Pulmonary embolism: Strategies to optimize pulmonary MDCT angiography studies

Transcription

1 Pulmonary embolism: Strategies to optimize pulmonary MDCT angiography studies Poster No.: C-0152 Congress: ECR 2013 Type: Educational Exhibit Authors: G. Viteri, A. Garcia-lallana, I. Simon Yarza, A. Villanueva Marcos, J. C. Pueyo, G. Bastarrika; Pamplona/ES Keywords: CT-Angiography, Pulmonary vessels, Thorax DOI: /ecr2013/C-0152 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 41

2 Learning objectives To review the MDCT pulmonary angiography protocols for the diagnosis of pulmonary embolism and the major optimization strategies available. Background Multidetector computed tomography (MDCT) has become the gold standard technique for the diagnosis of pulmonary embolism (PE). However, it is important to highlight that the wide availability and high performance of MDCT pulmonary angiography has led to a significant increase in the number of scans performed with the consequent increase in radiation exposure and contrast in the population. As a result, the development of optimized protocols to rule out PE that decrease the radiation and contrast dose has become a priority for thoracic radiologists. Summary of optimization strategies available: 1) Patient Selection Indication Body habitus 2) Parameters of the radiation beam Tube potential (kvp) Tube current modulation 3) Contrast Management Protocol Injection rate Contrast concentration Contrast volume 4) Direction of the acquisition of the study Craniocaudal Caudocranial Page 2 of 41

3 5) The Radiology report Presence or absence of PE Location Severity 6) Colour coded perfusion maps Which are the objectives of optimizing protocols for MDCT pulmonary angiography to rule out PE? To lower the doses of radiation and contrast while maintaining a sufficient image quality for the detection of PE so that it is comparable to that of conventional protocols. Why reduce the radiation dose? Several studies about the effects of exposure to ionizing radiation resulting from medical procedures have shown that radiation dose comparable to a few CT scans pose an increased risk of developing cancer. Although this increased risk is very small at the individual level, the rapid growth in the number of CT scans performed at the population level in recent decades has led to an increased interest in improving acquisition protocols to minimize the impact on the population level. Which patients will benefit most? The groups of patients that will benefit most from a reduction in the dose of radiation are; those with PE with low risk of mortality, patients with low clinical probability of having PE and younger patients, especially women, due to the greater radiosensitivity of the breast. Why lower the dose of iodinated contrast? There are many references in the literature about the nephrotoxic effect of intravenous administration of iodinated contrast, producing the condition known as contrast-induced nephropathy (CIN). In fact, the significant deterioration of renal function prior to the the performance of the study will be one of the criteria of contraindication. Not only the existence of a relationship between the administration of intravenous contrast and the deterioration of renal function has been demonstrated, but also has been found that this Page 3 of 41

4 relationship is dose dependent. Therefore, reducing the dose of intravenous iodinated contrast would be desirable for all patients and particularly for those who have a baseline impaired renal function and thus have an increased risk of developing CIN. Imaging findings OR Procedure details Optimization strategies 1. Patient Selection Proper selection of the patients that are going to undergo the MDCT pulmonary angiography to rule out PE is the best tool to avoid unnecessary tests and implement the most appropriate acquisition protocol. It has to obtain enough quality images with doses as low possible of radiation and contrast. Main factors to assess -Clinical indication of the study -Body habitus A) Clinical indication of the study The actual prevalence of PE in patients with clinical suspicion is low, according to several studies (10-35%). This is because both the clinical (symptoms and signs) and basic tests (blood analysis, a simple chest X-ray, ECG) are nonspecific in this disease. Therefore, a high percentage of patients with suspected PE actually have a very low probability of presenting this pathology, and thus not being indicated to undergo a MDCT pulmonary angiography. Consequently, to make the right clinical risk stratification of PE is essential to avoid performing unnecessary CT scans. Guides have been published offering numerous diagnostic algorithms which determine in which cases of suspected PE is indicated to perform a MDCT angiography based on risk stratification of patients. There are two types of risk stratification that must be performed. 1. Early mortality risk (Fig. 1 on page 15) Page 4 of 41

