Attenuation corrected cardiac perfusion SPECT

SPECT/CT: an update on technological developments and clinical applications

Functional nuclear medicine imaging with single-photon emission CT (SPECT) in combination with anatomical CT has been commercially available since the beginning of this century. The combination of the two modalities has improved both the sensitivity and specificity of many clinical applications and CT in conjunction with SPECT that allows for spatial overlay of the SPECT data on good anatomy images. Introduction of diagnostic CT units as part of the SPECT/CT system has also potentially allowed for a more cost-efficient use of the equipment. Most of the SPECT systems available are based on the well-known Anger camera principle with NaI(Tl) as a scintillation material, parallel-hole collimators and multiple photomultiplier tubes, which, from the centroid of the scintillation light, determine the position of an event. Recently, solid-state detectors using cadmium-zinc-telluride became available and clinical SPECT cameras employing multiple pinhole collimators have been developed and introduced in the market. However, even if new systems become available with better hardware, the SPECT reconstruction will still be affected by photon attenuation and scatter and collimator response. Compensation for these effects is needed even for qualitative studies to avoid artefacts leading to false positives. This review highlights the recent progress for both new SPECT cameras systems as well as for various data-processing and compensation methods.

The way to a man's heart is through his stomach: much 'diaphragmatic' attenuation is likely gastric, and effervescent granules enhance cardiac imaging

Avoidance of falsely positive results depends on distinguishing reality from artifact, in turn depending on images of highest quality. In radionuclide cardiac imaging, an inferior wall artifactual defect, so called "diaphragmatic attenuation", is particularly common and vexing. Despite the historically held view, analysis and review of the literature suggest the defect is likely not diaphragmatic but rather primarily due to attenuation by nearby stomach wall. The explanation is based on gravity and anatomy. With this improved understanding, effervescent granules were given as a clinical, nonresearch measure to nine patients during myocardial scanning. It was observed that two-thirds demonstrated moderate or marked lessening of attenuation. An additional benefit is lessening of artifact by extracardiac activity. These benefits may also apply to other sorts of cardiac radionuclide imaging. The significance of this new imaging method is discussed and various avenues of research are proposed.

[Diagnostic accuracy of the SPECT of post-stress myocardial perfusion with attenuation and scatter correction]

OBJECTIVE

To evaluate the effect of attenuation and scatter correction (AC-SC) on the diagnostic accuracy of post-stress myocardial perfusion (MP) SPECT.

MATERIAL AND METHODS

The retrospective analysis included 121 patients who had a non-corrected (NC) and AC-SC 99mTc-Tetrofosmin MP SPECT after stress. The left ventricle was divided into 13 segments. Two observers performed a visual assessment of the MP on a scale from 0 (perfusion defect) to 3 (normal uptake). A consensus on concordances and discordances between the NC and AC-SC images was established. Final diagnosis of coronary artery disease (CAD) was established by coronary angiography (CANG) (stenosis > or = 70 %).

RESULTS

The combined analysis of NC and AC-SC images produced 93 concordances and 28 discordances. Of the 93 concordances, both studies were abnormal in 67 patients (abnormal CANG in 57) and normal in 26 patients (normal CANG in 20). Among the 28 discordances, 23 were abnormal NC/normal AC-SC (normal CANG in 18) and 5 normal NC/abnormal AC-SC. In these 5 patients AC-SC generated anterior perfusion defects but the CANG was normal. Overall, the appearance of NC and AC-SC images were in agreement with the CANG findings in the 72 % (87/121) and 78 % (95/121) of the patients, respectively. Sixty-seven of the 90 patients with abnormal NC had also abnormal AC-SC (abnormal CANG in 57) and the other 23 had normal AC-SC (normal CANG in 18). The appearance of AC-SC was in agreement with CANG finding in the 83 % (75/90) of patients with abnormal NC. MP abnormalities in NC normalized by AC-SC were more frequently located in inferior wall

CONCLUSION

AC-SC improves the diagnostic accuracy of post stress NC MP SPECT for the diagnosis of CAD. From these results we consider that AC-SC is of clinical value for the correction of attenuation artifacts, more frequently observed in the inferior wall. The presence of antero-apical perfusion defects in AC-SC with normal NC does not mean CAD. So it is necessary to adjust the normalcy pattern of MP SPECT when AC-SC is performed.

The Assessment of Infarct Size in Postmyocardial Infarction Patients Undergoing Thallium-201 Tomographic Imaging Is Improved Using Attenuation Correction

PURPOSE

Attenuation correction (ATC) has been shown to improve the accuracy of thallium-201 single photon emission computed tomography (SPECT) for the detection and evaluation of patients with coronary artery disease. The purpose of this study was to evaluate the value of ATC for the assessment of infarct size in patients after myocardial infarction (MI).

MATERIALS AND METHODS

Tl-201 SPECT with ATC was performed on 39 patients with 49 previous MIs. This was followed by radionuclide ventriculography for the assessment of global and regional left ventricular function. Uncorrected and corrected 24-hour redistribution SPECT images were analyzed for regional perfusion using a 5-point segmental scoring scale from 0 (normal) to 4 (absent) thallium uptake.

