic root and visualization of the aorta and iliofemoral vessels within one scan, subsequently enabling accurate measurement of the aortic annulus and evaluation of the most appropriate vascular route for valve access in TAVI procedure. Optimal preoperative planning may reduce the risk of complications due to inaccurate valve sizing and positioning. Images for this section: Fig. 6: Balloon-inflation of a valve stent during a TAVI-procedure performed on Oslo University Hospital, Rikshospitalet. Page 12 of 13

13 Personal information Maren Agnete Ottesen, Radiographer Department for Radiology and Nuclear Medicine Oslo University Hospital, Rikshospitalet Oslo, Norway References 1. Nkomo VT, Gardin JM, Skelton TN, Gottdiener JS, Scott CG, Enriquez-Sarano M. Burden of valvular heart diseases. Lancet 2006;368: Leipsic J, Gurvitch R, LaBounty TM, Min JK, Wood D, Johnson M et al. Multidetector computed tomography in transcatheter aortic valve implantation. J Am Coll Cardiol Img 2011;4: Page 13 of 13