Clinical review of attenuation-corrected cardiac SPECT

QCard-NM: Developing a semiautomatic segmentation method for quantitative analysis of the right ventricle in non-gated myocardial perfusion SPECT imaging

BACKGROUND

Recent studies have shown that the right ventricular (RV) quantitative analysis in myocardial perfusion imaging (MPI) SPECT can be beneficial in the diagnosis of many cardiopulmonary diseases. This study proposes a new algorithm for right ventricular 3D segmentation and quantification.

METHODS

The proposed Quantitative Cardiac analysis in Nuclear Medicine imaging (QCard-NM) algorithm provides RV myocardial surface estimation and creates myocardial contour using an iterative 3D model fitting method. The founded contour is then used for quantitative RV analysis. The proposed method was assessed using various patient datasets and digital phantoms. First, the physician's manually drawn contours were compared to the QCard-NM RV segmentation using the Dice similarity coefficient (DSC). Second, using repeated MPI scans, the QCard-NM's repeatability was evaluated and compared with the QPS (quantitative perfusion SPECT) algorithm. Third, the bias of the calculated RV cavity volume was analyzed using 31 digital phantoms using the QCard-NM and QPS algorithms. Fourth, the ability of QCard-NM analysis to diagnose coronary artery diseases was assessed in 60 patients referred for both MPI and coronary angiography.

RESULTS

The average DSC value was 0.83 in the first dataset. In the second dataset, the coefficient of repeatability of the calculated RV volume between two repeated scans was 13.57 and 43.41 ml for the QCard-NM and QPS, respectively. In the phantom study, the mean absolute percentage errors for the calculated cavity volume were 22.6% and 42.2% for the QCard-NM and QPS, respectively. RV quantitative analysis using QCard-NM in detecting patients with severe left coronary system stenosis and/or three-vessel diseases achieved a fair performance with the area under the ROC curve of 0.77.

CONCLUSION

A novel model-based iterative method for RV segmentation task in non-gated MPI SPECT is proposed. The precision, accuracy, and consistency of the proposed method are demonstrated by various validation techniques. We believe this preliminary study could lead to developing a framework for improving the diagnosis of cardiopulmonary diseases using RV quantitative analysis in MPI SPECT.

Investigation of the physical effects of respiratory motion compensation in a large population of patients undergoing Tc-99m cardiac perfusion SPECT/CT stress imaging

BACKGROUND

Respiratory motion can deteriorate image fidelity in cardiac perfusion SPECT. We determined the extent of respiratory motion, assessed its impact on image fidelity, and investigated the existence of gender differences, thereby examining the influence of respiratory motion in a large population of patients.

METHODS

One thousand one hundred and three SPECT/CT patients underwent visual tracking of markers on their anterior surface during stress acquisition to track respiratory motion. The extent of motion was estimated by registration. Visual indicators of changes in cardiac slices with motion correction, and the correlation between the extent of motion with changes in segmental-counts were assessed.

RESULTS

Respiratory motion in the head-to-feet direction was the largest component of motion, varying between 1.1 and 37.4 mm, and was statistically significantly higher (p = 0.002) for males than females. In 33.0% of the patients, motion estimates were larger than 10 mm. Patients progressively show more distinct visual changes with an increase in the extent of motion. The increase in segmental-count differences in the anterior, antero-lateral, and inferior segments correlated with the extent of motion.

CONCLUSIONS

Respiratory motion correction diminished the artefactual reduction in anterior and inferior wall counts associated with respiratory motion. The extent of improvement was strongly related to the magnitude of motion.

Improved diagnosis of the number of stenosed coronary artery vessels by segmentation with scatter and photo-peak window data for attenuation correction in myocardial perfusion SPECT

BACKGROUND

Attenuation correction using segmentation of scatter and photo-peak window data (SSPAC) enables an evaluation of the attenuation map in a patient-specific manner without additional radiation exposure. We compared the accuracy of SSPAC and non-corrected myocardial perfusion scintigraphy methods for diagnosing the number of stenosed coronary artery vessels.