5 High (above 15%): When the patient has shock or hypotension. Intermediate or low: There may be signs of right ventricular dysfunction or myocardial injury but the patient is not in shock or hypertensive. MANAGEMENT If the risk of early mortality is high: indicative of immediate MDCT pulmonary angiography study if the patient's condition allows it (Fig. 2 on page 15). If the risk is intermediate or low: To assess the clinical probability of PE (Fig. 3 on page 16). 2. Clinical probability of PE Determines the attitude in patients at risk of early mortality intermediate or low. The clinical assessment of the probability of PE can be made based on clinical experience, although it is advisable to use one of the many risk scores that have been created for that purpose. One of the most widely used is the score of Wells Wells Score: (Fig. 4 on page 17, Fig. 5 on page 18) Risk factors: history of DVT or PE (1.5 points), surgery or immobililization (1.5 points), oncological disease (1 point) Symptoms: Haemoptysis (1 point) Signs: Tachycardia (1.5 points), signs of DVT (3 points) Clinical Trial: If the alternative is less likely than PE (3 points) MANAGEMENT High probability (7 or more points): Indication of MDCT pulmonary angiography. Intermediate (2-6 points) or low (0-1) probability: Analyze the dimer D. If the result is positive there is indication for MDCT pulmonary angiography. Conclusion: The best measure to reduce radiation dose and contrast is to avoid performing unnecessary examinations. This requires making a good pre-test risk stratification of PE. Page 5 of 41

6 B) Body habitus: The patient's body habitus expressed both in weight and body mass index (BMI) is the single largest factor that will influence vascular enhancement obtained during exploration. The relationship between body habitus and vascular enhancement is inversely proportional, so that the higher the weight or body mass index (BMI), the lower the degree of enhancement (Fig. 6 on page 19). This relationship justifies the need to adjust the contrast administration protocol depending on the patient's weight or BMI. In addition, body habitus influences also directly proportional to the noise of the image (Fig. 7 on page 20). Since both the degree of vascular enhancement and image noise will influence the quality of the images obtained and its diagnostic utility, it is essential to adapt the acquisition protocol of the study to the patient's body habitus. Thus, in patients with a high BMI and weight it will be necessary to place restrictions on the optimization strategies of radiation and contrast dose to maintain sufficient image quality for diagnosis (Fig. 8 on page 21): Patients with <80 kg or BMI <25 - It is possible to apply optimization measures as the decreased tube potential until 80 kvp tube and the automatic exposure control. - It is possible to apply protocols with low doses of contrast. Patients with> 100 kg or> 30 BMI -Restrictions to implement low-kilovoltage protocols: There are no studies that demonstrate the maintenance of the quality of the study with kvp protocols in these patients. -Restrictions on the application of the automatic exposure control: Various studies have demonstrated a lower efficiency of these systems in obese patients, reaching in some cases even an increase of the effective radiation dose administered. - Require high contrast dose protocols. 2. Radiation beam parameters Page 6 of 41

