Quantification of SPECT myocardial perfusion images: Methodology and validation of the Yale-CQ method

BACKGROUND: Quantification of single photon emission computed tomography (SPECT) images is important for reproducible and accurate image interpretation. In addition, SPECT quantification provides important prognostic information. The purpose of this study was to validate the Yale circumferential quantification (Yale-CQ) method in phantom studies. METHODS: Myocardial perfusion defects of varying extent and severities were simulated in a cardiac phantom with fillable defect inserts. Forty-five different phantom configurations simulated 45 different myocardial perfusion defect sizes, ranging from 1.6% to 32% of the cardiac phantom volume. Automatic processing was compared with manual processing in the phantom SPECT studies. RESULTS: The automatic Yale-CQ algorithm performed well in all phantom studies. Compared with manual processing, the mean absolute error for automatically determined center of short axis slices was 0.27 pixel in the x direction, 0.45 pixel in the y direction, and 0.15 pixel in radius. Quantification of phantom defects with the Yale-CQ method correlated well with actual defect sizes (R = 0.99), but there was a systematic underestimation (mean error = -7.9%). With derived correction factors the overall correlation between 45 phantom defects and actual defect sizes was excellent, and the estimation error was significantly improved (R = 0.98, mean error = -0.82% for manual method and -0.95% for automatic method). CONCLUSION: The automatic processing algorithm performs well for the phantom studies. Myocardial perfusion abnormalities can be quantified accurately by use of the Yale-CQ method. Quantified SPECT defect size can be expressed as a percentage of the left ventricle.     

Reproducibility of polar map generation and assessment of defect severity and extent assessment in myocardial perfusion imaging using positron emission tomography

The purpose of this study was to determine the reliability of new software developed for the analysis of cardiac tomographic data. The algorithm delineates the long axis and defines the basal plane and subsequently generates polar maps to quantitatively and reproducibly assess the size and severity of perfusion defects. The developed technique requires an initial manual estimate of the left ventricular long axis and calculates the volumetric maximum myocardial activity distribution. This surface is used to map three-dimensional tracer accumulation onto a two-dimensional representation (polar map), which is the basis for further processing. The spatial information is used to compute geometrical and mechanical properties of a solid model of the left ventricle including the left heart chamber. A new estimate of the axis is determined from this model, and the previously outlined procedure is repeated together with an automated definition of the valve plane until differences between the polar maps can be neglected. This quantitative analysis software was validated in phantom studies with defects of known masses and in ten data sets from normals and patients with coronary artery disease of various severity. We investigated the reproducibility of the maps with the introduction of a similarity criterion where the ratio of two corresponding polar map elements lies within a 10% interval. The maps were also used to measure intra–and interobserver variability in respect of defect size and severity. In the phantom studies, it was possible to reliably assess mass information over a wide range of defects from 5 to 60 g (slope: 1.02, offset –0.68, r = 0.972). Patient studies revealed a statistically significant increase in the reproducibility of the automatic technique compared with the manual approach: 54%±19% (manual) compared with 88%±9% (automatic) for observer 1 and 61%±20% vs 82%±5% for observer 2, respectively. The intervariability analysis showed a significant improvement from 59%±14% to 83%±7% in similar polar map elements and a significantly improved correlation in the calculation of severity (from r = 0.908 to 0.989) and extent (from r = 0.963 to r = 0.992) of the perfusion defects when the automated procedure was applied. It is concluded that, assuming a constant wall thickness and tissue density, absolute defect mass can be reliably estimated. Furthermore, the proposed software demonstrates a significant improvement in the generation of volumetric polar maps for the quantitative assessment of perfusion defects. Received 15 April and in revised form 27 May 1998     

Assessment of infarct size and severity by quantitative myocardial SPECT: results from a multicenter study using a cardiac phantom.

