Chapter 46 Cardiac MRI in Post-TOF Repair Adults

Chapter 46 Cardiac MRI in Post-TOF Repair dults RICH KOTHRI ONKR UTI VIML RJ INTRODUCTION Tetralogy of Fallot (TOF) is the most common cyanotic congenital heart disease (CHD) and accounts for approximately 6% 10% of all cases of CHD 1, 2. TOF is characterized by failure of fusion of the conal and ventricular septa. This antero-cephalad malalignment of the septum leads to the classic tetrad of subaortic ventricular septal defect (VSD), an overriding aorta, right ventricular (RV) infundibular stenosis and subsequent RV hypertrophy ( Fig. 46-1 ). The clinical presentation is variable and depends on the degree of RV outflow tract obstruction. The degree of cyanosis varies with the degree of pulmonary stenosis and the number and size of systemic-to-pulmonary collateral vessels. PREOPERTIVE SSESSMENT PRIOR TO TOF SURGERY Preoperative assessment is effectively accomplished by echocardiography. dditionally, multidetector Figure 46-1. ( and ) SSFP images of preoperative TOF showing subaortic VSD (black arrow) with overriding of aorta and RV infundibular stenosis (white arrow). Note the RV hypertrophy. computed tomography (MDCT) or magnetic resonance (MR) angiography can provide anatomical road map to aid in surgical repair by determining pulmonary vascular anatomy (specifically size and branching pattern of the pulmonary arteries), origin and course of the coronaries and other associated lesions like anomalies of the aortic arch 3, 4. Coronary artery that crosses anterior to the surface of the RV outflow tract may alter the surgical approach. SURGICL OPTIONS IN TOF There are two types of surgical approach, which are determined by presence and sizes of the branch pulmonary arteries. Size of the pulmonary arteries are standardized using McGoons ratio which is the ratio of sum of diameters of right and left pulmonary arteries just before the bifurcation and aortic diameter at the level of diaphragm. McGoons ratio above 1.2 is associated with acceptable postoperative outcome in TOF and the ratio below 0.8 is inadequate for complete repair of pulmonary arteries 5. Nakata index also can be used to assess pulmonary artery sizes. Palliative surgery is used for infants with pulmonary arterial hypoplasia, which necessitates postponing definite repair to allow for pulmonary arterial growth. These surgeries include mostly modified lalock Taussig shunt (shunt between subclavian artery to pulmonary artery). Other shunts like Waterston shunt (ascending aorta to right pulmonary artery) or rock operation (transpulmonary infundibular resection and pulmonary valvotomy) can also be done 4. When pulmonary arteries are adequate in size, definitive repair is done which is popularly known as intracardiac repair (ICR). ICR includes placement 381

