Effects of scatter correction on regional distribution of cerebral blood flow using I-123-IMP and SPECT

Optimization of the Attenuation Coefficient for Chang Attenuation Correction in 123I Brain Perfusion SPECT

N-isopropyl-p-¹²³I-iodoamphetamine brain perfusion SPECT has been used with various attenuation coefficients (μ-values); however, optimization is required. This study aimed to determine the optimal μ-value (μ_opt-value) for Chang attenuation correction (AC) using clinical data by comparing the Chang method and CT-based AC. Methods: We used 100 patients (reference group, 60; disease group, 40) who underwent N-isopropyl-p-¹²³I-iodoamphetamine SPECT. SPECT images of the reference group were obtained to calculate the AC using the Chang method (μ-values, 0.07-0.20; 0.005 interval) and the CT-based method, both without scatter correction (SC) and with SC. The μ_opt-value with the smallest mean percentage error for the brain regions of the reference group was calculated. Agreement between the Chang and CT-based methods applying the μ_opt-value was evaluated using Bland-Altman analysis. Additionally, the percentage error in the region of hypoperfusion in the diseased group was compared with the percentage error in the same region in the reference group when the μ_opt-value was applied. Results: The μ_opt-values were 0.140 for Chang without SC and 0.160 for Chang with SC. In the Chang method, with the μ_opt-value applied, fixed and proportional biases were observed in the Bland-Altman analysis (both P < 0.05), and there was a tendency for the percentage error to be underestimated in the limbic regions and overestimated in the central brain regions. There was no significant difference between the disease group and the reference group in the region of hypoperfusion in either Chang without SC or Chang with SC. Conclusion: The present study revealed that the μ_opt-values of the Chang method are 0.140 without SC and 0.160 with SC.

[Effect of Reconstruction Strategies for the Quantification and Diagnostic Accuracy of (123)I-FP-CIT SPECT]

PURPOSE

This study evaluates the effect of reconstruction strategies for the quantification and diagnostic accuracy of (123)I-FP-CIT SPECT.

METHODS

We evaluated the quantification of (123)I-FP-CIT SPECT obtained by several combinations of reconstruction using the striatal phantom. The phantom images were reconstructed using FBP and OSEM with/without attenuation correction (AC) and scatter correction (SC). We calculated the specific binding ratio (SBR) using volume of interest (VOI) analysis on each reconstructed images. For the clinical study, 40 patients who underwent (123)I-FP-CIT SPECT were selected. We grouped the patients into the normal binding group and decreased binding group according to their clinical diagnosis. The clinical images were reconstructed under the same conditions as the phantom study. The SBRs were calculated, and a receiver operating characteristic (ROC) analysis was performed to evaluate the diagnostic accuracy.

RESULTS

The SBRs with AC and SC significantly increased compared with no corrections. In the clinical study, although ROC analysis showed no significant difference in the all combinations of reconstruction, the area under the curve using SC and AC tended to be higher than that obtained by other reconstruction.

CONCLUSIONS

Quantification of (123)I-FP-CIT SPECT was affected by reconstruction strategies. In addition, both the AC and SC improved the diagnostic accuracy of (123)I-FP-CIT SPECT. Our results suggest that both the AC and SC are recommended for the improving the quantification and diagnostic accuracy in (123)I-FP-CIT SPECT.

Experimental determination of the weighting factor for the energy window subtraction-based downscatter correction for I-123 in brain SPECT studies

Correction for downscatter in I-123 SPECT can be performed by the subtraction of a secondary energy window from the main window, as in the triple-energy window method. This is potentially noise sensitive. For studies with limited amount of counts (e.g. dynamic studies), a broad subtraction window with identical width is preferred. This secondary window needs to be weighted with a factor higher than one, due to a broad backscatter peak from high-energy photons appearing at 172 keV. Spatial dependency and the numerical value of this weighting factor and the image contrast improvement of this correction were investigated in this study. Energy windows with a width of 32 keV were centered at 159 keV and 200 keV. The weighting factor was measured both with an I-123 point source and in a dopamine transporter brain SPECT study in 10 human subjects (5 healthy subjects and 5 patients) by minimizing the background outside the head. Weighting factors ranged from 1.11 to 1.13 for the point source and from 1.16 to 1.18 for human subjects. Point source measurements revealed no position dependence. After correction, the measured specific binding ratio (image contrast) increased significantly for healthy subjects, typically by more than 20%, while the background counts outside of all subjects were effectively removed. A weighting factor of 1.1–1.2 can be applied in clinical practice. This correction effectively removes downscatter and significantly improves image contrast inside the brain.

Regional cerebral blood flow in healthy volunteers measured by the graph plot method with iodoamphetamine SPECT

PURPOSE

The graph plot method, a technique that uses N-isopropyl-(123)I-p-iodoamphetamine (IMP) and single photon emission computed tomography (SPECT) for non-invasive measurement of regional cerebral blood flow (CBF), has been developed and applied in the clinical setting, although it has been performed without obtaining normal CBF values in normal, healthy subjects. The aim of this study was to measure normal regional CBF in older healthy subjects with IMP SPECT and the graph plot method.