The aim of this study was to determine the reproducibility of measurements of the size and severity of myocardial defects from 99mTc sestamibi cardiac phantom studies performed on multiple different gamma camera systems. METHODS: A total of 250 gamma camera systems were evaluated over a 5-y period as part of the validation process of multiple multicenter trials. Each laboratory performed 9 acquisitions of a cardiac phantom. Small myocardial defects (0%-30% of myocardial mass) were placed in the inferobasal region, whereas larger defects (40%-70%) were located in the anterior wall. Five representative short-axis slices were analyzed to determine defect size and severity (i.e., contrast in defect region) using circumferential short-axis count profiles. Defect size and severity were analyzed as a function of the type of collimator, gamma camera system, and type of orbit (180degrees versus 360degrees). RESULTS: Of the 250 systems, image data were acquired correctly and showed an acceptable correlation between true and measured defect size in 198 systems. For these systems, the slope of the regression line between true and measured defect size was 1.03 +/- 0.03, with an average absolute error in estimating defect size of 1.7% +/- 0.5% and a correlation coefficient r = 0.99 +/- 0.01. Results were independent of the gamma camera system, type of collimator, and orbit. Contrast in the defect region (minimum count/maximum count) showed a small dependence on collimator resolution and pixel size but was altered significantly by the type of acquisition orbit, with a 360 degrees orbit showing better contrast for defects located in the inferobasal wall than a 180degrees orbit. CONCLUSION: Measurement of defect size is independent of the gamma camera system, type of collimator, and orbit. Contrast in small defects located in the inferobasal wall of the heart is affected significantly by the type of acquisition orbit but not by the type of collimator.     

Cardiac phantom measurement validating the methodology for a cardiac multi-centre trial with positron emission tomography

In an ongoing international multi-centre trial, positron emission tomography (PET) is being used to evaluate the effect of a new P-selectin antagonist on the infarct size in patients with acute myocardial infarction, treated with thrombolysis. Although it is possible to correct for site-dependent factors, it is desirable to reduce these factors to a minimum. Therefore, acquisition and reconstruction protocols have been defined that can be closely followed by all participating centres. The resulting reconstructed images are transferred to the core centre for central processing with semi-automatic software. This paper reports on the multi-centre phantom experiment that was carried out to assess the inter-centre reproducibility of defect size determination with this protocol. Also, the spatial resolution of the short axis slices was examined. In addition, the analysis procedure was applied to normal PET studies to evaluate the specificity of perfusion defect detection. The transmural cold defect in the phantom occupied 14.8% of the left ventricular area. The automated analysis was applied to the phantom measurements from the 14 participating PET cameras. It yielded an accurate estimate of 15.1% with a standard deviation of 0.6%, indicating excellent reproducibility. The spatial resolution in the short axis slices was similar for all PET systems: 9.6+/-0.8 mm. The same procedure produced a defect size of zero in the studies of normal volunteers. This study indicates that cardiac studies from multiple PET systems can be pooled for statistical analysis.     

Differential effect of 180 degrees and 360 degrees acquisition orbits on the accuracy of SPECT imaging: quantitative evaluation in phantoms.

A circular 180 degrees acquisition orbit is considered standard for cardiac SPECT imaging. Theoretically, a 360 degrees acquisition orbit is preferred because of more complete Fourier spectral information on projection data. The differential effect of 180 degrees and 360 degrees acquisition orbits on image accuracy (homogeneity and defect size) was assessed quantitatively in phantom studies. METHODS: SPECT imaging with a dual-head gamma camera was performed on normal cardiac phantoms filled with a (99m)Tc solution, using 180 degrees and 360 degrees circular acquisition orbits. The phantoms were placed in the center of the orbit and at 5, 10, and 15 cm off center. Fillable defect inserts of different sizes were placed in the phantom to simulate myocardial perfusion defects. The homogeneity of count distribution in short-axis slices of the normal phantom was analyzed as the percentage of variability. Defects were quantified as a percentage of the entire phantom volume using circumferential count profiles and normal reference profiles. RESULTS: When normal phantoms were placed in the center of the orbit, percentage variability was not different whether a 180 degrees or 360 degrees acquisition orbit was used (4.2% +/- 0.1% vs. 4.4% +/- 0.2%, P = not statistically significant). However when normal phantoms were placed off center, SPECT imaging with a 180 degrees acquisition orbit showed increasing inhomogeneity, both visually and quantitatively (e.g., percentage variability for the 15-cm off-center position was 10.8% +/- 0.1% (P < 0.0001). SPECT imaging with a 360 degrees acquisition orbit showed similar homogeneity visually and quantitatively, whether the phantom was placed in or off the center (e.g., percentage variability for the 15-cm off-center position was 4.6% +/- 0.5%, P = not statistically significant). Quantification of phantom defects acquired with a 180 degrees orbit showed increasing overestimation of defect sizes with increasingly off-center positions. Quantification of phantom defects acquired with a 360 degrees orbit showed no effect from progressively off-center positions, although phantom defect sizes were mildly underestimated. CONCLUSION: SPECT images acquired with a 180 degrees orbit may have significant erroneous inhomogeneity and overestimation of defect size, in particular when the target object is off the center of the orbit, as is commonly seen in clinical cardiac imaging. In contrast, SPECT images acquired with a 360 degrees orbit may provide more accurate quantitative information.     