382 SECTION V Cardiac Imaging of RV to pulmonary artery conduit and the patch closure of VSD without the placement of pulmonary valve considering the increasing size of the pulmonary annulus compared to prosthetic valve 3, 5. If there are lung segments that are supplied by collateral vessels alone, attempts are made to incorporate these vessels into the repair procedure. Younger age at repair results in better outcomes, and there appears to be no benefit in delaying ICR for TOF after the end of the first year 4. If the pulmonary valve annulus is hypoplastic, a transannular patch (TP) is necessary. Early cardiac repair is associated with a favourable prognosis (early mortality rate 2%); however, there are increased residual defects, complications and sequelae 6 10. Post-ICR, due to lack of pulmonary valve, the patients usually present with a variable degree of pulmonary regurgitation (PR). Over long-term follow-up, RV dilatation and dysfunction can occur due PR. Therefore, optimal timing of pulmonary valve replacement (PVR) should be determined. Patient should be old enough to accept the valve and RV dysfunction/dilatation should not be fatal 11. Cardiac magnetic resonance (CMR) imaging is offered to follow-up these patients to assess RV size and function. CRDIC MRI IN PTIENTS WITH PREVIOUS TOF SURGERY CMR provides excellent anatomic and functional details with a large field of view, excellent inplane spatial resolution and good temporal resolution. In follow-up of repaired TOF patients, it provides precise measurements of RV volumes and function, PR fraction, anatomy of pulmonary arteries and left ventricular (LV) function. It is also useful in ascertaining the integrity of the VSD patch. 5. Cine SSFP right ventricular outflow tract (RVOT) view. 6. Phase-contrast imaging of main pulmonary artery (MP) and aorta. Cine axial images provide accurate functional assessment of RV and LV. It also provides anatomical details of the pulmonary arteries and other associated anomalies 4. LVOT cine view provides better visualization of VSD patch across the LVOT ( Fig. 46-2 ). It further gives the information of LVOT angulation and resultant obstruction which is important in case of long TP. Cine RVOT view ( Fig. 46-2 ) provides visualization and extent of akinesis or aneurysmal infundibulum. It allows direct visualization of PR and residual pulmonary stenosis if present 4. The phase-contrast imaging technique gives accurate quantification of vessel flow, including regurgitation volumes, especially PR ( Fig. 46-3 ) or tricuspid regurgitation (TR). It allows calculation of pulmonary-to-systemic flow ratio. This is in good agreement with catheter-derived pulmonary systemic flow ratio 12, 13. It also gives pressure gradient across pulmonary valve in case of residual stenosis; however, CMR underestimates the gradient compared to catheter-based studies. In patients with multiple sources of pulmonary blood flow, the phase-contrast imaging technique can be used to measure flow across any vessel to estimate the lung perfusion as in contrast to haemodynamic assessment with catheterization. dditional Sequences Optional dedicated views for branch pulmonary arteries can be taken in case of unclear P anatomy. Contrast administration with MR angiography can TECHNICL CONSIDERTIONS In routine follow-up of post-icr patients, CMR examination without contrast is sufficient to give all the necessary information. CMR protocol includes: 1. Localize rs. 2. xial cine SSFP (steady-state free precession) acquisition of entire heart with no slice gap. 3. Dedicated cine SSFP acquisition in two-chamber, four-chamber and three-chamber views. 4. Cine SSFP left ventricular outflow tract (LVOT) view. Figure 46-2. ( and ) SSFP images of post-icr case showing intact VSD patch (black arrow) and repaired capacious RVOT (white arrow).

Chapter 46 Cardiac MRI in Post-TOF Repair dults 383 C Figure 46-3. Phase-contrast images through main pulmonary artery ( and ) and analysis graph (C) showing free PR (positive peak in the graph denotes forward flow and trough denotes regurgitation) in a post-icr patient. be done to look for P anatomy and other associated anomalies. High-resolution gadoliniumenhanced 3D MR angiography is particularly valuable for imaging the systemic and pulmonary venous anatomy, branch pulmonary arteries ( Fig. 46-4 ) and the aorta for root dilatation 3. Late gadolinium enhancement imaging, which documents myocardial fibrosis, contributes to risk stratification for ventricular tachycardia and sudden cardiac death 4. INDICTIONS FOR CRDIC MRI ssessment or Residual PR and Guiding Its Management PR is the commonest sequel seen following RVOT repair ( Fig. 46-3 ). It is present in nearly all TOF patients undergoing ICR, as these patients do not have the pulmonary valve. PR can be accurately quantified with phase-contrast imaging. In addition, the effects of PR on RV size and function can be serially measured. Growing evidence during the last decade supports the rationale for PVR after TOF repair. Chronic PR in patients with repaired TOF is though well tolerated, without intervention may lead to severe RV dilatation and dysfunction, tricuspid valve regurgitation, LV dysfunction, tachyarrhythmias, diminished exercise tolerance, heart failure symptoms and death 6 8,14,15. If the decision for PVR is based solely on appearance of symptoms, patients usually develop marked RV dilation and RV and/or LV dysfunction 16. Thus, the timing and indications for PVR after TOF repair needs to be based on avoidance of setting in of irreversible

