Kei Kikuchi

Acquisition Protocols and Correction Methods for Estimation of the Heart-to-Mediastinum Ratio in 123I-Metaiodobenzylguanidine Cardiac Sympathetic Imaging

Septal penetration of high-energy photons affects quantitative results in imaging of 123I-labeled tracers. We investigated acquisition protocols (collimator choice and energy window setting) and correction methods for estimating the heart-to-mediastinum (H/M) ratio in cardiac 123I-metaiodobenzylguanidine (MIBG) imaging. Methods: Four hours after 123I-MIBG injection, 40 patients successively underwent planar anterior chest imaging with the medium-energy (ME) (ME method) and low-energy high-resolution (LEHR) (LEHR method) collimators. A 20% energy window was used for both collimators. Another 40 patients were imaged successively with the ME collimator and a 20% window (ME method), the low-medium-energy (LME) collimator and a 20% window (LME20 method), and the LME collimator and a 15% window (LME15 method). The H/M ratios obtained by the LEHR, LME20, and LME15 methods were corrected using their correlations with the H/M ratio obtained by the ME method (empiric correction). The 123I-dual-window (IDW) correction was also applied to remove the influence of high-energy photons. Results: Without correction, severe underestimation of the H/M ratio was shown for the LEHR method using the ME method as a standard, and this underestimation increased with increasing H/M ratios. Underestimation substantially decreased using the LME20 method and further using the LME15 method. Empiric correction reduced the error in the H/M ratio by the LEHR method, but the error was still evident. After empiric correction, the H/M ratios with the LME collimator were comparable to those with the ME collimator. The IDW correction only partially reduced underestimation by the LEHR method and caused a small overestimation for the LME15 method. Conclusion: The use of an LME collimator appears to be acceptable for cardiac 123I-MIBG imaging as an alternative to an ME collimator, and the application of a 15% energy window is recommended when an LME collimator is used. Empiric correction is also expected to improve exchangeability between H/M ratios calculated with ME and LME collimators. Neither the use of an LEHR collimator nor the use of IDW correction is recommended.

Interictal crossed cerebellar hyperperfusion on Tc-99m ECD SPECT

Crossed cerebellar hyperperfusion (CCH) in epilepsy is a rare condition that is observed on ictal cerebral perfusion SPECT. The mechanism of CCH assumes that hyperperfusion in the epileptic foci of the unilateral supratentorium causes hyperperfusion secondary to the corticopontocerebellar pathway (CPCP)-mediated remote effect in the contralateral cerebellar hemisphere. This phenomenon is similar to that of crossed cerebellar diaschisis (CCD). In this report we demonstrated interictal CCH in a patient with epilepsy in technetium-99m-ethyl cysteinate dimer (Tc-99m ECD) SPECT of the brain. To the best of our knowledge, interictal CCH has not been reported in the literature. This is the first report to describe the phenomenon with interictal Tc-99m ECD SPECT.

Correction of collimator-dependent differences in the heart-to-mediastinum ratio in 123I-metaiodobenzylguanidine cardiac sympathetic imaging: Determination of conversion equations using point-source imaging

BackgroundSeptal penetration causes collimator-dependent differences in the heart-to-mediastinum (H/M) ratio in 123I-metaiodobenzylguanidine (MIBG) cardiac imaging. We investigated generally applicable methods to correct such differences.Methods and ResultsFour hours after 123I-MIBG injection, 40 patients underwent anterior chest imaging successively with medium-energy (ME) and various non-ME collimators. The H/M ratios obtained with the non-ME collimators before and after 123I-dual-window penetration correction were compared with the ME-derived standard values to determine patient-based conversion equations for empiric and combined corrections, respectively. A 123I point source was imaged with various collimators, and the central ratio, the ratio of count in a small central region of interest to count in a large one, was calculated. The method of predicting the conversion equations from the central ratios was determined. Correction using the patient-based conversion equations removed systematic underestimation of the H/M ratios obtained with the non-ME collimators, and combined correction depressed residual random errors to some degree. Point-source-based equations yielded results comparable to the patient-based equations.ConclusionsEmpiric and combined corrections effectively reduce collimator-dependent differences in the H/M ratio. The conversion equations can be predicted from simple point-source imaging, which would allow to apply these corrections to data obtained with various collimators.