Phantom studies for estimation of defect size on cardiac (18)F SPECT and PET: implications for myocardial viability assessment.

SPECT with (18)F-FDG has emerged as an alternative to dedicated PET for the assessment of myocardial viability. However, whether FDG SPECT can reliably quantify the extent of viable and scarred myocardium is uncertain. The aim of this study was to investigate whether SPECT with an (18)F-labeled agent would provide information on defect size similar to that provided by dedicated PET. METHODS: Imaging was performed using an elliptic cylinder chest phantom with simulated bone, lung, mediastinum, liver, and heart. (18)F was administered into the myocardium, mediastinum, right and left ventricular cavities, and liver. Plastic inserts (n = 11) ranging in size from 2% to 60% of the myocardium were used to simulate transmural myocardial infarctions. The chest phantom was imaged with a dedicated PET camera and with a double-head SPECT camera equipped with ultra-high-energy collimators. Both SPECT and PET data were analyzed using a semiquantitative polar map approach. Defects were quantified using various cutoff thresholds ranging from 30% to 80% of peak activity and were expressed as a percentage of the left ventricular myocardium. Defect size as measured by SPECT or PET was compared with true defect size. RESULTS: The measured SPECT defect size was highly variable depending on the cutoff used, whereas PET defect size was relatively constant over the range of cutoffs tested. The mean absolute difference between measured and true defect sizes was minimal at a cutoff of 50% of peak activity for both SPECT (3.3% +/- 3.3%) and PET (2.7% +/- 2.5%). For this threshold, both SPECT and PET measurements showed an excellent correlation with true defect size (r = 0.98 for SPECT and 0.99 for PET). The correlation between SPECT and PET measurements was also excellent (r = 0.99; P < 0.01). CONCLUSION: If an appropriate threshold is used to define a defect, SPECT with an (18)F-labeled agent can accurately measure defect size similarly to the manner of PET.     

Quantitative evaluation of acute myocardial infarction by In-111 antimyosin Fab myocardial imaging]

For quantitative evaluation of acute myocardial infarction, In-111 antimyosin Fab myocardial imaging (InAM) was performed in 17 patients with myocardial infarction who underwent Tl-201 (TL) and Tc-99m pyrophosphate (PYP) myocardial imaging in acute phase. For calculating the infarct size, voxel counter method was used for analysis in PYP and InAM, and extent and severity score were used on bull's-eye polar map in TL. The most appropriate cut-off level ranged from 65 to 80% by the fundamental experiment using cardiac phantom. The cut-off level of 0.70 (InAM) and 0.65 (PYP) were used for clinical application of voxel counter analysis. The infarct size calculated by InAM and PYP was compared with wall motion abnormality index by echocardiography (WMAI), TL extent score, TL severity score, peak CK and sigma CK. Infarct size by InAM showed the following correlations with other indices. PYP: r = 0.26 (ns), TL extent score: r = 0.72 (p less than 0.01), TL severity score: r = 0.65 (p less than 0.05), WMAI: r = 0.69 (p less than 0.05). The infarct size by PYP did not show any correlations with these indices. Therefore, the infarct size by InAM showed better correlations with TL and WMAI than that of PYP. So InAM was considered superior to PYP for quantitative evaluation of acute myocardial infarction.     