7 A. Tube potential (kvp) It determines the energy of the photons (kev) of the X-ray beam and the total energy flux. A decreased tube potential is one of the strategies most used, both for its effectiveness and for its ease of implementation. More recent protocols propose the use of kvp compared with previous protocols that used Kvp (Fig. 9 on page 22, Fig. 10 on page 23). Consequences of using a decreased kilovoltage: a) Effect on image quality Since a decreased kilovoltage produces both increased vascular enhancement and noise compared to conventional protocols, the parameters used to compare the quality of the image should be contrast-to-noise ratio and signal-to-noise ratio. A review of the literature shows that the noise and contrast ratios obtained with protocols of 120, 100 and 80 kvp show no significant differences at the level of the central or peripheral vessels when applied in selected patients (weight restriction). When analyzing only the degree of vascular enhancement, it is confirmed when kilovoltage is lowered (maximum at 80 kvp protocols). By analyzing the sensitivity, specificity, and positive and negative predictive values in simulation studies, no differences were found between conventional and optimized protocols. b) Reduction of the radiation dose The decrease of radiation dose depends on the selected kvp. Many studies in the literature have assessed the differences in effective dose of radiation according to different kilovoltages. The effective dose reduction between 120 and 100 kvp protocols is approximately 50%. The decrease in effective dose between 80 and 100 kvp protocols is up to 47%. c) Increased vascular enhancement The decrease in the photon energy (kev) increases the likelihood of occurrence photoelectric interactions with the atoms of iodine, resulting in increased vascular Page 7 of 41

8 enhancement. This increased vascular enhancement occurs in both central and peripheral vessels. It is one of the factors that makes possible to lower contrast dose. d) Increased image noise The decrease in total energy flow will be responsible for the increased image noise. The increased noise will result in deterioration of the image quality, becoming the main limiting factor in applying the kilovoltage reduction. e) Restriction on weight / BMI As discussed above, the patient's body habitus has a direct relationship with the image noise. Thus, by increasing the patient's weight increases the image noise and decreases the quality of the study. Given that the image noise is greater the lower the kvp (maximum at 80 kvp with protocols), a high weight of the patient may cause the image noise to become excessive. It is therefore essential to establish a limit of weight or BMI for implementing low-kilovoltage protocols. Although studies with phantoms have shown that there is a significant increase in noise above 80 Kg, literature often set as a cut-off of 100 Kg for implementing protocols of kvp. Optimizing kilovoltage Patients under 80kg-80 kvp Patients kg- 100 kvp Patients> 100 kg,> 120 kvp B) Tube current modulation The tube current modulation is based on the existence of large variations in absorption of radiation by the patient according to the angle of the tube in different anatomical regions in each time of the study. One advantage of modulation is that the current can be reduced in some projections without significantly increasing the noise in the final image. There are several mechanisms to set mas regarding the patient's anatomy Page 8 of 41

9 1 - Angular modulation (X-Y axes) ma 2 - Longitudinal modulation (Z-axis) ma 3 - Longitudinal and angular combined modulation These mechanisms of current modulation are implemented in what is known as automatic exposure control Systems (AEC). To use these systems, the operator must define the parameters of image quality desired (in terms of noise or contrast-to-noise ratio) and the system will determine the current-time product. Consequences of the current modulation a) Reduction of the radiation dose The decrease of the radiation dose obtained at a scan will depend on the anatomic region under study and patient's body habitus. - Anatomic Region: In the lung fields dose reduction reaches 30%. - Body habitus (adult patients): In thin patients can achieve reduction of 44.9%. In obese patients the reduction can be as low as 3.1%. b) Effect on image quality Since the noise level of the examination is selected by the operator, the decrease in quality will never exceed a limit previously established. Therefore, once defined the parameters of image quality considered acceptable for the detection of PE (when the objective of the study is the detection of pulmonary embolism, the tolerable noise limit will be higher than a study to detect mediastinal pathology). If current modulation is combined with low kilovoltage protocols (80 kvp kvp), it is possible to observe a noise study heterogeneity. The presence of relatively low current values in some areas like the upper lung fields (region of high density due to the presence of the shoulders) will case and increased noise, that results in the previously mentioned heterogeneity. Tips for using AEC systems: Page 9 of 41

