A fast cardiac gamma camera with dynamic SPECT capabilities: design, system validation and future potential

Abstract

Purpose: The goal of this study is to present the Discovery NM 530c (DNM), a cardiac SPECT camera, interfacing multi-pinhole collimators with solid-state modules, aiming at slashing acquisition time without jeopardizing quality. DNM resembles PET since it enables 3-D SPECT without detector motion. We further envision how these novel capabilities may help with current and future challenges of cardiac imaging.

Methods: DNM sensitivity, spatial resolution (SR) and energy resolution (ER), count rate response, cardiac uniformity and cardiac defect contrast were measured and compared to a dedicated cardiac, dual-head standard SPECT (S-SPECT) camera.

Results: DNM sensitivity was more than threefold higher while SR was notably better. Significantly, SR was the same for (99m)Tc and (201)Tl. ER was improved on DNM and allowed good separation of (99m)Tc and (123)I spectral peaks. Count rate remained linear on DNM up to 612 kcps, while S-SPECT showed severe dead time limitations. Phantom studies revealed comparable uniformity and defect contrast, notwithstanding significantly shorter acquisition time for the DNM. First patient images, including dynamic SPECT, are also presented.

Conclusion: DNM is raising the bar for expedition and upgrade of practice. It features high sensitivity as well as improved SR, temporal resolution and ER. It enables reduction of acquisition time and fast protocols. Importantly, it is potentially capable of dynamic 3-D acquisition. The new technology is potentially upgradeable and may become a milestone in the evolution of nuclear cardiology as it assumes its key role in molecular imaging of the heart.

Fig. 1

Left: the Discovery NM 530c (DNM) featuring a gantry that is similar to a conventional cardiac SPECT camera, with a different detector assembly that is completely stationary during acquisition. Right: hybridization of the DNM camera with a multidetector CT camera

Fig. 2

Components of the detector assembly. a Multi-pinhole collimator covering the heart volume. b Pinhole collimation and miniaturization of the detector enable proximity to the heart with minification of the heart image, utilizing most of the detector surface to cover the heart projection. c A 4 × 4 cm CZT detector size as compared to a classic photomultiplier tube of a conventional SPECT camera. d The detector box with its rear electronic connections. e The CZT is pixelated featuring intrinsic resolution of about 2.5 mm irrespective of photon energy

Fig. 3

Imaging of anthropomorphic torso phantom by the DNM camera with male (upper right) and female (upper left) configurations. Bottom: transaxial CT slice of the phantom with middle-sized breast extensions and defect inserts in septal and lateral walls

Fig. 4

Tc-99m energy spectra of DNM (blue) and S-SPECT (pink). Better energy resolution is demonstrated for the DNM (5.4%) as compared to S-SPECT (9.4%)

Fig. 5

Clear separation of ^99mTc and ¹²³I energy peaks on DNM as contrasted with S-SPECT. In addition, the S-SPECT spectra show two fluorescence peaks for Pb at 75 and 85 keV originating at the parallel-hole collimator. Septal penetration and scatter of higher energy photons associated with the ¹²³I dose are also noted on S-SPECT alone

Fig. 6

¹²³I images. Left: planar image from S-SPECT system demonstrates a “star effect” due to parallel-hole collimator septal penetration and scatter of higher energy photons associated with the ¹²³I. Right: “planar” image on a single collimator-detector module of DNM system has no “star effect”

Fig. 7

²⁰¹Tl point sources as imaged on a dedicated conventional cardiac SPECT camera (a) and DNM (b), clearly demonstrating the higher resolution of the DNM. In addition corresponding line profiles reveal the degrading effect of septal penetration of the parallel hole collimator on S-SPECT resolution (c), while no septal penetration is evident on DNM (d)

Fig. 8

Count rate performance with both systems operating at optimal mode for high count rate using ^99mTc sources. According to NEMA guidelines the incident rate is determined by assuming that it is equal to the rate observed on the camera at low doses, while higher doses are normalized accordingly. The curves demonstrate linear count rate response of the DNM system (blue line) up to the system computer limit and non-linear response with saturation and even paralysis of S-SPECT (pink line). The maximal rate achieved with DNM is almost twice of what can be achieved with S-SPECT. No count loss is noted with DNM, while NEMA criterion for 20% loss was reached at 210 kc/s on S-SPECT

Fig. 9

Samples of phantom comparisons: tomographic slices and corresponding polar plots obtained using a low-dose rest acquisition. Left panel: DNM (3-min acquisition). Right panel: S-SPECT (15-min acquisition). Upper row: male configuration without a defect. Middle row: female configuration with medium-sized breast inserts and with perfusion defects in the septal and lateral walls. Lower row: male configuration with hot liver and without a defect. Images are quite comparable in spite of fivefold shorter acquisition time on the DNM

Fig. 10

Distribution of segmental scores for a series of ten uniform phantoms acquired under conditions mimicking low-dose rest acquisition. Blue: DNM short acquisition time. Orange: S-SPECT (Ventri) with longer acquisition time. Error bars represent two standard deviation values showing variability within corresponding series. Most segments show comparable mean scores and the differences between the scores of the two modalities are similar to their intrinsic variability

Fig. 11

Upper panel: ROI within the left ventricular cavity. Lower panel: dynamic time-activity curve of the same ROI, reflecting the first 90 s of bolus passage

Fig. 12

A 1-day ^99mTc-tetrofosmin study. Upper two rows: DNM images employing short acquisition times. Lower two rows: images of the same patient acquired by conventional dedicated cardiac SPECT with significantly longer acquisition times. Images are comparable and both are compatible with inferoseptal wall ischaemia of the left ventricular wall extending to the apical area. Courtesy of the Department of Nuclear Medicine, Rambam Hospital, Israel

Fig. 13

A treadmill stress/rest ²⁰¹Tl study. a Conventional SPECT images employing 20 min acquisition for each stress or rest data set. b DNM image employing only 6 min acquisition for each data set. Myocardial ischaemia of the inferolateral wall of the left ventricle is noted on both modalities. VLA vertical long axis, HLA horizontal long axis. Courtesy of Dr. Iftikhar Ali from Ottawa Heart Institute, Canada