METHODS AND RESULTS

We retrospectively reviewed the data from 183 consecutive patients who underwent 99mTc-tetrofosmin stress/rest SPECT examination and a coronary angiography within 3 months. The MPS images were reconstructed with and without SSPAC attenuation correction. We examined the accuracy of the quantitative interpretation using summed differential score in the detection of coronary artery disease (CAD). The attenuation maps were successfully determined in 179 of 183 patients (98%). In terms of the vessel-based diagnostic ability, sensitivity, specificity, positive predictive and negative predictive values of the SSPAC and non-correction methods for diagnosing CAD in individual coronary territories were 77%*, 89%, 74%*, and 90%* vs 51%, 87%, 62%, and 82%, respectively (*P < .05). In 35 patients with multi-vessel CAD, those values were 78%*, 81%, 93%, and 55%* vs 49%, 81%, 89%, and 34%, respectively (*P < .05; AUC: 0.82 vs 0.62, P < .05).

CONCLUSION

SSPAC-corrected SPECT myocardial perfusion images exhibit improved accuracy in the detection of the number of stenosed coronary artery vessels, even in patients with multi-vessel CAD.

Taking the perfect nuclear image: quality control, acquisition, and processing techniques for cardiac SPECT, PET, and hybrid imaging

Nuclear Cardiology for the past 40 years has distinguished itself in its ability to non-invasively assess regional myocardial blood flow and identify obstructive coronary disease. This has led to advances in managing the diagnosis, risk stratification, and prognostic assessment of cardiac patients. These advances have all been predicated on the collection of high quality nuclear image data. National and international professional societies have established guidelines for nuclear laboratories to maintain high quality nuclear cardiology services. In addition, laboratory accreditation has further advanced the goal of the establishing high quality standards for the provision of nuclear cardiology services. This article summarizes the principles of nuclear cardiology single photon emission computed tomography (SPECT) and positron emission tomography (PET) imaging and techniques for maintaining quality: from the calibration of imaging equipment to post processing techniques. It also will explore the quality considerations of newer technologies such as cadmium zinc telleride (CZT)-based SPECT systems and absolute blood flow measurement techniques using PET.

Adding attenuation corrected images in myocardial perfusion imaging reduces the need for a rest study

BACKGROUND

The American Society of Nuclear Cardiology and the Society of Nuclear Medicine conclude that incorporation of attenuation corrected (AC) images in myocardial perfusion scintigraphy (MPS) will improve diagnostic accuracy. The aim was to investigate the value of adding AC stress-only images for the decision whether a rest study is necessary or not.

METHODS

1,261 patients admitted to (99m)Tc MPS were studied. The stress studies were interpreted by two physicians who judged each study as "no rest study necessary" or "rest study necessary", by evaluating NC stress-only and NC + AC stress-only images. When there was disagreement between the two physicians, a third physician evaluated the studies. Thus, agreement between 2 out of 3 physicians was evaluated.

RESULTS

The physicians assessed 214 more NC + AC images than NC images as "no rest study necessary" (17% of the study population). The number of no-rest-study-required was significantly higher for NC + AC studies compared to NC studies (859 vs 645 cases (p < 0.0001). In the final report according to clinical routine, ischemia or infarction was reported in 23 patients, assessed as "no rest study necessary" (22 NC + AC cases; 8 NC cases), (no statistically significant difference). In 11 of these, the final report stated "suspected/possible ischemia or infarction in a small area".

CONCLUSIONS

Adding AC stress-only images to NC stress-only images reduce the number of unnecessary rest studies substantially.

Normal stress databases in myocardial perfusion scintigraphy--how many subjects do you need?

BACKGROUND

Commercial normal stress databases in myocardial perfusion scintigraphy (MPS) commonly consist of 30-40 individuals. The aim of the study was to determine how many subjects are needed.

METHODS

Four normal stress databases were developed using patients who underwent 99mTc MPS: non-corrected images (NC) for male, NC for female, attenuation-corrected images (AC) for male and AC for female subjects. 126 male and 205 female subjects were included. The normal database was created by alternatingly computing the mean of all normal subjects and normalizing the subjects with respect to this mean, until convergence. Coefficients of variation (CV) were created for increasing number of included patients in the four different normal stress databases.

RESULTS

Normal stress databases with < 35 subjects had a high CV. Mean CV -2 standard deviations (SD) decreased with 28% between two and five included subjects, 71% between two and 35 subjects and 83% between two and 100 included subjects for NC man.

CONCLUSIONS

We conclude that the commonly used 30-40 individuals for making a normal stress database might not be enough due to the high CV. We propose that normal stress databases should consist of more than 30-40 individuals, preferably more than 50 individuals, both for NC and AC studies.