SUBJECTS AND METHODS

Eleven healthy volunteers (mean age: 63.5 ± 8.9 years; six males and five females) were recruited and regional CBF was measured using IMP SPECT and the graph plot method.

RESULTS

The averaged global CBF was 45.4 ml/100 g/min. The distribution of regional CBF was almost homogenous in the cortices. There was no significant correlation between the global CBF and age in subjects aged 50-80 years.

CONCLUSION

We used the IMP graph plot method to measure regional CBF in normal healthy subjects, without arterial blood sampling, and obtained compatible CBF values. This method is non-invasive and convenient for determination of regional CBF in the clinical setting.

I-123 MIBG cardiac uptake measurements: limitations of collimator choice and scatter correction in the clinical context

OBJECTIVE

Low uptake of metaiodobenzylguanidine (MIBG) in patients with heart failure generally indicates poor prognosis. Our objective was to determine the best method for calculating I-123 MIBG uptake. MIBG uptake as a percentage of the injected dose is presented as an alternative method for serial assessment.

METHODS

Patients with chronic heart failure were imaged with I-123 MIBG with both a medium-energy (ME) collimator and a low-energy high-sensitivity (LEHS) collimator. Scatter correction was used to correct the LEHS images. Heart-to-mediastinal (H/M) ratio and the percentage of myocardial uptake of MIBG were obtained.

RESULTS

Mean H/M ratios for the ME images, LEHS images and scatter-corrected LEHS images were 2.45+/-0.61, 2.22+/-0.47 and 2.51+/-0.62, respectively. Mean H/M ratio was significantly different among all the three sets (P<0.001) of images. The average difference in H/M ratios between the ME images and LEHS images was lower when scatter correction was applied (4.95% vs. 9.79%). The error in calculating the myocardial uptake as a percentage of the injected dose was significantly lower than the error in calculating H/M ratio (0.2 vs.10.2% LEHS; 0.3 vs.16.0% ME; 0.2 vs.11.8% LEHS scatter corrected).

CONCLUSION

For quantitative assessment of H/M ratio in I-123 MIBG imaging a LEHS collimator can be used in place of a ME collimator to achieve better counting statistics, but scatter correction must be used. The calculation of the myocardial uptake as a percentage of the injected dose has potential as an alternative method of measurement, particularly for serial assessment.

Database of normal human cerebral blood flow measured by SPECT: I. Comparison between I-123-IMP, Tc-99m-HMPAO, and Tc-99m-ECD as referred with O-15 labeled water PET and voxel-based morphometry

OBJECTIVES

Three accumulative tracers, iodine-123-labeled N-isopropyl-p-iodoamphetamine (I-123-IMP), technetium-99m-labeled hexamethylpropyleneamineoxime (Tc-99m-HMPAO), and technetium-99m-labeled ethyl cysteinate dimer (Tc-99m-ECD) are widely used to measure cerebral blood flow (CBF) in single-photon emission computed tomography (SPECT). In the present study, normal regional distribution of CBF measured with three different SPECT tracers was entered into a database and compared with regional distribution of CBF measured by positron emission tomography (PET) with H2(15)O. The regional distribution of tissue fractions of gray matter determined by voxel-based morphometry was also compared with SPECT and PET CBF distributions.

METHODS

SPECT studies with I-123-IMP, Tc-99m-HMPAO, and Tc-99m-ECD were performed on 11, 20, and 17 healthy subjects, respectively. PET studies were performed on 11 healthy subjects. Magnetic resonance (MR) imaging studies for voxel-based morphometry were performed on 43 of the 48 subjects who underwent SPECT study. All SPECT, PET, and MR images were transformed into the standard brain format with the SPM2 system. The voxel values of each SPECT and PET image were globally normalized to 50 ml/100 ml/min. Gray matter, white matter, and cerebrospinal fluid images were segmented and extracted from all transformed MR images by applying voxel-based morphometry methods with the SPM2 system.

RESULTS

Regional distribution of all three SPECT tracers differed from that of H2150 in the pons, midbrain, thalamus, putamen, parahippocampal gyrus, posterior cingulate gyrus, temporal cortex, and occipital cortex. No significant correlations were observed between the tissue fraction of gray matter and CBF with any tracer.

CONCLUSION

Differences in regional distribution of SPECT tracers were considered to be caused mainly by differences in the mechanism of retention of tracers in the brain. Regional distribution of CBF was independent of regional distribution of gray matter fractions, and consequently the blood flow per gray matter volume differed for each brain region.

Influence of photon scattering and attenuation on ROI analysis in brain perfusion single-photon emission tomographic imaging of normal subjects

OBJECTIVE

The aim of this study was to evaluate the effect of scatter and attenuation correction in region of interest (ROI) analysis in brain perfusion single-photon emission tomography (SPECT) and to assess the influence of selecting the reference area on semi-quantification.

