Hybrid PET/CT Scanning for Functional and Molecular Imaging

BIRC team has developed a functional CT imaging technique (CT Perfusion) for measuring absolute myocardial perfusion (MP), from which the risk of heart attack (acute myocardial infarction) in patients with coronary artery disease (CAD) can be predicted to avoid unnecessary interventional reperfusion procedures. They are investigating the implementation of several novel CT technologies and algorithmic advancements in CT Perfusion to facilitate low-dose diagnosis of CAD and therapeutic guidance for its related complications.

Dual-energy Scanning to Improve Diagnostic Accuracy

The iodinated contrast agent circulates in the heart chambers during CT MP Imaging induces spatially inconsistent changes in myocardial enhancement that is unrelated to MP. This ‘beam hardening’ image artifact (error) may lead to misdiagnosis of myocardial ischemia if uncorrected (FIG 1A, red arrow). We have shown that virtual monochromatic imaging from rapid kV (tube voltage) switching dual-energy scanning can eliminate this image artifact (FIG 1B) leading to a more accurate assessment of time-dependent myocardial enhancement from which MP can be estimated with tracer kinetic modeling.

FIG 1. Contrast-enhanced CT image of a normal porcine heart (A) with and (B) without beam hardening artifact.

Iterative Image Reconstruction to Reduce Radiation Dose

Radiation dose of a quantitative CT MP study can be reduced by scanning with less X-rays, but the profound image noise leads to large fluctuation in myocardial enhancement and hence inaccurate MP calculation with CT Perfusion.

Preliminary data showed that image noise in low dose scanning can be effectively minimized with adaptive statistical iterative image reconstruction (ASIR). Specifically, the diagnostic quality of MP map acquired with a low dose quantitative CT MP protocol (2 mSv for whole heart assessment) is unaffected compared with that from a high dose (8 mSv) protocol when ASIR is applied (FIG 2).

FIG 2. MP maps of a normal pig acquired with (A) high-dose and (B) low-dose CT MP imaging. 

Simultaneous Perfusion and Edema Imaging to Guide Therapeutic Treatment after Heart Attack

We have expanded CT Perfusion beyond perfusion measurement to include myocardial edema assessment which together can be used to delineate the extent of salvageable myocardium (hypo-perfused but still living) after heart attack (FIG 3). This technique does not require additional contrast administration to MP imaging and can be very useful to triage late-presenting patients towards interventional versus medical therapy.

FIG 3. (A) Edema and (B) perfusion maps of a pig with acute myocardial infarction. ROI 1 (red) outlines the tissue at risk of infarction and ROI 2 (purple) delineates the tissue within the area at risk that can be rescued from timely and successful reperfusion.

Heart Attack Risk Assessment without the use of Pharmacologic Stress Agent

The most commonly used method to predict the risk of heart attack from CAD is by measuring MP at rest and during which the heart is stressed with pharmacologic agents such as adenosine or dipyrdaimole (vasodilators). The disadvantages of the rest/stress protocol include patient discomfort from stressing the heart and two contrast injections and scans are required. Our initial data suggested that the baseline vascular mean transit time (MTT) calculated with CT Perfusion reflects the degradation of perfusion pressure arising from a coronary stenosis, which can be used to predict the resulting ischemia and heart attack (FIG 4) and therefore stress MP scan may not be required.

FIG 4. (A) Rest MTT map and (B) stress MP map of a patient whose left circumflex (LCx) artery was 75% occluded and left anterior descending (LAD) artery was normal. The LCx territory (yellow arrow) exhibited a higher baseline MTT and lower hyperemic MP compared with those in the LAD territory (green arrow).

Ting-Yim Lee, PhD, Imaging
Aaron So, PhD, Imaging
Gerald Wisenberg, MD, Cardiology
Patrick Teefy, MD, Cardiology