RESULTS

The mean number of segments with scores of >1 and 2 was significantly higher without ATC than with ATC (5.3 +/- 3.6 vs. 3.5 +/- 3.6, P = 0.0001 for scores >1; 3.8 +/- 3.6 vs. 2.5 +/- 3.0, P = 0.0001 for scores of >2, respectively). The mean total number of segments with scores of >1 assessed without ATC was significantly higher compared with that assessed with ATC (16.9 +/- 13.5 vs. 11.2 +/- 12.2, P = 0.0001). Evaluation without ATC demonstrated only a fair correlation between the SPECT parameters (number of segments with scores of >1 and >2, and total score of segments with scores of >1) and left ventricular regional and global function, whereas there was a clear improvement in all the parameters after ATC. With ATC, a decrease in infarct size was demonstrated in 27 of the 49 infarcts (55%).

CONCLUSIONS

The improved correlation with left ventricular function indicates that SPECT imaging with ATC provides a more accurate assessment of infarct size in post-MI patients. The use of nonattenuation-corrected SPECT imaging overestimates infarct size in a majority of patients.

Chronic effects of transmyocardial laser revascularization in the nonischemic myocardium: a word of caution

BACKGROUND

Transmyocardial laser revascularization (TMLR) has been shown to induce angiogenesis and improve regional myocardial blood flow. However, the biological response to laser of different energy doses in nonischemic myocardium has not been well studied. We study the time course of histological appearance following high- and low-energy TMLR in a rodent model of normal myocardium.

METHODS

The animals were randomized to two groups (high-energy (1.4 J/pulse) TMLR group and low-energy (0.6 J/ pulse) TMLR group). TMLR was applied to the anterolateral wall of the left ventricle. Then the animals were sacrificed (one every 2 or 3 days up to 6 weeks, then one every week) to give a series of hearts from 1 week to 12 weeks following TMLR.

RESULTS

No patent laser channels were identified. All laser channel remnants were composed of granulation tissue, fibrosis, and various amounts of vascular structures. Changes in vascular density demonstrated a similar pattern in both groups over time. The initial increase in vascular density within the laser channel remnants and the surrounding areas was more marked in group I than in group II. Gradual decline of vascular density occurred during a later follow-up. More fibrosis and less vasculature were demonstrated 12 weeks after laser therapy in group I.

CONCLUSION

Angiogenic response to the Holmium:YAG laser in the nonischemic myocardium is nonspecific and unsustainable. TMLR might chronically impair the myocardium function by enhancing regional scar formation. Therefore, we should clearly identify the ischemic area for laser therapy. Reduction of laser energy might prevent this potential complication.

Clinical significance of apical thinning after attenuation correction

BACKGROUND

Apical thinning and other image changes at the apex have been described after attenuation correction of myocardial perfusion single photon emission computed tomography (SPECT) studies, but their clinical significance is unknown.

METHODS AND RESULTS

We studied 102 subjects from a multicenter trial of attenuation correction, 46 with angiographic coronary artery disease and 56 normal subjects. We graded the presence and magnitude (on a 4-point scale) of apical thinning (decrease in wall thickness, best assessed in the vertical long-axis view) in both noncorrected and attenuation-corrected images. In attenuation-corrected images, apical thinning of any degree was present in 78% of the abnormal patients and 63% of the normal subjects (P = not significant [NS]). However, moderate or severe apical thinning was present in 30% of the abnormal patients compared with 5% of the normal subjects (relative risk = 2.2, P <.001). In noncorrected images, apical thinning of any degree was present in 87% of the abnormal patients and 71% of the normal subjects (P = NS). However, moderate or severe apical thinning was present in 28% of the abnormal patients compared with 4% of the normal subjects (relative risk = 2.3, P <.001).

CONCLUSION

The presence of mild apical thinning is common in both noncorrected and attenuation-corrected SPECT images and does not imply coronary artery disease. Moderate or severe apical thinning is 7 times more common in patients than in normal subjects, but it is relatively uncommon and thus is not a generally useful clinical tool.

Patient effective radiation dose and associated risk from transmission scans using 153Gd line sources in cardiac spect studies

The aim of the present study was to determine the contribution of transmission measurements acquisition to total patient effective dose from cardiac SPECT studies. A dual-head L-shaped gamma camera equipped with a transmission scan acquisition system based on two 153Gd line sources was used to simulate transmission measurements acquisition exposure on an anthropomorphic phantom. Thermoluminescence dosimeters were used to directly monitor the dose to 550 measuring points in the phantom. The effective dose and associated risk from transmission scans acquisition were estimated and compared to those associated to the radiopharmaceutical injected. The maximum effective dose from a typical transmission measurements acquisition was 1.3 microSv and 1.9 microSv for male and female patients, respectively. The contribution of the typical transmission scans acquisition to total patient radiation risk from a cardiac SPECT study is less than 10(-3). Thus, radiation exposure may not be considered as a limiting factor for the clinical application of attenuation correction methods based on transmission measurements in cardiac SPECT.