Single photon tomographic imaging of a standard heart phantom with 201Tl: A gamma camera based system

A standard heart phantom (University of lowa design), including discrete myocardial walls, a central blood pool, and a 24-cc transmural cold defect, was studied with both planar and transverse tomographic imaging. The heart phantom was filled with ²⁰¹Tl and placed within a cylindrical tank containing water and ²⁰¹Tl to simulate nonmyocardial background activity from the thorax. The tomographic imaging system used was a commercially available, rotating, large field-of-view gamma camera. Image reconstruction from 64 sampling angles was performed in a nuclear medicine minicomputer system. The percentage activity in the region of the defect (actual activity of 0) contrasted to the normal wall was compared between planar and 1.25-cm transaxial tomographic slices. Defect activity fell to between 65% and 85% of that of the opposing normal wall in planar images, whereas it fell to between 26% and 49% of that of the normal wall in the tomographic images. In most cases, tomographic defect activity was half or less than that in the planar image. The geographic extent of the defect was seen in an appropriate number of tomographic slices; i.e., the geographic 3.2-cm defect length was predominantly seen in three 1.250cm transverse slices. We conclude that camera-based tomographic systems show promise for improved ²⁰¹Tl myocardial defect detection and quantitation over conventional planar images.Supported in part by the Medical Research Service of the Veterans Administration.     

Validation of threshold method for myocardial control database by use of clinical data

A database is an important factor in the statistical analysis of myocardial scintigraphy. Our aim in this study was to verify the validity of the threshold method using phantoms and to create a clinical database using this method. Since this method involves artificially excluding a low count area on a polar map, we created a myocardial phantom with defects. Then, we applied this method to the construction of a control database (CDB) for which we used stress–rest scans of 152 male and 52 female Japanese patients. The clinical relevance of this database was investigated by comparison of the values between the CDB and a Japanese normal database. In the study evaluation, we mainly used the summed extent score (SES) and a severity map (severity). Data from the phantom with defects demonstrated that the threshold method could compensate for defective areas, enabling the use of data for the creation of the CDB. Comparison of the CDB with the Japanese normal database showed a good relationship with respect to the SES and severity (Initial post-stress: SES: r = 0.978; severity: r = 0.997, Redistribution: SES: r = 0.944; severity: r = 0.993). The threshold method facilitates the effective creation of a database by use of clinical data. This enables individual institutions to build their own databases, taking into account differences in collection and processing conditions between institutions as well as the characteristics of individual equipment.     

Quantitative analysis of myocardial ischemia by technetium-99m sestamibi exercise scintigraphy: A new method for change rate mapping

In order to quantitatively assess the extent and severity of myocardial ischemia by Tc-99m sestamibi exercise myocardial scintigraphy, we developed a new method of change rate (CR) mapping and examined its efficiency. CR was calculated to divide the counts per pixel in the stress polar map by that in the rest polar map at each corresponding pixel. The CR map showed the CR values at each pixel. To correct the differences between the stress and rest images for the dose of Tc-99m sestamibi administered, the mean counts per pixel in the stress polar map and the rest map were adjusted to the same level. Regarding the regions in which the CR value was less than 1 as ischemia, we compared the abilities of the CR map and the polar map to detect coronary artery stenosis in 5 patients with angina pectoris. The sensitivity for coronary artery stenosis was 80% in the CR map, and 40% in the polar map. The specificity for both was 75%. We concluded that the CR map was effective in assessing the extent and severity of myocardial ischemia in Tc-99m sestamibi exercise myocardial scintigraphy.     

The influence of gating on measurements of myocardium at risk and infarct size during acute myocardial infarction by tomographic technetium 99m-labeled sestamibi imaging

Background

Serial perfusion imaging with ^99mTc-labeled sestamibi has been useful in the assessment of myocardial salvage from reperfusion therapy during acute myocardial infarction. Studies in animal models have shown that discernible perfusion defects can be created by left ventricular asynergy from partial volume effects in the setting of homogenous perfusion tracer distribution. The purpose of this study was to examine the influence of gating on serial perfusion images during acute myocardial infarction to determine the magnitude of potential partial volume effects.

Methods and Results

^99mTc-labeled sestamibi was injected into 18 patients during acute myocardial infarction and 29 patients 5 to 8 days after myocardial infarction. Tomographic imaging was acquired in gated format (16 frames per R-R cycle of the electrocardiogram) for each set of images. All frames were summed to produce ungated images. Tomographic images were quantified on three different thresholds to define the perfusion defect: 50%, 60%, and 70% of maximal counts. Severity of perfusion defects was calculated as the lowest ratio of minimum/maximum counts on five short-axis slices. Regional wall motion was assessed subjectively on the gated images by cine-loop display. Radionuclide ventriculography was performed at 6 weeks. There was a close correlation between perfusion defect size on ungated images and end-diastolic and end-systolic images independent of the quantitative threshold used (r=0.90 to 0.93; p<0.0001 for all correlations). Gated images provided consistently significantly greater estimates of perfusion defect size and severity by a small increment (3% to 9% of the left ventricle; p<0.05 for all comparisons) independently of the quantitative threshold used or the time of imaging (acute or late). Ungated images provided slightly better correlations with left ventricular ejection fraction at 6 weeks independently of the quantitative threshold used and despite significant wall motion abnormalities present on both the acute and final studies.