10 -Have a lower efficacy in obese patients, but there is no reference value from which its use is inadvisable. -When combined with low kilovoltage protocols ( kvp) may be advisable not to use them to avoid heterogeneity in the noise of the study. 3. Contrast protocols The optimization of contrast administration protocol aims to achieve a sufficient pulmonary vascular enhancement for the diagnosis of PE (reference value 250 UH optimal) with minimum necessary dose of contrast. Basic rules of iodinated contrast dynamic 1 - Vascular enhancement is directly proportional to the flow rate of administration, which will depend on the injection rate and the concentration of contrast. 2 - Vascular enhancement increases with increasing duration of contrast injection, which depends on the injection rate and the volume of contrast. 3 - Vascular enhancement is influenced by: patient factors (cardiac function, weight) and the tube potential (kvp). Factors to be considered to optimize contrast administration protocol: Injection rate (Fig. 11 on page 24) Contrast concentration (Fig. 12 on page 25) -Contrast volume (Fig. 13 on page 26) Main limiting factors for low contrast protocols - Patient Weight - Acquisition time of the study Optimizing contrast protocols in non-obese patients (80-100kg) In these patients the weight does not have a significant impact on vascular enhancement so optimization of contrast dose can be combined with low kilovoltage (80 kvppp) Page 10 of 41

11 obtaining images of sufficient quality. The main objective of the optimization will be reducing to a minimum the amount of administered contrast. Features High-rate injection: Recommended 4 ml / s. Concentration of contrast- Recommended medium concentration (300 mg I / ml). Contrast Volume: Calculated based on weight following a 1:1 ratio. Considerations: - If the acquisition is performed with low kilovoltage (80 kvp), in patients between 60 and 80 kg it is usually enough with 60 ml of contrast if he length of the helix of acquisition is around 5-7 seconds. -The length of injection should be higher that the time of image acquisition. If not, it is necessary to decrease the injection rate or increase the volume of contrast. Optimizing contrast protocols in obese patients (> 100Kg/IMC> 30) In these patients the weight has a significant impact on vascular enhancement and it is not recommended to combine low doses of contrast with low kilovoltages ( kw). The main objective of the optimization will be sufficient to ensure vascular enhancement is enough to obtain diagnostic quality. Features High-rate injection: Recommended 4-5 ml / s. Concentration of contrast- Recommended medium or high concentration ( mg I / ml). Contrast volume: It is advisable to calculate volume based on BMI. If weight is used following a rule 1:1, it usually leads to an overestimation of necessary volume. Considerations: -The length of injection should be higher at the time of image acquisition. If not, it is necessary to decrease the injection rate or increase volume of contrast. Page 11 of 41

12 4. Direction of acquisition A) Cranio-caudal direction - Advantages: Improved vascular enhancement in the upper lobes, decreasing the number of nondiagnostic/inconclusive studies. - Disadvantages: - Increased susceptibility to the occurrence of respiratory artifacts in the lung bases in explorations with long acquisition times or dyspnoeic patients. - Increased risk of beam hardening artifacts if not using a bolus of saline after administration of the contrast bolus B) Caudo-cranial direction - Advantages - Lower risk of respiratory artifacts in the lung bases. When breathing artifacts appear, they usually take place in the upper lobes, where its impact on the diagnostic capacity is less. - Reduced risk of beam hardening artifacts. - Disadvantages - Minor vascular enhancement in the upper lobes. Optimizing the acquisition: If short acquisition times and serum bolus are available - cranio-caudal If patient dyspnoeic, long acquisition time of bolus or serum is not available - caudocranial. 5. The Radiology report Page 12 of 41

13 When reporting a MDCT pulmonary angiography for suspected PE, it is important to make a systematic report, which must include the following items: 1. Presence or absence of PE When the study presents a sufficient diagnostic quality, implies the presence of a positive and negative predictive value high enough to confirm or exclude the presence of PE. When the study has an insufficient diagnostic quality, its positive predictive value is high but its negative predictive value is low. Thus, it is possible to confirm the presence of PE, but it cannot rule out the presence of PE. This latter case is what is called a non-diagnostic or inconclusive study and if that is the case, it should be mentioned in the report. 2. Location of the PE Level of the pulmonary vasculature (Fig. 14 on page 27) Central arteries: -Pulmonary artery trunk -Main arteries (Fig. 15 on page 28) -Lobe arteries (Fig. 16 on page 29) Peripheral arteries: -Segmental arteries (Fig. 17 on page 30) -Subsegmental arteries (Fig. 18 on page 31) Laterality: -Right -Left -Bilateral 3. Severity Page 13 of 41