Small average differences in attenuation corrected images between men and women in myocardial perfusion scintigraphy: a novel normal stress database

Background

The American Society of Nuclear Cardiology and the Society of Nuclear Medicine state that incorporation of attenuation-corrected (AC) images in myocardial perfusion scintigraphy (MPS) will improve image quality, interpretive certainty, and diagnostic accuracy. However, commonly used software packages for MPS usually include normal stress databases for non-attenuation corrected (NC) images but not for attenuation-corrected (AC) images. The aim of the study was to develop and compare different normal stress databases for MPS in relation to NC vs. AC images, male vs. female gender, and presence vs. absence of obesity. The principal hypothesis was that differences in mean count values between men and women would be smaller with AC than NC images, thereby allowing for construction and use of gender-independent AC stress database.

Methods

Normal stress perfusion databases were developed with data from 126 male and 205 female patients with normal MPS. The following comparisons were performed for all patients and separately for normal weight vs. obese patients: men vs. women for AC; men vs. women for NC; AC vs. NC for men; and AC vs. NC for women.

Results

When comparing AC for men vs. women, only minor differences in mean count values were observed, and there were no differences for normal weight vs. obese patients. For all other analyses major differences were found, particularly for the inferior wall.

Conclusions

The results support the hypothesis that it is possible to use not only gender independent but also weight independent AC stress databases.

Quantification of myocardial perfusion SPECT studies in Chinese population with Western normal databases

BACKGROUND

The purpose of this study is to assess diagnostic performance of the current quantification packages using Western normal databases in detecting coronary artery disease in Chinese population.

METHODS

Seventy-five patients who underwent rest/stress myocardial perfusion SPECT and coronary angiography (CAG) were enrolled. Emory Cardiac Toolbox (ECTb) and Quantitative Perfusion SPECT (QPS) with its standard (QPS-standard) and simplified (QPS-simplified) methods were used to quantify these studies. A preliminary Chinese normal database was created from 80 normal subjects (QPS-simplified-Chinese). Receiver operator characteristic (ROC) was used to assess the accuracy of the four normal databases in detecting >or=50% or >or=70% stenosis given by CAG.

RESULTS

The enrolled cohorts had lower body mass index (BMI) and smaller heart size than Western population. The areas under ROC curve of ECTb, QPS-standard, and QPS-simplified were significantly lower than QPS-simplified-Chinese in detecting >or=50% stenosis, but not in detecting >or=70% stenosis. Diagnostic accuracy was much lower in the RCA and LCX territory.

CONCLUSION

Chinese normal database is needed for accurately applying these quantification methods to Chinese population, especially for detecting moderate defects in regions with relatively greater attenuation impact. An alternative approach could be modification of the existing Western normal databases for low-BMI and/or small-heart subjects.

C-SPECT - a Clinical Cardiac SPECT/Tct Platform: Design Concepts and Performance Potential

Because of scarcity of photons emitted from the heart, clinical cardiac SPECT imaging is mainly limited by photon statistics. The sub-optimal detection efficiency of current SPECT systems not only limits the quality of clinical cardiac SPECT imaging but also makes more advanced potential applications difficult to be realized. We propose a high-performance system platform - C-SPECT, which has its sampling geometry optimized for detection of emitted photons in quality and quantity. The C-SPECT has a stationary C-shaped gantry that surrounds the left-front side of a patient's thorax. The stationary C-shaped collimator and detector systems in the gantry provide effective and efficient detection and sampling of photon emission. For cardiac imaging, the C-SPECT platform could achieve 2 to 4 times the system geometric efficiency of conventional SPECT systems at the same sampling resolution. This platform also includes an integrated transmission CT for attenuation correction. The ability of C-SPECT systems to perform sequential high-quality emission and transmission imaging could bring cost-effective high-performance to clinical imaging. In addition, a C-SPECT system could provide high detection efficiency to accommodate fast acquisition rate for gated and dynamic cardiac imaging. This paper describes the design concepts and performance potential of C-SPECT, and illustrates how these concepts can be implemented in a basic system.

Improved quantification in single-pinhole and multiple-pinhole SPECT using micro-CT information

PURPOSE

The purpose of this study was to demonstrate the feasibility of accurate quantification in pinhole SPECT using micro-CT information.

