High-speed SPECT: Higher Resolution in One-fourth the Time
Equal diagnostic confidence by high-speed SPECT compared with conventional SPECT is shown by similar number of patients within definitely normal and abnormal scan categories and similar number of borderline or equivocal scans. EQ = equivocal; PA = probably abnormal; PN = probably normal. (Source: JACC Cardiovascular Imaging)  
Conventional SPECT (top panel) and high-speed SPECT (bottom panel) images showing infarct and ischemia of the left anterior descending coronary artery and ischemia of the right coronary artery. (Source: JACC Cardiovascular Imaging)
The first clinical experience with high-speed myocardial perfusion SPECT imaging indicates that the technology delivers high image quality and up to eight times increased system sensitivity compared with conventional SPECT.

Researchers also found the amount of perfusion abnormality visualized by the D-SPECT scanner (Spectrum Dynamics, Caesarea, Israel) was highly correlated to conventional SPECT, with an equivalent level of diagnostic confidence.

Despite hardware and software advances to conventional SPECT systems, they still must deal with a trade-off between resolution and sensitivity, which dictates the need for relatively large doses of radiopharmaceuticals and prolonged imaging time of 15 to 20 minutes.

“Current nuclear cardiology practice continues to struggle with the yin and yang of gamma camera performance—that is, the difficult balance between resolution and sensitivity,” wrote Robert O. Bonow, MD, from Northwestern University, in an accompanying editorial to the study published in the March JACC Cardiovascular Imaging.

The D-SPECT scanner introduces a new design of a photon acquisition system, as well as reconstruction algorithm, that “enables significantly reduced imaging time or, alternatively, reduced radiation exposure, while providing higher resolution than the Anger camera approach,” according to the study.

The new system uses nine pixilated solid-state detector columns, cadmium zinc telluride crystals and wide-angle tungsten collimators that, combined with the novel image reconstruction algorithm, provide patient-specific images localized to the heart (region of interest-centric scanning).

Researchers, led by Tali Sharir, MD, from Procardia-Maccabi Healthcare Services in Tel-Aviv, Israel, hypothesized that high-speed SPECT, acquired over a shorter time, will be comparable to conventional SPECT in detecting perfusion defects and would provide similar diagnostic confidence.

For the study, investigators first imaged 44 patients with a conventional two-detector gamma camera (Axis, Philips Medical Systems or CardiaL, GE Healthcare), followed 30 minutes later with high-speed imaging.

Acquisition protocol included a low dose of 11 mCi Tc-99m sestamibi injected at peak stress, and eight-frame gated SPECT imaging (100% acceptance window) 15 to 30 minutes later. A second dose of 28 mCi Tc-99m sestamibi was injected after two hours, and eight-frame gated SPECT imaging was started one hour later.

Scanning protocol for the high-speed imaging included a 30-second pre-scan acquisition to identify the location of the heart within the chest and to set the angle limits of scanning for each detector (ROI-centric scanning).

High-speed acquisition times for stress and rest were four minutes and two minutes, respectively, compared with 16 minutes and 12 minutes, respectively, for conventional SPECT.

The investigators found that the high-speed technology provided an eight- to 10-fold increase in sensitivity, coupled with a two-fold improvement in spatial resolution, enabling a significant reduction in imaging time and dose of radio-isotopes.

The summed stress score (SSS) and summed rest score (SRS) of high-speed SPECT linearly correlated with conventional SPECT scores (r = 0.93 for both).

Readers rated image quality good and higher in 94 percent of cases for high-speed SPECT and 89 percent of cases for conventional SPECT.

“Importantly, all patients with low likelihood of CAD had normal perfusion by high-speed SPECT, suggesting high specificity,” Sharir said. “Diagnostic confidence was similar for the two imaging methods, with about 80% of the studies categorized as definitely normal or abnormal.”

Bonow, in his commentary, suggested that higher image quality that detects smaller perfusion defects might also result in a greater number of false positive findings, thereby reducing the specificity high-speed SPECT.

On the bright side, Bonow said the faster system could promote the development of new short-lived radiotracers or even resurrect an agent such as technetium-99m teboroxime (8), whose short physical half-life proved too rapid for standard gamma cameras to image appropriately

The study was the first of its kind and limited in many respects. More research is needed, according to the authors. Whether the high-speed camera will replace many entrenched systems, depends on many factors, including price.

Time, effort, and experience will decide whether this new scanner will revolutionize the field of MPI, Bonow said, adding that “it certainly has the potential to do so.”