14 There are many scoring systems developed and validated for objective risk stratification of patients with PE (Mastora, Quanadli, etc.). Their aim is to identify those patients with increased risk of mortality that require urgent treatment. However, the application of most scoring systems is complicated and laborious, requiring time and training. Since the objective of assessing the severity of PE is to identify those patients with a poor prognosis, a simpler and equally effective system is to underline the presence of those poor prognostic factors presented by the patient. Poor prognostic factors (Fig. 19 on page 32): A) Right ventricular heart failure It is the most accurate predictor of the severity of PE. It is produced by overload and right ventricular dilatation secondary to pulmonary hypertension. Findings suggestive of right heart failure (Fig. 20 on page 33, Fig. 21 on page 34): - RV/LV short diameters > 1.5 (major factor). - Flattening of the interventricular septum. - Dilated superior vena cava and azygos vein. - Reflux of contrast into the inferior vena cava. B) Pulmonary hypertension It can be combined or not to signs of right heart failure. Findings suggestive of pulmonary hypertension (Fig. 22 on page 35, Fig. 23 on page 36): - Diameter of the pulmonary artery trunk > 30 mm. - Mosaic perfusion. C) Massive central PE Can be considered a simpler implementation of the severity scoring systems. The presence of large central arterial PE always going to have high value in the scoring systems, indicating poor prognosis regardless of whether or not signs of pulmonary hypertension or right heart failure are present. Page 14 of 41

15 6. Colour-coded perfusion maps They allow the radiologist to know the state of the pulmonary vasculature. The presence of perfusion defects of the microcirculation in the map will help identify sub-segmental emboli in very small branches which are not detectable by conventional sectional images, and assess the impact of those emboli on the perfusion of the parenchyma. The acquisition of these maps is possible with a single energy MDCT, but is on dual energy MDCT where this tool is becoming more important. The maps obtained with dual energy protocols offer higher quality with lower radiation dose (Fig. 24 on page 37) Images for this section: Fig. 1: Early mortality risk stratification Page 15 of 41