Conclusions

The differences between perfusion defect size for gated and ungated images were highly significant as a group, but the magnitude of difference was small and not clinically relevant. The larger estimates provided by end-diastolic gated images are opposite the difference expected if partial volume effects were significantly influencing perfusion defect size. Partial volume effects appear to have minimal impact on clinical tomographic imaging during acute myocardial infarction for the quantification of myocardium at risk and infarct size.     

Interdependence between measures of extent and severity of myocardial perfusion defects provided by automatic quantification programs

AIM: To evaluate the accuracy of the values of lesion extent and severity provided by the two automatic quantification programs AutoQUANT and 4D-MSPECT using myocardial perfusion images generated by Monte Carlo simulation of a digital phantom. The combination between a realistic computer phantom and an accurate scintillation camera simulation tool allows the generation of realistic single-photon emission computed tomography (SPECT) images similar to those obtained in clinical patient studies. METHODS: The NCAT phantom and the SIMIND Monte Carlo program were used to simulate myocardial perfusion studies. Perfusion defects with sizes ranging from 5 to 17% of the left ventricular wall volume and reductions in tracer uptake of 20, 60 and 100% were simulated in three vascular territories. RESULTS: The values of the extent provided by the programs were dependent on the reduction in tracer uptake, i.e. the severity. Similarly, the measures of severity were dependent on the size of the lesions. The severity provided by AutoQUANT for different defects was not dependent on the location, whereas 4D-MSPECT presented different values depending on the location in the left ventricle. The measures of extent and severity of the defects with the same true extent and activity uptake reduction provided by the two programs were different. CONCLUSIONS: The NCAT phantom and the SIMIND Monte Carlo program were shown to be useful in simulating clinical myocardial SPECT studies. The quantification programs gave values of lesion extent that were dependent on the magnitude of the severity. Users should therefore consider this dependence when interpreting results from these programs.     

体位改变对SPECT心肌显像中左室下壁衰减校正的价值

目的 探讨右侧卧位（RL）和俯卧位采集对^99Tc^m－甲氧基异丁基异腈（MIBI）心肌显像左室下壁衰减校正的价值。方法 对31例正常者进行了仰卧位、RL和俯卧位^99Tc^m-MIBI静息心肌显像。在靶心图上测定各区域像素平均计数,在断面图上对下壁显示断层形态和扫描过程中位移情况进行分级,在不同体位间进行比较。结果 （1）仰卧位有58．1％的下壁显示不全,体位变化后88．9％有不同程度的改善。（2）俯卧位和RL对于下壁的形态显示和计数均优于仰卧位。（3）各种体位间位移差异无显著性。结论 俯卧位和RL可使下壁衰减得以校正,计数和断层明显改善,且前者更好。     

Optimum threshold values of 201Tl SPET in the determination of the left ventricular border and extent of myocardial infarction.

The aim of this study was to determine the optimum threshold value of the left ventricular border and the extent of myocardial infarction using quantitative 201Tl single photon emission tomography (SPET). We used the unfolded map method to determine the size of the left ventricle and the extent of myocardial infarction in 10 patients, because it has been shown to be superior to the conventional polar map method. The relative differences in the size of the left ventricle and extent of infarction between 201Tl SPET and post-mortem examination were calculated. The optimum threshold value was determined when the relative difference = 0%. There was an excellent correlation between scintigraphic and post-mortem left ventricular size at a threshold value of 53% (r = 0.91, P < 0.001); an excellent correlation was also observed between scintigraphic and post-mortem infarct size at a threshold value of 55% (r = 0.93, P < 0.03). The optimum threshold value in determining left ventricular size using 201Tl SPET is 53% and that in determining infarct size is 55%.     