384 SECTION V Cardiac Imaging Figure 46-4. MR angiography images showing proximal LP narrowing (arrow) in axial () and coronal oblique view (). dysfunction due to RV volume overload versus the disadvantages of a premature surgical or transcatheter procedure. Several different modalities are available for assessment of PR severity and determining the need for PVR in these patients. Echocardiography remains the initial modality because it is readily available, noninvasive, free of radiation exposure and is much less expensive. However, accurate assessment of RV volumes and function by echocardiography is challenging and remains a major limitation of this modality. In contrast, CMR can accurately help to quantify RV volumes and ejection fraction as well as PR fraction. For this reason, CMR plays a critical role in decision of timing of PVR in post-tof repair patients 11. During the early stages after TOF repair (usually the first decade), CMR is indicated only if diagnostic uncertainty persists after initial assessment by echocardiography. However, from second decade onwards, routine surveillance with CMR is indicated because of high incidence of significant RV dilatation and dysfunction which may warrant early PVR. Following are the currently accepted indications for PVR in patients with ICR 11 1. symptomatic patient with moderate to severe PR (regurgitation fraction 25%) two or more of the following criteria: (a) RV end-diastolic volume index 150 ml/m 2 or Z-score 4. In patients whose body surface area falls outside published normal data: RV/LV end- diastolic volume ratio 2. (b) RV end-systolic volume index 80 ml/m 2. (c) RV ejection fraction 47%. (d) LV ejection fraction 55%. (e) Large RVOT aneurysm. (f) QRS duration 140 ms. (g) Sustained tachyarrhythmia related to right heart volume load. (h) Other haemodynamically significant abnormalities: RVOT obstruction with RV systolic pressure more than two-thirds of systemic pressure. Severe branch pulmonary artery stenosis ( 30% flow to affected lung) not amenable to transcatheter therapy. Moderate to severe TR. Left-to-right shunt from residual atrial or VSDs with pulmonary-to-systemic flow ratio 1.5. Severe aortic regurgitation. Severe aortic dilatation (diameter 5 cm). 2. Symptomatic patients Symptoms and signs attributable to severe RV volume load documented by CMR or alternative imaging modality, fulfilling 1 of the quantitative criteria detailed above. Residual Pulmonary Stenosis pproximately 10% 15% of patients will have residual ( Fig. 46-5 ) or recurrent branch pulmonary stenosis, some of whom will require additional surgery or catheter-directed angioplasty 8. The level of stenosis varies from the proximal RV outflow tract to the distal branch pulmonary arteries, including surgically placed RV pulmonary artery conduits. CMR can correctly localize and characterize the stenosis.

Chapter 46 Cardiac MRI in Post-TOF Repair dults 385 C Figure 46-5. case of post-icr SSFP RVOT image () showing residual pulmonary stenosis with flow acceleration (block arrow) and ostial LP stenosis (arrow) in black blood imaging () and SSFP image (C) showing flow acceleration. Figure 46-6. SSFP axial images of two different post-icr patients showing RV dilatation and TR jet (arrow). Tricuspid Regurgitation TR occurs due to annular dilatation of the tricuspid valve due to progressive RV dilatation ( Fig. 46-6 ). Phase-contrast imaging data give an idea of the magnitude of the regurgitation and the effects of TR on RV size can be assessed with RV volumetric data. RV Outflow Tract neurysm Transannular or RV outflow tract patch is usually related to the aneurysmal RV outflow tract dilatation. Extensive infundibular muscle resection and ischaemic insult may also be responsible in some cases 17. Presence of RV outflow tract aneurysm results in reduced RV ejection fraction 18. The size of the RV outflow tract is easily and accurately evaluated with cine SSFP imaging. Conduit Obstruction Patients who require a conduit as a part of initial repair may develop obstruction over time. This can be seen on multiple imaging sequences. rtefacts related to metal within the conduit and to turbulent flow often interfere with this assessment. If conduit obstruction is not assessed by SSFP images, contrast MR angiography helps to delineate the RVOT. Phase-contrast imaging helps to quantify the gradient across the obstruction. Residual VSD Cine LVOT view demonstrates residual VSD across the LVOT ( Fig. 46-7 ). Phase-contrast imaging can quantify the magnitude of the shunt in residual VSDs. Prior to the advent of CMR, reliable shunt quantification was possible only with invasive catheterization. LV Dysfunction The LV ejection fraction (LVEF) derived from CMR retains its accuracy in the presence of RV volume overload (diastolic septal flattening) and abnormal