METHODS

Pinhole SPECT scans were performed using a clinical dual-head gamma camera. Each pinhole SPECT scan was followed by a micro-CT acquisition. Functional and anatomical images were coregistered using six point sources visible with both modalities. Pinhole SPECT images were reconstructed iteratively. Attenuation correction was based on micro-CT information. Scatter correction was based on dual and triple-energy window methods. Phantom and animal experiments were performed. A phantom containing nine vials was filled with different concentrations of (99m)Tc. Three vials were also filled with CT contrast agent to increase attenuation. Activity concentrations measured on the pinhole SPECT images were compared with activity concentrations measured by the dose calibrator. In addition, 11 mice were injected with (99m)Tc-labelled Nanobodies. After acquiring functional and anatomical images, the animals were killed and the liver activity was measured using a gamma-counter. Activity concentrations measured on the reconstructed images were compared with activity concentrations measured with the gamma counter.

RESULTS

The phantom experiments demonstrated an average error of -27.3 +/- 15.9% between the activity concentrations measured on the uncorrected pinhole SPECT images and in the dose calibrator. This error decreased significantly to -0.1 +/- 7.3% when corrections were applied for nonuniform attenuation and scatter. The animal experiment revealed an average error of -18.4 +/- 11.9% between the activity concentrations measured on the uncorrected pinhole SPECT images and measured with the gamma counter. This error decreased to -7.9 +/- 10.4% when attenuation and scatter correction was applied.

CONCLUSION

Attenuation correction obtained from micro-CT data in combination with scatter correction allows accurate quantification in pinhole SPECT.

Attenuation-corrected 99mTc-MIBI SPECT in overweight patients with chronic ischaemic dysfunction: a comparison to NH3 PET and implications for the diagnosis of myocardial viability

OBJECTIVE

We determined the value of attenuation correction (AC) of myocardial perfusion estimation with (99m)Tc-MIBI SPECT in overweight patients by comparison of uncorrected (filtered back-projection (FBP) and corrected (an iterative algorithm with a measured attenuation coefficients map (FL-AC)) (99m)Tc-MIBI relative uptake to perfusion data obtained in the same patients with NH3 PET. In addition, the impact of attenuation correction for the assessment of myocardial viability with (99m)Tc-MIBI SPECT was determined using FDG PET as the reference method.

METHODS

Thirty consecutive overweight patients (BMI=28+/-4) with left ventricular dysfunction underwent a resting (99m)Tc-MIBI SPECT and a PET study (NH3 and FDG). (99m)Tc-MIBI SPECT scans were reconstructed without attenuation correction (FBP) and with attenuation correction (FL-AC). The left ventricle was divided into 16 segments, in which the relative uptake was quantified using circumferential profiles. A relative uptake > or = 60% was considered consistent with viable myocardium for FDG and MIBI.

RESULTS

The absolute difference between (99m)Tc-MIBI SPECT and NH3 PET uptakes was less pronounced in the inferior (12+/-10% vs. 17+/-12%, P<0.001), anteroseptal (12+/-11% vs. 16+/-12%, P=0.009) and septal (15+/-12% vs. 18+/-14%, P=0.003) regions (FL-AC vs. FBP, respectively). The sensitivity of MIBI for diagnosing myocardial viability increased from 83 to 100% (P=0.034), without loss in specificity.

CONCLUSION

Attenuation correction improves myocardial perfusion estimation by (99m)Tc-MIBI SPECT in the inferior, anteroseptal and septal regions and increases its sensitivity for the diagnosis of myocardial viability.

Imaging techniques in nuclear cardiology for the assessment of myocardial viability

The assessment of myocardial viability has become an important aspect of the diagnostic and prognostic work-up of patients with ischemic cardiomyopathy. Although revascularization may be considered in patients with sufficient viable myocardium, patients with predominantly scar tissue should be treated medically. Patients with left ventricular dysfunction who have viable myocardium are the patients at highest risk because of the potential for ischemia but at the same time benefit most from revascularization. It is important to identify viable myocardium in these patients, and radionuclide myocardial scintigraphy is an excellent tool for this. Single-photon emission computed tomography perfusion scintigraphy (SPECT), whether using (201)thallium, (99m)Tc-sestamibi, or (99m)Tc-tetrofosmin, in stress and/or rest protocols, has consistently been shown to be an effective modality for identifying myocardial viability and guiding appropriate management. Metabolic and perfusion imaging with positron emission tomography (PET) radiotracers frequently adds additional information and is a powerful tool for predicting which patients will have an improved outcome from revascularization. New techniques in the nuclear cardiology field, like attenuation corrected SPECT, dual isotope simultaneous acquisition (DISA) SPECT and gated FDG PET are promising and will further improve the detection of myocardial viability. Also the combination of multislice computed tomography scanners with PET opens possibilities of adding coronary calcium scoring and non-invasive coronary angiography to myocardial perfusion imaging and quantification. Evaluation of the clinical role of these creative new possibilities warrants investigation.

