A new method for SPECT quantification of targeted radiotracers uptake in the myocardium

New approach to quantification of molecularly targeted radiotracer uptake from hybrid cardiac SPECT/CT: methodology and validation

Quantification of molecularly targeted radiotracer uptake in the myocardium from SPECT remains challenging in part due to potentially low levels of focal tracer uptake of presently available molecularly targeted agents and further degradation of cardiac SPECT by extracardiac radioactivity and partial-volume effect. The purpose of this study was to derive and validate a new SPECT quantification method for assessments of absolute radiotracer uptake in the myocardium.

METHODS

The method was integrated with a hybrid micro-SPECT/CT imaging protocol to calculate radiotracer uptake of a molecularly targeted agent in the ischemic myocardium. CT coregistered with SPECT was used to identify the position and orientation of the left ventricle. Corrections for extracardiac activity and partial-volume errors were performed via a heuristic method derived with a total count sampling scheme. Myocardial radiotracer uptake was quantified from SPECT using an external point source as a known reference. Methods were validated using an ischemic rat model injected with a (99m)Tc-labeled SPECT radiotracer targeted at αvβ3 integrin. SPECT-quantified myocardial radiotracer uptake was compared with postmortem myocardial tissue well-counted radioactivity.

RESULTS

Initial correlation between SPECT-quantified and well-counted radioactivity was fair (R(2) = 0.19, y = 0.50x + 0.05, P = 0.06) when no correction was applied to SPECT quantification. Correlation was significantly improved with tissue weight correction (R(2) = 0.84, y = 1.82x - 0.01, P < 0.001), and a trend toward the improvement of correlation was observed with extracardiac activity correction (R(2) = 0.85, y = 1.54x - 0.01, P < 0.001) and partial-volume correction (R(2) = 0.86, y = 1.68x - 0.01, P < 0.001). Reproducibility of the SPECT quantification was excellent, either with no correction (R(2) = 0.99, y = 1.00x + 0.00, P < 0.001) or with all corrections (R(2) = 1.00, y = 1.00x - 0.00, P < 0.001).

CONCLUSION

Corrections for the myocardial tissue weight, extracardiac activity, and partial-volume errors are crucial for precise assessments of myocardial radiotracer uptake using micro-SPECT/CT. The quantitative SPECT/CT approach developed provides a reasonable and reproducible in vivo estimation of absolute radiotracer uptake in a model of myocardial injury and should permit quantitative serial monitoring of subtle changes in the myocardial uptake of targeted radiotracers.

Multimodality imaging of myocardial injury and remodeling

Advances in cardiovascular molecular imaging have come at a rapid pace over the last several years. Multiple approaches have been taken to better understand the structural, molecular, and cellular events that underlie the progression from myocardial injury to myocardial infarction (MI) and, ultimately, to congestive heart failure. Multimodality molecular imaging including SPECT, PET, cardiac MRI, and optical approaches is offering new insights into the pathophysiology of MI and left ventricular remodeling in small-animal models. Targets that are being probed include, among others, angiotensin receptors, matrix metalloproteinases, integrins, apoptosis, macrophages, and sympathetic innervation. It is only a matter of time before these advances are applied in the clinical setting to improve post-MI prognostication and identify appropriate therapies in patients to prevent the onset of congestive heart failure.

A novel three-dimensional image reconstruction method for near-field coded aperture single photon emission computerized tomography

Coded aperture imaging for two-dimensional (2D) planar objects has been investigated extensively in the past, whereas little success has been achieved in imaging 3D objects using this technique. In this article, the authors present a novel method of 3D single photon emission computerized tomography (SPECT) reconstruction for near-field coded aperture imaging. Multiangular coded aperture projections are acquired and a stack of 2D images is reconstructed separately from each of the projections. Secondary projections are subsequently generated from the reconstructed image stacks based on the geometry of parallel-hole collimation and the variable magnification of near-field coded aperture imaging. Sinograms of cross-sectional slices of 3D objects are assembled from the secondary projections, and the ordered subset expectation and maximization algorithm is employed to reconstruct the cross-sectional image slices from the sinograms. Experiments were conducted using a customized capillary tube phantom and a micro hot rod phantom. Imaged at approximately 50 cm from the detector, hot rods in the phantom with diameters as small as 2.4 mm could be discerned in the reconstructed SPECT images. These results have demonstrated the feasibility of the authors' 3D coded aperture image reconstruction algorithm for SPECT, representing an important step in their effort to develop a high sensitivity and high resolution SPECT imaging system.

SPECT attenuation correction: an essential tool to realize nuclear cardiology's manifest destiny

Single photon emission computed tomography (SPECT) myocardial perfusion imaging has attained widespread clinical acceptance as a standard of care for cardiac patients. Yet, physical phenomena degrade the accuracy of how our cardiac images are visually interpreted or quantitatively analyzed. This degradation results in cardiac images in which brightness or counts are not necessarily linear with tracer uptake or myocardial perfusion. Attenuation correction (AC) is a methodology that has evolved over the last 30 years to compensate for this degradation. Numerous AC clinical trials over the last 10 years have shown increased diagnostic accuracy over non-AC SPECT for detecting and localizing coronary artery disease, particularly for significantly increasing specificity and normalcy rate. This overwhelming evidence has prompted our professional societies to issue a joint position statement in 2004 recommending the use of AC to maximize SPECT diagnostic accuracy and clinical usefulness. Phantom and animal studies have convincingly shown how SPECT AC recovers the true regional myocardial activity concentration, while non-AC SPECT does not. Thus, AC is also an essential tool for extracting quantitative parameters from all types of cardiac radionuclide distributions, and plays an important role in establishing cardiac SPECT for flow, metabolic, innervation, and molecular imaging, our manifest destiny.