Septal penetration in iodine-123 metaiodobenzylguanidine cardiac sympathetic imaging using a medium-energy collimator

BackgroundSeptal penetration of high-energy photons affects the estimation of the heart-to-mediastinum (H/M) ratio in cardiac 123I-metaiodobenzylguanidine (MIBG) imaging, and the use of a medium-energy (ME) collimator has been shown to improve quantitative accuracy. We investigated the effect of septal penetration on the estimation of H/M ratios using an ME collimator.Methods and ResultsPoint sources of 99mTc and 123I were imaged using various collimators, which indicated that the effect of high-energy photons with the ME collimator was relatively small but larger than that with the high-energy (HE) collimator. Four hours after 123I-MIBG injection, 20 patients underwent planar anterior chest imaging by different methods in succession. The ME collimator gave lower H/M ratios (mean 2.52) than the HE collimator (2.57), indicating influence of septal penetration in the ME collimator; however, the difference was limited. Although narrowing the energy window from 20% (2.51) to 15% (2.54) increased the H/M ratios in imaging with the ME collimator, the difference was quite limited.ConclusionsSeptal penetration affects the estimation of the H/M ratios using an ME collimator; however, this influence is small and would not have clinical significance.

Validity of the mediastinum as a reference region to evaluate cardiac accumulation of iodine-123 metaiodobenzylguanidine

ObjectiveThe heart-to-mediastinum (H/M) ratio, the heart count normalized for the mediastinum count, is commonly used in cardiac 123I-metaiodobenzylguanidine (MIBG) imaging. However, there are reports describing age-dependent increases in the mediastinum count with or without correction for the injected dose (ID) and a poor correlation between the H/M ratio and heart count normalized for ID. We evaluated the validity of the mediastinum count as a reference in comparison with the ID.MethodsResults of cardiac 123I-MIBG imaging in 200 patients were analyzed. The mean counts for the heart and mediastinum were estimated to calculate the H/M ratio. Additionally, the heart and mediastinum counts were normalized for ID measured with a dose calibrator. ID was corrected for body size represented by body weight, body surface area, or lean body mass.ResultsThe coefficient of variance of the ID-normalized mediastinum count was reduced by correcting ID for body size. The indicators of body size showed significant negative correlations with age. Although a positive correlation was found between age and the ID-normalized mediastinum counts, the age-dependence was reduced by body size correction. There was a close correlation between the H/M ratio and ID-normalized heart counts, and body size correction improved the correlation.ConclusionThe results of this study indicate the validity of the mediastinum as a reference region and support the use of the H/M ratio as an index of cardiac accumulation of 123I-MIBG.

New calibration and evaluation method for PET scanners using point-like radioactive sources

Objective: In order to improve the convenience and reliability of the calibration procedure with respect to the quantitative aspects of PET scanners, we have been developing new methods that rely on the use of point-like radioactive sources. The aim of this study is to report the results of the application of the proposed method for obtaining the cross-calibration factors (CCFs) of a clinical PET scanner for the first time. Methods: A 22Na point-like source was moved in the axial direction to cover the axial field-of-view (FOV) of a commercial PET scanner, SET-2400W (Shimadzu). From reconstructed images, saturated regions-of-interest (ROI) values were used to calculate the CCFs that were defined slice by slice. Results: It was possible to obtain CCFs that were practically equivalent to those obtained by the standard method with cylindrical water phantoms. The variations in the CCFs obtained over about a half-year were well reproduced by the proposed method. Conclusion: The proposed calibration method, which is based on the use of a point-like radioactive source, is useful for determining CCFs and their variations for a clinical PET scanner.

Application of novel calibration scheme based on traceable point-like 22 Na sources to various types of PET scanners

Purpose: We have been developing a practical and reliable calibration scheme based on the use of traceable pointlike sources. In using 22Na sources, special care should be taken to avoid the effects of 1.275-MeV γ rays accompanying β+ decays. The purpose of this study is to validate this calibration method with traceable point-like 22Na sources on various types of PET scanners. Method: The traceable point-like 22Na sources with a spherical absorber design used in this study were of the same type as those used in a previous study. The tested PET scanners included one clinical whole-body PET scanner, four types of clinical PET/CT scanners from different manufacturers, and one small-animal PET scanner. The ROI (region of interest) diameter dependence of ROI values were represented with a fitting function, which was assumed to consist of a recovery part due to spatial resolution and a quadratic background part originating from the scattered γ rays. Results: The calibration factors determined using the point-like source were consistent with those by the standard cross-calibration method within ±4%, which was comparable to the uncertainty of the standard cross-calibration method. Conclusion: The novel calibration scheme based on the use of traceable 22Na point-like sources was validated for six types of commercial PET scanners.