EANM/ESC procedural guidelines for myocardial perfusion imaging in nuclear cardiology

The European procedural guidelines for radionuclide imaging of myocardial perfusion and viability are presented in 13 sections covering patient information, radiopharmaceuticals, injected activities and dosimetry, stress tests, imaging protocols and acquisition, quality control and reconstruction methods, gated studies and attenuation-scatter compensation, data analysis, reports and image display, and positron emission tomography. If the specific recommendations given could not be based on evidence from original, scientific studies, we tried to express this state-of-art. The guidelines are designed to assist in the practice of performing, interpreting and reporting myocardial perfusion SPET. The guidelines do not discuss clinical indications, benefits or drawbacks of radionuclide myocardial imaging compared to non-nuclear techniques, nor do they cover cost benefit or cost effectiveness.

Contributions of subdiaphragmatic activity, attenuation, and diaphragmatic motion to inferior wall artifact in attenuation-corrected Tc-99m myocardial perfusion SPECT

BACKGROUND

Subdiaphragmatic activity and diaphragmatic motion both contribute to inferior wall artifacts in technetium 99m myocardial perfusion single photon emission computed tomography (SPECT).

METHODS AND RESULTS

We used an anthropomorphic phantom with ventricular wall activity, liver/spleen inserts containing variable Tc-99m activity, and variable vertical (diaphragmatic) motion amplitude. SPECT and transmission scans were obtained on a GE Optima NX camera. Data were processed by use of filtered backprojection or attenuation correction (AC). Resulting myocardial activity maps were analyzed with standardized inferior-anterior and anterior-lateral wall ratios. At a subdiaphragmatic-myocardial activity ratio of 0.5:1, inferior wall attenuation predominates, producing a cold artifact. AC corrects inferior wall activity to the level of the anterior wall irrespective of diaphragmatic motion. At a subdiaphragmatic-myocardial activity ratio of 1:1, inferior wall counts vary widely depending on the proximity of subdiaphragmatic activity to the ventricle. With increasing diaphragmatic amplitude, the overlap of subdiaphragmatic activity and inferior wall worsens, leading to a complex mixture of cold and hot artifacts, not corrected by AC.

CONCLUSIONS

Concentration and proximity of subdiaphragmatic Tc-99m activity relative to myocardium comprise a major factor in the nature and severity of inferior wall artifacts. If the subdiaphragmatic Tc-99m concentration is equivalent to that in the myocardium, complex, potentially uninterpretable hot and cold inferior wall artifacts are produced.

[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.

Added value of attenuation-corrected Tc-99m tetrofosmin SPECT for the detection of myocardial viability: comparison with FDG SPECT

BACKGROUND

The aim of this study was to evaluate the value of attenuation correction of technetium 99m tetrofosmin single photon emission computed tomography (SPECT) imaging for the detection of myocardial viability.

METHODS AND RESULTS

A head-to-head comparison between resting Tc-99m tetrofosmin SPECT and fluorine 18 fluorodeoxyglucose (FDG) SPECT was performed. Both the noncorrected and attenuation-corrected Tc-99m tetrofosmin SPECT images were compared with the FDG images that served as the reference for viability. Consecutive patients (n = 33) with chronic coronary artery disease and left ventricular dysfunction were included. Segmental Tc-99m tetrofosmin and FDG data were displayed in polar maps (17-segment model), and the segments were normalized to peak activity by use of the 4D-MSPECT software program. Segments with normalized FDG activity greater than 50% were considered viable. A similar cutoff value to assess viability was used for the noncorrected and attenuation-corrected Tc-99m tetrofosmin images. Regional contractile function was determined from the gated Tc-99m tetrofosmin images and scored as normokinesia, hypokinesia, or akinesia/dyskinesia. Of all segments, 482 (85%) were viable on FDG SPECT. Of these, 427 (89%) were classified as viable with noncorrected Tc-99m tetrofosmin. Thus 55 (11%) were underestimated with noncorrected Tc-99m tetrofosmin SPECT; these segments were mainly located in the inferior and inferoseptal regions. Attenuation correction changed the classification of 39 (70%) of the underestimated segments to viable. By use of attenuation correction, the agreement between Tc-99m tetrofosmin and FDG imaging improved from 84% to 90%. Similar observations were made when the analysis was restricted to the dysfunctional segments.

