Materials and Methods: Forty-seven patients were enrolled in this comparative study. Tc-99m-HMPAO brain SPET was performed. For semiquantitative analysis, two orbitomeatal (OM) composite slices were obtained. Regional cerebral perfusion data were semi-quantitatively computed by the ratio between each cerebral ROI mean-counts and the reference region mean-counts. Mean counts of the cerebellum, mean-counts of lower OM slice and contralateral ROI mean-counts were used as reference regions. Two different nuclear physicians evaluated all images. It was considered 0 (=hypoperfusion area); 1 (=normal perfusion area) and 2 (=hyperperfusion area) by visual evaluation. SPET slice’s region of interest (ROI) data were analyzed by receiver operator characteristics (ROC) method to establish the optimal decision threshold for different quantitative methods.

Results: Our results observed that usage of small ROIs were better than larger ones. The mean-counts of lower OM slice had a positive correlation with the cerebellum mean-count. Additionally, as reference regions, mean-counts of lower OM slice was slightly better than cerebellum and evidently better than symmetric region of the contralateral hemisphere.

Conclusion: We did not found the expected high correlation among visual analysis and these quantitative methods. We believe that new quantitative methods on the Tc-99m-HMPAO brain perfusion SPET are required. ©2006, Fırat Üniversitesi, Tıp Fakültesi

Standardization techniques in quantitative evaluation of the cerebral perfusion SPECT require a number of control cases to be matched to the clinical studies, allowing for evaluation of a patient scan by comparison of the results with a reference database for diagnostic accuracy in cerebral disorders. Knowledge of the normal distribution of the radiopharmaceuticals in the brain is mandatory when estimating pathological alterations in rCBF ².

For the planning of future quantization protocols, comparison of these methods are becoming ever more important to identify and characterize the “probable” changes in the regional cerebral blood flow (rCBF) distribution.

We aimed to compare different quantitative methods on Tc-99m-HMPAO SPET based on visual evaluation.

Radiopharmaceuticals and SPECT examination: The radiopharmaceuticals was prepared strictly according to the manufacture’s instructions. Radiochemical purity exceeded 85% at the time of injection. For brain SPECT perfusion imaging, 740 MBq Tc-99m-HMPAO (Frederic Joliot-Curie National Research Institute, Budapest, Hungary) was injected intravenously in a tranquil place with eyes closed and ears occluded and dimmed light after about 30 min rest, within 15 min after placement of an intravenous line. Patients were examined in a supine position with a head holder to avoid motion artifact. Imaging was initiated approximately between 20 min to 90 min after injection. SPECT brain imaging was performed using a two-headed gamma camera (Adac vertex plus V-60) equipped with a high resolution low energy collimator. The projection data were acquired for 25 s per projection at 60 equal angles of a complete revolution (0-360). Data were obtained from the 140 KeV photo peak (20% window) and a 64 x 64 matrix and zoom factor of 1.85. Reconstruction was performed by filtered back-projection using a Gaussian filter (cut off frequency 0.38 cycle/cm, order 20) with attenuation correction by the Chang method. Slice thickness of SPECT samples was 6.3 mm. After reconstruction, orbitomeatal (OM) transaxial, coronal and sagittal images were obtained.

For semi-quantitative analysis of neuroanatomical region of interest, two OM composite slices were obtained. Lower OM transaxial composite slice was obtained by summing up the three consecutive well-seen basal ganglia and thalamus slices. Upper OM transaxial composite slices were obtained by summing up the two consecutive slices and distance to lower OM transaxial composite slice was 44.1mm (Fig. 1- 3).

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Figure 1: Orbitomeatal (OM) slices, upper (U) and lower (L) orbitomeatal slice.

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Figure 2: Lower OM composite slice: 1 (4 and 5), frontal; 2 (7 and 8), temporal; 3 (10 and 11), occipital; 6, frontotemporal; 9, temporooccipital.

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Figure 3: Upper OM slice: 1, right hemisphere; 2, left hemisphere; 3 and 4, frontal lobe; 5 and 6, parietal lobe

Basal nucleus, thalamus and other cortical region’s of interest (ROI) were drawn manually on the slices.

For visual interpretation, two different nuclear physicians evaluated all images. The images were viewed on a color monitor using a spectrum scale of 255 colors (thermal) and gray scale. All OM slices and two composite OM slices were displayed. Two experienced nuclear medicine physicians who were unaware of the patient’s diagnosis visually assessed the SPET slices. Disagreement was resolved by discussion to reach a consensual interpretation. To facilitate determination of perfusion, all ROI’s activity uptake was compared with general cortical uptake. If ROI contained decreased or increased perfusion area, it was considered 0 (=hypoperfusion area) and 2 (=hyperperfusion area). If ROI did not contain 0 or 2, it was considered 1 (=normal perfusion area).

On the semiquantitative evaluation, all ROI’s were divided into two groups, large ROI’s group and small ROI’s group. Total frontal lobe, hemi-frontals, temporal lobes and hemi-occipitals on the lower OM composite slice (fig. 2) and right and left hemispheres on the upper OM composite slice (fig. 3) were drawn manually as large ROIs.