16 Fig. 2: Algorithm for patients with a high risk of early mortality Page 16 of 41

17 Fig. 3: Algorithm for patients with low-intermediate risk of early mortality Page 17 of 41

18 Fig. 4: Wells score to determine the clinical probability of PE (I) Page 18 of 41

19 Fig. 5: Wells score to determine the clinical probability of PE (II) Page 19 of 41

20 Fig. 6: Inverse relationship between weight and vascular enhancement Page 20 of 41

21 Fig. 7: Directely proportional relationship between weight and image noise Page 21 of 41

22 Fig. 8: Optimization strategies according to body habitus Page 22 of 41

23 Fig. 9: Low kilovoltage Protocols: 80 vs 120 kvp Page 23 of 41

24 Fig. 10: Low kilovoltage Protocols: 80 vs 100 kvp Page 24 of 41

25 Fig. 11: Influence of the injection rate in the vascular enhancement Page 25 of 41

26 Fig. 12: Influence of the concentration of the contrast medium in the vascular enhancement Page 26 of 41

27 Fig. 13: Influence of the volume of contrast in the vascular enhancement Page 27 of 41

28 Fig. 14: Schematic of the pulmonary vasculature Page 28 of 41

29 Fig. 15: Central PE: main pulmonary arteries Page 29 of 41

30 Fig. 16: Central PE: Lobar pulmonary arteries Page 30 of 41

31 Fig. 17: Peripheral PE: segmental pulmonary arteries Page 31 of 41

32 Fig. 18: Peripheral PE: subsegmental pulmonary arteries Page 32 of 41

33 Fig. 19: Factors of poor prognosis in patients with PE Page 33 of 41

34 Fig. 20: Sign of right ventricular failure (I) Page 34 of 41

35 Fig. 21: Signs of right ventricular failure (II) Page 35 of 41

36 Fig. 22: Signs of pulmonary hypertension (I) Page 36 of 41

37 Fig. 23: Signs of pulmonary hypertension (II) Page 37 of 41

38 Fig. 24: Color-coded perfusion map with single and dual energy energy Page 38 of 41

39 Conclusion -The purpose of the optimization MDCT pulmonary angiography protocols to rule out PE is to decrease of radiation and contrast dose administered to the patient while maintaining the diagnostic quality of the study. -There are several strategies allowing optimization of the protocol focusing on patient selection, technical parameter modification or interpretation of the study: 1) Patient Selection - Most important measure: To avoid unnecessary testing. Body habitus: Determines which group of patients (< kg) can undergo protocols with low kilovoltage and low contrast dose without significant loss of quality. 2) Contrast protocol There are three main parameters of which will depend on the vascular enhancement of the study: the injection rate, the concentration of the contrast and contrast volume. 3) Parameters of the radiation beam There are two main parameters of the radiation beam can be modified to decrease the radiation dose to the patient: the tube potential (kilovoltage) and the tube current. 4) Direction of acquisition The direction of acquisition will impact on the appearance of respiratory artifacts, beam hardening and the degree of vascular enhancement in certain lung regions. 5) The radiology report A structured report ensures that the responsible physician is transmitted all the necessary information on the presence or absence of PE, its location, severity and the presence of poor prognostic factors. 6) Color-coded maps Page 39 of 41

40 Tool for the assessment of the microcirculation and pulmonary vasculature will be gaining importance with the introduction of dual energy MDCT. -The application of these strategies in properly selected patients can obtain diagnostic quality studies with a significant decrease of the effective radiation dose and volume of contrast. References 1. Torbicki A. et al. (2008) Guidelines on the diagnosis and management of acute pulmonary embolism. The Task Force for the Diagnosis and Management of Acute Pulmonary Embolism of the European Society of Cardiology (ESC). European Heart Journal 29, Bae KT. (2010) Intravenous contrast medium administration and scan timing at CT:considerations and approaches. Radiology. Jul;256(1): Bae KT, Tao C, Gürel S, Hong C, Zhu F, Gebke TA, Milite M, Hildebolt CF. (2007) Effect of patient weight and scanning duration on contrast enhancement during pulmonary multidetector CT angiography. Radiology. Feb;242(2): Schaefer-Prokop C, Prokop M. (2005) MDCT for the diagnosis of acute pulmonary embolism. Eur Radiol. 15 Suppl 4:D Heyer CM, Mohr PS, Lemburg SP, Peters SA, Nicolas V. (2007) Image quality and radiation exposure at pulmonary CT angiography with 100- or 120-kVp protocol: prospective randomized study. Radiology. Nov;245(2): Holmquist F, Hansson K, Pasquariello F, Björk J, Nyman U. (2009) Minimizing contrast medium doses to diagnose pulmonary embolism with 80-kVp multidetector computed tomography in azotemic patients. Acta Radiol. Mar;50(2): Björkdahl P, Nyman U. (2010) Using 100- instead of 120-kVp computed tomography to diagnose pulmonary embolism almost halves the radiation dose with preserved diagnostic quality. Acta Radiol. Apr;51(3): Page 40 of 41

41 8. Kristiansson M, Holmquist F, Nyman U. (2010) Ultralow contrast medium doses at CT to diagnose pulmonary embolism in patients with moderate to severe renal impairment: a feasibility study. Eur Radiol. Jun;20(6): Personal Information Page 41 of 41