CONCLUSION

The addition of attenuation correction to Tc-99m tetrofosmin SPECT significantly improved detection of myocardial viability in patients with chronic coronary artery disease, although minimal underestimation of viability remained as compared with FDG SPECT imaging.

Quantitative Tc-99m sestamibi attenuation-corrected SPECT: development and multicenter trial validation of myocardial perfusion stress gender-independent normal database in an obese population

BACKGROUND

A gender-independent stress normal database and criteria for abnormality for attenuation-corrected rest-stress technetium 99m sestamibi same-day myocardial perfusion imaging were developed by evaluation of 112 patients, validated against an obese population of 95 patients from four different clinical sites, and compared with conventional gender-matched database quantification of non-attenuation-corrected studies.

METHODS AND RESULTS

These 95 validation patients (63 men) were used for prospective quantitative evaluation (mean weight, 213 +/- 57 lb; mean body mass index, 32 +/- 9 kg/m(2)). This group included 21 patients (12 men) with a lower than 5% likelihood of coronary artery disease (mean weight, 226 +/- 72 lb; mean body mass index, 34 +/- 13 kg/m(2)) and 74 who underwent cardiac catheterization within 2 months (35 with normal coronaries or coronary lesions <70%). These studies were processed twice, once by use of conventional reconstruction and gender-specific database quantification and a second time by use of attenuation correction and a single gender-independent attenuation-corrected normal database. The attenuation-corrected normal database and criteria for abnormality were developed by evaluation of 48 and 78 patients, respectively. No statistically significant differences were found when comparing attenuation-corrected perfusion distributions of normal men and women, whereas significant differences were found in the same uncorrected studies. Compared with quantitative analysis of the uncorrected studies, quantitative analysis of the attenuation-corrected studies by use of a gender-independent normal database demonstrated a significant improvement in normalcy rate (90% vs 52%, P =.006) and specificity (57% vs 29%, P =.015) in this obese population at no significant loss in sensitivity (90% vs 97%, P = not significant).

CONCLUSION

Attenuation-corrected studies can be quantified with a single gender-independent normal database and a single criterion for abnormality without loss of sensitivity and with significantly better specificity and normalcy rate.

Prospective clinical comparison of non-corrected and attenuation- and scatter-corrected myocardial perfusion SPECT in patients with suspicion of coronary artery disease

Attenuation artefacts decrease the specificity of myocardial perfusion single-photon emission computed tomography (SPECT). In this paper, the results of a prospective study evaluating the clinical applicability of attenuation and scatter correction in myocardial perfusion SPECT are presented. Of 607 patients in whom post-stress 99mTc-tetrofosmin myocardial perfusion SPECT was performed due to suspicion of coronary artery disease, 99 also underwent coronary angiography (CAG). A simultaneous emission/transmission acquisition was performed. A multiple linear array of 153Gd sources and four independent energy windows were used for attenuation and scatter correction. A blind separate analysis of non-corrected (NC) and attenuation- and scatter-corrected (AC-SC) images was performed with scores of zero (no uptake) to three (normal uptake). The final diagnosis was based on CAG findings, and stenoses of > or =70% were considered to be significant. NC images had a sensitivity of 92% and a specificity of 46%. In AC-SC images, the sensitivity decreased to 76%, but the specificity increased to 71%. The decrease in the sensitivity of AC-SC images was observed in all three coronary regions. Attenuation and scatter correction increased the specificity in the right coronary region, but decreased the specificity in the left anterior descending coronary region. In 13 of the 99 patients, AC-SC images showed false positive findings due to count deficiency in the anterior wall with normal CAG. The size of perfusion defects was decreased in AC-SC images (from 5.01 +/- 2.74 to 3.15 +/- 2.50 segments). The severity of perfusion defects was higher in NC (1.10 +/- 0.60) than in AC-SC (1.28 +/- 0.56) images. The combined evaluation of NC and AC-SC images was in agreement with the CAG findings in 79% of patients. It can be concluded that, when compared with NC images, AC-SC images improved the specificity in the right coronary region and decreased the sensitivity in all three coronary regions. Attenuation and scatter correction may generate anterior wall defects with normal CAG. The analysis of AC-SC images cannot be used alone for the diagnosis of coronary artery disease. In the clinical setting, combined NC and AC-SC images are recommended for the evaluation of post-stress myocardial perfusion SPECT.