Semi-quantitative ROI count ratios were obtained by following formula:

                                          Mean-counts of ROI
ROI count ratio=-----------------------------------------------
                               Mean-count of reference region

Cerebellum and lower OM slice (as a whole brain) and symmetric ROI on the contralateral hemispheres were used as reference regions.

Mean-count of the symmetric ROI on the contralateral hemisphere were used as a reference region, (<1) ratios were standardized by (1/ratio) value on the ROC analysis and symmetric hyperperfusion were considered as a normal perfusion in the visual evaluation.

SPET slices region of interest (ROI) data were analyzed by receiver operator characteristics (ROC) method to: establish the optimal decision threshold for different quantitative methods. Receiver-operating characteristics (ROC) curves and the area under the ROC curve (Az) were calculated. A maximum value of (specificity + sensitivity) was considered as a cutoff value for each quantitative method. The method, which had larger areas under curve and smaller statistical significant p value, was considered better.

The Pearson correlation test was used to examine the relationship between cerebellum and lower OM slice meancounts.

The SPSS version 11.0 (SPSS Inc, Chicago, USA) was used for all statistical analyses. The level for statistical significance was set at p<0.05 for all test.

As a reference region, preference of the lower OM slice was slightly better than cerebellum and evidently better than symmetric ROI on the contralateral hemisphere for evaluation of hypoperfusion on the lower and upper OM slice (Table 1, 2).

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Table 1: Lower OM slice ROC analysis results (R=Right; L= left; LOSAM= mean-counts of lower OM slice mean-count; SM= cerebellum meancount and Az=Area under the curve).

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Table 2: Upper OM slice ROC analysis results (R=Right; L= left; LOSAM= mean-counts of lower OM slice mean-count; SM= cerebellum meancount and Az = Area under the curve).

The mean-count of lower OM slice had a positive correlation with the cerebellum mean-count (Pearson correlation test: r=0.874, P=0.001).

Symmetrical hyperperfusion was seen on 46 ROIs (8.3%) on the lower OM slice and on 23 ROIs (9.8%) on the upper OM slice. Symmetrical hypoperfusion was not seen.

Thalamic hyperperfusion and basal ganglia’s hypoperfusion was not determined with these quantitative methods (p>0.05).

Uptake counts of ROIs do not use direct quantification on Tc-99m-HMPAO cerebral perfusion SPET. ROI counts must be scaled to a reference region (mean-count of cerebellum, mean-count of whole brain or mean-count of the symmetric ROI on the contralateral hemisphere) ^3-5.

Soonawala et al reported that cerebellar normalization produces marginally more accurate diagnostic results in singlescan statistical parametric mapping analysis of Alzheimer-type disease patients than did mean-count of whole brain ⁵. In our study, we found that mean-count of lower OM slice was slightly better than mean-count of cerebellum as a reference region.

Another comparison methods was an asymmetry index, [(right ROI – left ROI) x 0,5/ (right ROI + left ROI)] x 100, was calculated on SPET slices ^2,6. Asymmetry index is similar to our third method.

It appears that the assessment of rCBF varies depending on the radiopharmaceuticals used, the mean age of the investigated groups and the employed methodology. In addition, rCBF analysis has been carried out with different manual, semi-automatic and automatic methods for outlining the regions of interest, and the choice of method has also been shown to effect the results ^2,7,8. One of the problems faced when attempting to properly map the CBF distribution is the large variety of regions under study hence the large number of anatomically defined functional variables obtained. The issue of how to deal with this statistical problem is still under discussion ⁹.

In same diseases, many studies showed that visually interpreted hypo or hyperperfusion areas were slightly or evidently different from statistically significant semiquantitative analysis areas. For example, Chiu et al reported that abnormal hypoperfusion pattern was seen in 22 of 27 (82%) patient who had Tourette’s syndrome. The involved area was the left frontal cortex (73%), left temporal cortex (50%), left basal ganglion (32%), right basal ganglion (9%), right temporal (4.5%) and left thalamus (4.5%). However, children with Tourette’s syndrome had only a significant decreased perfusion in the left anterior temporal cortex with semiquantitative analysis on the transverse images ³. This opposition between visual and semiquantitative evaluation may be related with complex organization of brain, hand dominance, hemispheric dominance, taken ROI areas or standardization methods, different somatotrophic reorganization or SPET acquisition and processing method (especially attenuation correction methods and back-projection filters).

Our study has three main limitations. First, visual assessment of abnormal cerebral uptake was often difficult in patients with mild reductions. Second, inter-observer variations may be larger in visual interpretation than quantitative evaluation. Three, as gold standard, quantization of the regional cerebral blood flow determined by PET can produce objective results and prevent these subjective errors.

Quantitative brain perfusion SPECT has been widely used for decrease of inter-observer variation and establishment of non visualized lesions. In our study, area under the ROC curve was evidently different among the quantization methods. Neither size of the ROIs nor preferred reference region was better in all brain regions for quantitative evaluation.

Conclusion: Our results established that using of the small ROIs was better than larger ones. The mean-counts of lower OM slice had a positive correlation with the cerebellum mean-count. Additionally, as reference regions, preference of lower OM slice was slightly better than cerebellum and evidently better than symmetric ROI on the contralateral hemisphere. Expected high correlation among visual analysis and the quantitative methods was not found. We believe that new quantization methods on the Tc-99m-HMPAO brain perfusion SPET are required.

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