METHODS

Ten normal subjects were enrolled and injected with 123I-iodoamphetamine to undergo simultaneous emission and transmission scanning for scatter and attenuation correction. We reconstructed three SPECT images from common projection data of each subject: with scatter correction and non-uniform attenuation correction, with scatter correction and uniform attenuation correction, and with uniform attenuation correction applied to data without scatter correction. A program for automated ROI drawing was used to set ROIs on various regions in brain images. Regional count ratios were compared in images with different correction procedures by using three different reference areas.

RESULTS

The effect of the combination of scatter and attenuation correction was marked in the precentral, temporal, posterior, hippocampus and especially in the cerebellum. In contrast, it was not appreciable in the central and parietal areas. When using the cerebellar ROI as the reference, the count ratio varied widely depending on the correction procedures. On the other hand, the whole brain reference offered the least variation in the count ratio.

CONCLUSIONS

The influence of photon scattering and attenuation was dependent on regions. Since the count in the cerebellar ROI is greatly affected by photon scattering and attenuation, nonuniform attenuation correction combined with scatter correction deserves consideration when using the cerebellar ROI as the reference.

Effect of scatter correction on the compartmental measurement of striatal and extrastriatal dopamine D2 receptors using [123I]epidepride SPET

Prior studies with anthropomorphic phantoms and single, static in vivo brain images have demonstrated that scatter correction significantly improves the accuracy of regional quantitation of single-photon emission tomography (SPET) brain images. Since the regional distribution of activity changes following a bolus injection of a typical neuroreceptor ligand, we examined the effect of scatter correction on the compartmental modeling of serial dynamic images of striatal and extrastriatal dopamine D(2) receptors using [(123)I]epidepride. Eight healthy human subjects [age 30+/-8 (range 22-46) years] participated in a study with a bolus injection of 373+/-12 (354-389) MBq [(123)I]epidepride and data acquisition over a period of 14 h. A transmission scan was obtained in each study for attenuation and scatter correction. Distribution volumes were calculated by means of compartmental nonlinear least-squares analysis using metabolite-corrected arterial input function and brain data processed with scatter correction using narrow-beam geometry micro (SC) and without scatter correction using broad-beam micro (NoSC). Effects of SC were markedly different among brain regions. SC increased activities in the putamen and thalamus after 1-1.5 h while it decreased activity during the entire experiment in the temporal cortex and cerebellum. Compared with NoSC, SC significantly increased specific distribution volume in the putamen (58%, P=0.0001) and thalamus (23%, P=0.0297). Compared with NoSC, SC made regional distribution of the specific distribution volume closer to that of [(18)F]fallypride. It is concluded that SC is required for accurate quantification of distribution volumes of receptor ligands in SPET studies.

Scatter correction by two-window method standardizes cardiac I-123 MIBG uptake in various gamma camera systems

Heart to mediastinum count ratio (H/M) has been commonly utilized as an indicator of myocardial I-123 MIBG uptake. However, normal ranges of H/M were markedly different among various gamma camera systems. The purpose of this study was to clarify whether scatter correction by two-window method standardizes H/M among various gamma camera systems.

METHODS

Scatter uncorrected and corrected MIBG imaging was acquired in phantom and human studies in combination with low energy high-resolution collimator (LEHR) and medium energy collimator (MEC). For scatter correction, energy window width of 159 keV +/- 10% was applied to main window imaging and 193 keV +/- 9.5% was applied to upper window imaging for scatter correction.

RESULTS

In phantom study, a significant difference was observed in uncorrected H/M among three gamma camera systems using LEHR or MEC (2.09 +/- 0.06 vs. 2.58 +/- 0.03 in GCA7200 camera, 2.00 +/- 0.07 vs. 2.42 +/- 0.06 in DS7 camera and 2.16 +/- 0.04 vs. 2.67 +/- 0.07 in Vertex plus camera). However, there was no significant difference in corrected H/M among the three gamma camera systems, either with LEHR or MEC (2.70 +/- 0.07 vs. 2.69 +/- 0.07 in GCA7200 camera, 2.66 +/- 0.08 vs. 2.61 +/- 0.05 in DS7 camera and 2.66 +/- 0.05 vs. 2.61 +/- 0.05 in Vertex plus camera). In human study, uncorrected H/M in DS7 camera with LEHC was significantly lower than that in GCA7200 camera with MEC (1.60 +/- 0.37 vs. 1.85 +/- 0.54, N = 14). In contrast, the difference was insignificant in corrected H/M (2.12 +/- 0.59 vs. 2.16 +/- 0.68). There was a very excellent correlation in corrected H/M between DS7 and GCA7200 cameras (r = 0.991, p < 0.001).

CONCLUSION

This study demonstrated that scatter correction by the two-window method standardizes the H/M in MIBG scintigraphy either with LEHR or MEC. Scatter corrected H/M can be applied to measure a standardized parameter of MIBG uptake in human clinical studies using various gamma camera systems.