Navigating the Procedure of Myocardial Viability Imaging in Infant

INTRODUCTION

Reimplanted coronary arteries during arterial switch operation (ASO) for the transposition of great vessels are susceptible to abnormal vasodilation, kinking, or failure to grow at the anastomotic level.^[1] Survival outcomes depend on perfusion of these transferred coronary arteries.^[2-4] Coronary-related mortality reaches 10% in symptomatic patients.^[5] Perfusion defects diagnosed by myocardial perfusion imaging (MPI) are present in 5%–24% of patients during follow-up and persist more than 10 years after ASO.^[6,7] Since revascularization by mammary bypass grafting and percutaneous transluminal coronary angioplasty are now therapeutic options in infancy, information on viability is mandatory.^[8] MPI by single-photon emission computed tomography (SPECT) and myocardial viability assessment by fluorodeoxyglucose (FDG) positron emission tomography (PET) scanning are standards of care in adult patients with coronary artery disease for medical management and before revascularization procedures.^[9,10] However, such a procedure for perfusion and viability assessment in an infant is rare and complicated, as no standardized protocols are established. Potential challenges included patient preparation, achieving metabolic setting for stimulating cardiac FDG uptake, adequate small-organ SPECT imaging in the baby.

CASE REPORT

A male infant of 10 months was diagnosed with cyanotic congenital heart disease on day 10 of life, with no features of failure to thrive. The patient underwent ASO with LeCompte maneuver, atrial septal defect, and patent ductus arteriosus repair in the neonatal period. The patient presented to our institute at 10 months of age with progressive left ventricular dysfunction. Echocardiogram stated left ventricular ejection fraction of 40% along with moderate-to-severe mitral regurgitation and lateral wall hypokinesia. Invasive angiography showed a common origin for left anterior descending (LAD) and right coronary artery from the right (anterior) sinus with normal flow, left circumflex (LCx) faintly visualized through collaterals from LAD. With high suspicion and known complications of switched arteries leading to reduced perfusion of the left ventricular wall, a revascularization surgery was being planned; however, it was essential to know whether the LCX territory was viable, to put the child under a high-risk surgery. Hence, a requisition for myocardial perfusion and viability imaging was raised.

After a discussion with the interventional cardiologist, cardiothoracic surgeon, and pediatric intensivist, nuclear MPI and viability procedure were planned under short anesthesia.

For the myocardial perfusion scan, following fasting of 4 h, 2.5 mCi of 99 mTc-tetrofosmin was administered intravenously. The dose was calculated using a European Association of Nuclear Medicine dose card. After 60 min, gated SPECT was acquired on Symbia-T from Siemens Healthineers, using the multifocal collimator (SMARTZOOM).

For FDG viability scan, 2 g/kg glucose was administered as a concentrated (low volume) oral solution. After 30 min, 18F-FDG (5MBq/kg) was administered intravenously. 90 min following 18F-FDG administration, cardiac PET scan was acquired on Siemens Biograph mCT. Processing was done on Corridor 4DM on Syngo via.

Processed images are depicted in Fig 1. On the perfusion scan, left ventricle appeared moderately dilated. There was severe hypoperfusion in the mid and basal lateral wall segments. Gated images showed a severely hypokinetic lateral wall with a resting ejection fraction of 45%. FDG-PET metabolic imaging showed preserved glucose metabolism in the hypoperfused segments of lateral wall suggestive of mismatched perfusion-metabolism defect. It was concluded that severely hypoperfused lateral wall of the myocardium (LCx territory) was viable. A multidisciplinary cardiac team, thus, planned for revascularization by mammary bypass grafting.

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Fig 1:

Image rows marked A from above to below show myocardial perfusion scan with Tc-99m tetrofosmin in short axis, vertical long axis, and horizontal long axis; intervening rows marked B show matched slices of reformatted cardiac FDG PET images in the same respective axes.