[Effect of attenuation correction and scatter compensation on the 99mTc-MIBI myocardial perfusion spect in patients without coronary artery disease]

OBJECTIVE

Attenuation correction (AC) and scatter compensation (SC) techniques are recent developments of myocardial perfusion SPECT. Our aim was to evaluate the effect of AC + SC on the myocardial distribution of 99mTc-MIBI in a population without coronary artery disease.

MATERIAL AND METHODS

A multiarray of Gd-153 linear sources was used for simultaneous transmission/emission 99mTc-MIBI myocardial perfusion SPECT in 27 patients without coronary artery disease. A visual analysis and polar map quantification was performed. Changes between non-corrected (NC) and corrected (AC + SC) studies were compared.

RESULTS

AC + SC produced an increase in liver activity and better visualization of the right ventricle. Intestinal activity increased in six patients. Myocardial homogeneity was increased by AC + SC. No differences by gender were observed after AC + SC. In females AC + SC led to a decrease of uptake in the anterior wall, apex and apical segments of the lateral wall and septum, and an increase in the inferior wall. In males AC + SC caused an increase of uptake in the inferior wall and in the basal segments of septum and a decrease of uptake in apex and apical segments of anterior and lateral walls. AC + SC generated false defects in the anterior wall of five patients.

CONCLUSIONS

Our results show the usefulness of AC + SC for compensating the interferences produced by attenuation on the myocardial distribution of 99mTc-MIBI. Because AC + SC may introduce false defects, it must not be applied to normal perfused myocardium.

Low event rate for stress-only perfusion imaging in patients evaluated for chest pain

OBJECTIVES

We sought to demonstrate the safety of stress-only perfusion imaging among patients with low to medium probability of coronary disease being evaluated for chest pain.

BACKGROUND

The need for performing rest in addition to stress perfusion imaging to confirm normalcy is due largely to defects created on the stress images by attenuation artifacts. A low cardiac event rate among patients undergoing stress-only imaging with attenuation correction (AC) would validate the safety of stress-only imaging.

METHODS

Patients with low to medium pretest probability for coronary artery disease (CAD) referred for chest pain evaluation from July 1, 1997 to July 1, 1999, were scheduled for a two-day stress/rest tomographic (single photon emission computerized tomography, or SPECT) perfusion imaging study with Tc-99m sestamibi. Patients were imaged on a variable angle camera with AC using Gd-153 scanning line sources (Vantage ADAC, Milpitas, California). If the stress scan was normal without AC or corrected with AC, the patient did not return for rest scan and was followed.

RESULTS

Seven hundred twenty-nine patients underwent stress-only imaging, and follow-up was obtained on 652 (89%) of those patients. There were 224 males and 428 females with mean age of 52 +/- 13 years. Mean follow-up was 22.3 +/- 6.4 months. The mean pretest probability was 37 +/- 24%. The non-AC images showed breast and/or diaphragmatic attenuation artifacts severe enough to have required the patient to return for rest imaging in 37% of patients, and all corrected completely with AC. During follow-up, there were two noncardiac deaths and no cardiac deaths. There was one myocardial infarction; three patients with progressive unstable angina underwent diagnostic coronary angiography showing significant CAD. The overall cardiac event rate was 0.6%.

CONCLUSIONS

These results support stress-only imaging in patients with low to medium probability for CAD as a safe, time- and cost-efficient imaging modality.

Modeling the axial extension of a transmission line source within iterative reconstruction via multiple transmission sources

Reconstruction algorithms for transmission tomography have generally assumed that the photons reaching a particular detector bin at a particular angle originate from a single point source. In this paper, we highlight several cases of extended transmission sources, in which it may be useful to approach the estimation of attenuation coefficients as a problem involving multiple transmission point sources. Examined in detail is the case of a fixed transmission line source with a fan-beam collimator. This geometry can result in attenuation images that have significant axial blur. Herein it is also shown, empirically, that extended transmission sources can result in biased estimates of the average attenuation, and an explanation is proposed. The finite axial resolution of the transmission line source configuration is modeled within iterative reconstruction using an expectation-maximization algorithm that was previously derived for estimating attenuation coefficients from single photon emission computed tomography (SPECT) emission data. The same algorithm is applicable to both problems because both can be thought of as involving multiple transmission sources. It is shown that modeling axial blur within reconstruction removes the bias in the average estimated attenuation and substantially improves the axial resolution of attenuation images.