Tomographic and planar quantitation of perfusion defects on technetium 99m-labeled sestamibi scans: Evaluation in patients treated with thrombolytic therapy for acute myocardial infarction

Background

Our segmentation algorithm for single-photon emission computed tomographic perfusion studies was, tested in 244 patients treated by thrombolysis within 5 hours after onset of symptoms. This algorithm, uses radial slices to approximate true three-dimensional gradients, determines the apex and basal plane, and creates a perfusion and volume polar map.

Methods and Results

Perfusion defect size was compared with enzymatic infarct size and global and regional function. All patients underwent rest planar and tomographic ^99mTc-labeled sestamibi scanning, contrast coronary angiography, and ventriculography 10 to 14 days after the start of treatment. Manual correction had to be performed in only 10% of the cases and presented no problems. The correlation coefficients (r) between planar and relative tomographic perfusion defects versus enzymatic infarct size were 0.71 and 0.73. A negative correlation was found with left ventricular ejection fraction: r=-0.65 and r=-0.60. A comparable correlation was also found between regional wall motion and perfusion defect size. Most correlations were higher in the case of anterior infarction. An excellent correlation was found between planar and tomographic defect size (r=0.83).

Conclusions

In most cases, our segmentation algorithm delineates myocardial edges and basal plane automatically. A good correlation was found between perfusion defect size, enzymatic infarct size, and global and regional ventricular function.     

CT-based SPECT attenuation correction and assessment of infarct size: results from a cardiac phantom study

Rationale

Myocardial perfusion SPECT is a commonly performed, well established, clinically useful procedure for the management of patients with coronary artery disease. However, the attenuation of photons from myocardium impacts the quantification of infarct sizes. CT-Attenuation Correction (AC) potentially resolves this problem. This contention was investigated by analyzing various parameters for infarct size delineation in a cardiac phantom model.

Methods

A thorax phantom with a left ventricle (LV), fillable defects, lungs, spine and liver was used. The defects were combined to simulate 6 infarct sizes (5–20% LV). The LV walls were filled with 100120 kBq/ml ^99mTc and the liver with 10–12 kBq/ml ^99mTc. The defects were filled with water of 50% LV activity to simulate transmural and non-transmural infarction, respectively. Imaging of the phantom was repeated for each configuration in a SPECT/CT system. The defects were positioned in the anterior as well as in the inferior wall. Data were acquired in two modes: 32 views, 30 s/view, 180° and 64 views, 15 s/view, 360° orbit. Images were reconstructed iteratively with scatter correction and resolution recovery. Polar maps were generated and defect sizes were calculated with variable thresholds (40–60%, in 5% steps). The threshold yielding the best correlation and the lowest mean deviation from the true extents was considered optimal.

Results

AC data showed accurate estimation of transmural defect extents with an optimal threshold of 50% [non attenuation correction (NAC): 40%]. For the simulation of non-transmural defects, a threshold of 55% for AC was found to yield the best results (NAC: 45%). The variability in defect size due to the location (anterior versus inferior) of the defect was reduced by 50% when using AC data indicating the benefit from using AC. No difference in the optimal threshold was observed between the different orbits.

Conclusion

Cardiac SPECT/CT shows an improved capability for quantitative defect size assessment in phantom studies due to the positive effects of attenuation correction.

     

Apical thinning: real or artefact?

BACKGROUND AND OBJECTIVE: Apical thinning is a well-known phenomenon in myocardial perfusion SPECT, often attributed to reduced myocardial thickness at the apex of the left ventricle. Attenuation correction processing appears to exaggerate this effect. Although currently there is agreement that reduced apical counts are not a diagnostic indicator, opinions differ over the cause of this effect; we sought to clarify this using results from a phantom study. METHODS: A commercially available anthropomorphic torso phantom was expanded using attachments mimicking tissue and bone to create three phantoms of increasing size. These were imaged using a dual-headed gamma camera and low-dose CT-based attenuation correction. The data were processed using iterative reconstruction, with and without attenuation correction. RESULTS: The cardiac insert had a uniform wall thickness and yet defects characteristic of apical thinning appeared after attenuation correction, increasing in severity with phantom size. Before attenuation correction, a flare of activity was seen at the apex corresponding in position and size to the defect after attenuation correction. Further investigations showed the following: depth-dependent resolution was not responsible; the severity of the defect was more noticeably dependent on the addition of breast activity than the addition of attenuating material; the artefact was not unique to one particular algorithm; increasing the number of iterations reduced the severity of the artefact. CONCLUSION: Data acquisition and processing methods are thought to be responsible for the apparent apical defect. This phantom study therefore demonstrates that apical thinning is not simply an anatomical feature but can also be an artefact introduced by the use of attenuation correction.     