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DISCUSSION

ASO is the treatment of choice in patients with simple transposition of great arteries.^[11] Multiple complications can follow such a major cardiothoracic surgery, concerning ones being coronary ischemia and left ventricular dysfunction, with incidences of 3%–18%^[12] and 13%, respectively. The major reasons for coronary arterial stenosis include anastomotic stenosis, mechanical distortion and high tension of coronary artery, or operative injury. Left ventricular dysfunction is attributed to the myocardial ischemia caused by postoperative coronary arterial stenosis, insufficient intraoperative myocar-dial protection, and the left ventricular atrophy induced by late timing of surgery. Coronary ischemia can be reversed by revascularization procedures, which are life-threatening in infants. Hence, preoperative evaluation of ischemia and confirmation of viable myocardium is cardinal.

Our case highlights a patient who developed progressive deterioration of the left ventricular function because of coronary ischemia [Fig 1].

The procedure needed optimum tailoring and planning as follows:

1. Managing pediatric immobilization and anesthesia: Since there were two scans involved with a gap between procedures, it would have been a challenge to give anesthesia/sedation twice. The appropriate time and duration of anesthesia had to be discussed. Though the standard preparation for hyperinsulinaemic euglycaemic clamp involves an oral glucose feed - because the baby was to possibly undergo anaesthesia, “Nil by mouth” for 4 hours prior was required by anaesthesia team. For our patient, the SPECT gamma camera imaging was managed by good immobilization of the child using pediatric palate and could be done without anesthesia. The shorter scan duration of only 8 min on the IQ-SPECT proved pivotal.

2. Myocardial SPECT imaging in infants is compromised by the small size of the heart and limited dose administration. Dose of administered radionuclide had to be low to reduce radiation exposure. Yet, one needs to plan pre-emptively that mobilization for a long time may be difficult without anaesthesia, and hence a shorter scan acquisition time on scanner is preferable - which would need good level of count rate. Enough activity should also be planned to cover for any delay in scan acquisition. To overcome the same, dose of 2.5 mCi Tc-tetrofosmin was decided. The advantage of the IQ-SPECT cardiac imaging detectors became apparent in this case. In an advantage over the parallel hole collimator, the acquisition was done on this multifocal collimator (SMARTZOOM), which employs the magnifying properties of a focusing collimator near the center of the field of view (FoV) and behaves like a parallel hole collimator toward the edge of the FoV, reducing truncation. This enables the detectors to zoom in on the heart for four times higher sensitivity. The dedicated reconstruction method (IQ-SPECT) enabled higher count which translated to reduced duration of acquisition and reduced motion artifact without degrading image quality.

3. Infant glucose metabolism and demands are significantly different than adults; they also are at higher risk of hypoglycemia which could even manifest as seizures.^[13] Strenuous blood glucose monitoring during the entire procedure is important. Insulin is not administered to children <10 years of age, because of their higher insulin sensitivity. Few international studies have documented the utility of MPI and viability assessment following ASO^[14] and their pertinent role before revascularization procedures. As per advice of Endocrinology team, the peak insulin secretion time in a baby, after an oral feed of glucose is 20–30 min after meal. Keeping in mind that the baby may need anesthesia during PET scan acquisition, we dissolved 10 g glucose in a very small volume (10 ml) of water. The child was fed via dropper feed. At 30 min after feed assuming a good insulin peak, we injected (5MBq/kg) FDG. There was no need of glucose monitoring. PET-CT scan was acquired on Siemens MCT 40 Flow after 90 min, with imaging time of 10 min. Using this protocol and relying solely on the endogenous insulin response in the child, good stimulation of GLUT receptors in myocardium was achieved. The scan showed high FDG avidity in the ischemic but viable lateral wall of the myocardium.

Other non Nuclear Medicine methods to assess Cardiac Viability include Cardiac magnetic resonance imaging (CMR). However, it is also cumbersome - requires significant software training and expertise,^[15] data acquisition time is long (20–50 min), requires long anesthesia for infants and separate set of CMR compatible monitoring equipment.

CONCLUSION

This case report provides an insight on the important role Nuclear Medicine can play in guiding further management decisions in cases of Ischemia following ASO in infants. We share our experience to approaching this otherwise common scan procedure but in a rare age group - and practical ways to overcome technical and clinical difficulties in this age group.