The current status of CZT SPECT myocardial blood flow and reserve assessment: Tips and tricks

Abstract

Cardiac PET-derived measurements of myocardial blood flow (MBF) and myocardial flow reserve (MFR) are proven robust indexes of the severity of coronary artery disease (CAD). They facilitate the diagnosis of diffuse epicardial and microvascular disease and are also of prognostic significance. However, low availability and high cost have limited their wide clinical implementation. Over the last 15 years, cadmium zinc telluride (CZT)-based detectors have been implemented into SPECT imaging devices. Myocardial perfusion scintigraphy can be performed faster and with less radiation exposure as compared with standard gamma cameras. Rapid dynamic SPECT studies with higher count rates can be performed. This technological breakthrough has renewed the interest in SPECT MBF assessment in patients with CAD. Currently, two cardiac-centered CZT gamma cameras are available commercially—Discovery NM530c and D-SPECT. They differ in parameters such as collimator design, number of detectors, sensitivity, spatial resolution and image reconstruction. A number of publications have focused on the feasibility of dynamic CZT SPECT and on the correlation with cardiac PET and invasive coronary angiography measurements of fractional flow reserve. Current study reviews the present status of MBF and MFR assessment with CZT SPECT. It also aims to provide an overview of specific issues related to acquisition, processing and interpretation of quantitative studies in patients with CAD.

Introduction

PET with ¹⁵O-water along with ¹³N-ammonia and ⁸²Rb are the gold standard of myocardial blood flow (MBF) and myocardial flow reserve (MFR) assessment. However, PET is not widely available due to high costs or need for an on-site cyclotron (for ¹³N-ammonia and ¹⁵O-water).¹ Therefore, approaches using single photon emitting radiotracers have been considered as an alternative to PET. Their feasibility has been proven in a series of experimental studies based on dynamic planar and static SPECT techniques.2, 3, 4 Studies in humans have also suggested that SPECT quantification of MBF and MFR can be performed using NaI-based general-purpose SPECT systems.4, 5, 6, 7, 8 The most important limitation of this method is the fact that simultaneous acquisition of all projections during the rapid transit of the radiotracer through the left ventricle (LV) and the myocardium cannot be performed.⁹ It would be possible to compensate the angular inconsistency of SPECT cameras with the capability of rapid rotation gantry (RRG) even though rapid tracer movement occurs in the early phase after tracer injection. This correction is important to improve MBF measurement agreement between RRG and CZT SPECT in addition to full physical corrections.¹⁰ Additional limitations of this technique include the fact that obtaining high-quality dynamic images requires relatively big amounts of radioactivity, 370 and 1000 MBq for the first and second injection, respectively. Moreover, the lack of dedicated widely available software for dynamic planar and SPECT MBF calculation is also an impediment to its wide use.

The implementation of the new generation of SPECT cameras represents a technological breakthrough.¹¹ These devices are equipped with more sensitive solid-state cadmium zinc telluride (CZT) detectors¹¹ and are characterized by higher spatial and energy resolution as compared to NaI detectors. Currently, there are two commercially available models of such cameras—D-SPECT (Spectrum Dynamics, Caesarea, Israel) and Discovery 530 or Discovery 570c (GE Healthcare, Waukesha, Wi, USA).

The design of the gantry of cardiac CZT-based cameras focuses on the heart. There is no need for rotation of detectors around the human body in order to obtain all projections of the heart simultaneously. D-SPECT cameras have nine internal axially swivel detector’s modules that perform multiple forward and backward sweeps.12, 13, 14 The Discovery 530c design includes fixed detectors and heart-centered multi-pinhole collimators.¹⁵ These features enable to perform dynamic SPECT in order to assess MBF. Table 1 provides a brief summary of studies investigating the feasibility and clinical value of CZT SPECT. The technical differences between both cameras can explain differences in MBF values in the literature.

In this review, we aim to describe the state-of-the-art MBF assessment with CZT SPECT and point toward several technical issues related to image acquisition, data post-processing and interpretation.