Evaluation of a multicrystal gamma camera and rotating chair for tomographic studies of the heart

Background

This study evaluates the feasibility of performing tomographic studies with a multicrystal gamma camera combined with a rotating chair.

Methods and Results

Tomographic acquisitions were performed with a cardiac phantom containing eight defects of different sizes. Defect size was determined from the fraction of counts in the short-axis slices that fell below a fixed threshold value. Image contrast was determined from the ratio of minimum/maximum counts. Images of an American College of Nuclear Physicians cardiac single-photon emission computed tomographic phantom were acquired and the results were compared with those obtained from 194 centers in the United States. For cardiac studies with ²⁰¹TI and ^99mTc, threshold values of 65% to 70% gave the best correlation (R ²>0.94) between true and measured defect sizes, although the slope of the regression line was less than 0.95 for both isotopes. Small inferior defects demonstrated poor image contrast, particularly for ^99mTc. Of the three defects in the American College of Nuclear Physicians phantom, the two largest were identified in the tomographic images.

Conclusions

A multicrystal gamma camera system coupled with a rotating chair can be used for tomographic studies of the heart. Image quality is poorer than that seen on conventional single-photon emission computed tomographic systems, particularly for ^99mTc.     

Estimation of thyroid mass in Graves' disease by a scintigraphic method

scintigraphic method for the estimation of the thyroid mass in patients with Graves' disease is described. The method was first standardized using thyroid phantoms with eight different volumes ranging from 5 to 110 cm(3). The planar and single photon emission computed tomography (SPECT) images of each phantom were acquired with four different activities [3.7 MBq (100 microCi), 11.1 MBq (300 microCi), 22.2 MBq (600 microCi) and 37 MBq (1.0 mCi) of 99mTc-pertechnetate] with a 20% window symmetrically placed over the photopeak of 99mTc. The thyroid lobes were enclosed with the help of regions of interest (ROI) tools and a threshold was selected to identify the thyroid boundaries. The same threshold was used in all slices of an image. In the phantom study, a 20% threshold for planar images and a 30% threshold for SPECT were found to be optimum for measuring the thyroid volume. The volume from planar images was calculated by the formula described by Allen and Goodwin (The scintillation counter as an instrument for in-vivo determination of thyroid weight. Radiology 1952; 58: 68-79), whereas, in SPECT images, the sum of the slice areas was multiplied by the slice thickness. The estimated volume of each phantom was compared and correlated with its actual volume. After standardization of the technique with phantom studies, planar scintigraphy (with 20% threshold) in 51 patients and SPECT (with 30% and 35% threshold) in 40 patients with Graves' disease were performed to estimate the thyroid size. The thyroid size was also estimated by ultrasonography, which showed good agreement with the scintigraphic method, particularly with SPECT.     

A comparison of adenosine and arbutamine for myocardial perfusion imaging

We have compared our standard stress protocol (adenosine combined with exercise) with the new stress agent arbutamine, for thallium-201 myocardial perfusion imaging (MPI) in order to assess the comparative value of arbutamine. We studied 23 patients referred for MPI, and each patient had two studies (18 males, median age 66 years, five with previous myocardial infarction). Uptake scores were assigned to each of nine segments, and the extent and severity of defects were measured using a polar plot. Haemodynamic changes were greater with arbutamine (rate-pressure product increase 78% vs 51%, P = 0.003). Symptoms were experienced by 21 patients with arbutamine and 16 with adenosine (P = 0.07). Agreement between the techniques for classification of patients as normal or as having reversible, fixed or mixed defects was good (19 of 23 studies, 83%, κ = 0.76). Agreement for similar classification of segments was also good (82%, κ = 0.71). Segmental agreement for stress scores was good (86%, κ = 0.77). However, mean size of stress defect was larger with adenosine (83±52 pixels vs 65±48 pixels, P<0.05), though severity and reversibility were similar (P = NS). We conclude that arbutamine provides comparable results to those obtained with adenosine and exercise and that the observed differences are not clinically significant. Received 6 October and in revised form 23 December 1997