386 SECTION V Cardiac Imaging CMR requires remaining still and holding the breath during the procedure, the technique can be modified for a less cooperative younger patient, for free breathing sequences. Figure 46-7. SSFP LVOT image showing residual VSD (arrow) in a post-icr patient. septal motion, unlike echocardiography-derived shortening fraction 19. It has recently been found to be the strongest predictor of poor clinical status 20. The proposed mechanisms include akinesia resulting from the VSD patch, septal fibrosis, chronic volume overloading from early palliative shunt creation, abnormal septal motion and myocardial injury at the time of repair. The decrease noted in LVEF after repair of TOF may be too late to enable intervention as the patient may already have an increased risk of sudden cardiac death. CMR can detect early regional LV dysfunction. CMR is a noninvasive option for the pre- and postoperative evaluation of patients with TOF as compared to transoesophageal echocardiography or cardiac catheterization. There are no ionizing radiation and hence have a distinct advantage over catheterization and CT, especially in paediatric patients who may require multiple examinations. It is particularly useful in older postsurgical patients, who often have poor acoustic windows that severely limit the diagnostic quality of ultrasonography. CMR does not need contrast for routine followup of patients with ICR, so it can be safely done in patients with renal dysfunction. vailability of MR safe pacemakers has increased the scope, as it has been seen that some types of implanted metallic stents and coils affect the image quality due to susceptibility artefacts. lthough REFERENCES 1. Helbing, W.., & de Roos,. ( 2000 ). Clinical applications of cardiac magnetic resonance imaging after repair of tetralogy of Fallot. Pediatric Cardiology, 21, 70 79. 2. Nadas,. S. ( 1992 ). Tetralogy of Fallot. In D. C. Fyler, (Ed.), Nadas pediatric cardiology. Philadelphia, Pa : Hanley & elfus. 3. Norton, K., Tong, C., Glass, R., & Nielsen, J. ( 2006 ). Cardiac MR imaging assessment following tetralogy of Fallot repair. Radiographics, 26, 197 211. 4. abu-narayan, S.V., Shore, D., Chung, N., & Gatzoulis, M. ( 2010 ). Tetralogy of Fallot and truncus arteriosus. In M. Crawford, J. DiMarco, & W. Paulus, (Eds.), Cardiology ( 3rd ed., pp. 1471 1484 ). Philadelphia, P: Mosby Elsevier Ltd. [Chapter 109]. 5. alaguru, D., & Dilawar, M. ( 2007 ). Pulmonary atresia with ventricular septal defect: Systematic review. Heart Views, 8, 52 61. 6. Nollert, G., Fischlein, T., outerwek, S., öhmer, C., Klinner, W., & Reichart,. ( 1997 ). Long-term survival in patients with repair of tetralogy of Fallot: 36-year follow-up of 490 survivors of the first year after surgical repair. Journal of the merican College of Cardiology, 30, 1374 1383. 7. Murphy, J. G., Gersh,. J., Mair, D. D., Fuster, V., McGoon, M. D., Ilstrup, D. M., et al. ( 1993 ). Long-term outcome in patients undergoing surgical repair of tetralogy of Fallot. New England Journal of Medicine, 329, 593 599. 8. Gatzoulis, M.., alaji, S., Webber, S.., Siu, S. C., Hokanson, J. S., Poile, C., et al. ( 2000 ). Risk factors for arrhythmia and sudden cardiac death late after repair of tetralogy of Fallot: multicentre study. Lancet, 356, 975 981. 9. acha, E.., Scheule,. M., Zurakowski, D., Erickson, L. C., Hung, J., Lang, P., et al. ( 2001 ). Long-term results after early primary repair of tetralogy of Fallot. Journal of Thoracic and Cardiovascular Surgery, 122, 154 161. 10. Lillehei, C. W., Varco, R. L., Cohen, M., Warden, H. E., Gott, V. L., DeWall, R.., et al. ( 1986 ). The first open heart corrections of tetralogy of Fallot: 26 31 year follow-up of 106 patients. nnals of Surgery, 204, 490 502. 11. Geva, T. ( 2011 ). Repaired tetralogy of Fallot: The roles of cardiovascular magnetic resonance in evaluating pathophysiology and for pulmonary valve replacement decision support. Journal of Cardiovascular Magnetic Resonance, 13, 9. 12. Powell,. J., Maier, S. E., Chung, T., & Geva, T. ( 2000 ). Phase- velocity cine magnetic resonance imaging measurement of pulsatile blood flow in children and young adults: In vitro and in vivo validation. Pediatric Cardiology, 21, 104 110.

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