Should SPET attenuation correction be more widely employed in routine clinical practice? Against

The development of reliable and accurate devices for the correction of nonuniform soft tissue attenuation is essential for the future clinical use of SPET myocardial perfusion imaging. In addition to abolishing false-positive defects, which is the chief goal, such corrected SPET images may allow for improved detection of coronary artery disease and perhaps ultimately for true quantification of regional myocardial blood flow. Although progress has been made, most existing attenuation correction devices are not yet ready for prime time. To date the literature shows as many positive results as negative results. There is considerable uncertainty, confusion, and skepticism about the true reliability and value of currently available attenuation correction packages. Although commonly referred to as "attenuation correction devices," these packages are in fact much more complex systems and contain novel mechanical designs, novel image acquisition and image reconstruction algorithms, scatter correction, and depth-dependent resolution compensation, in addition to attenuation correction. Each of these variables needs to be better understood and tested prior to clinical implementation. Although the general concepts are shared, there are as may different approaches to attenuation correction as there are vendors. In order to minimize the confusion of potential buyers about such complex systems, it is desirable that, before attenuation correction is implemented in routine clinical practice, each attenuation correction device is rigorously tested using a standardized testing protocol. Potential buyers of equipment should be able to compare the results of testing with various devices against predefined criteria in order to make an educated decision. Such standards have as yet not been developed. At the present time it is unclear whether attenuation correction of cardiac SPET will remain the emperor's new clothes or will develop into a fashionable Armani suit. Until further progress has been made, one cannot recommend attenuation correction devices for routine clinical practice.

Performance of simultaneous emission-transmission systems for attenuation-corrected SPEct: a method for validation applied to two camera systems

Several commercially available systems for attenuation correction in single photon emission computed tomography (SPECT) based on a transmission scan have been introduced that vary in performance. A test procedure for attenuation correction in SPECT is described and applied to two principally different gamma camera systems (the Siemens Multispect 3 triple-headed system [3HS] and the ADAC Genesys Vertex double-headed system [2HS]). The test procedure was based on geometrically well-defined phantoms. A torso phantom was used to illustrate the attenuation correction methods. The test procedure can be used without detailed knowledge of or access to the algorithms used for attenuation correction. The influence on the transmission measurement of radioactivity in a phantom was higher for the 2HS than for the 3HS. The 3HS produced satisfactory attenuation maps and corrected emission count rates to a constant value independent of phantom density and size. With the 2HS, there was a progressive decrease in the correction of emission count rates with increasing phantom density, and about 30% lower corrected count rates in the large compared with the small phantom. A decrease in measured attenuation coefficients in the vicinity of an emission source was demonstrated in large but not small phantoms. A likely explanation is erroneous correction of downscatter into the transmission energy window. This study demonstrates the need for independent evaluation of systems for attenuation correction in SPECT.

An EM algorithm for estimating SPECT emission and transmission parameters from emissions data only

A maximum-likelihood (ML) expectation-maximization (EM) algorithm (called EM-IntraSPECT) is presented for simultaneously estimating single photon emission computed tomography (SPECT) emission and attenuation parameters from emission data alone. The algorithm uses the activity within the patient as transmission tomography sources, with which attenuation coefficients can be estimated. For this initial study, EM-IntraSPECT was tested on computer-simulated attenuation and emission maps representing a simplified human thorax as well as on SPECT data obtained from a physical phantom. Two evaluations were performed. First, to corroborate the idea of reconstructing attenuation parameters from emission data, attenuation parameters (mu) were estimated with the emission intensities (lambda) fixed at their true values. Accurate reconstructions of attenuation parameters were obtained. Second, emission parameters lambda and attenuation parameters mu were simultaneously estimated from the emission data alone. In this case there was crosstalk between estimates of lambda and mu and final estimates of lambda and mu depended on initial values. Estimates degraded significantly as the support extended out farther from the body, and an explanation for this is proposed. In the EM-IntraSPECT reconstructed attenuation images, the lungs, spine, and soft tissue were readily distinguished and had approximately correct shapes and sizes. As compared with standard EM reconstruction assuming a fix uniform attenuation map, EM-IntraSPECT provided more uniform estimates of cardiac activity in the physical phantom study and in the simulation study with tight support, but less uniform estimates with a broad support. The new EM algorithm derived here has additional applications, including reconstructing emission and transmission projection data under a unified statistical model.