Collimator choice in cardiac SPECT with I-123–labeled tracers

doi:10.1016/j.nuclcard.2004.04.009

Journal of Nuclear Cardiology

Volume 11, Issue 4, July–August 2004, Pages 433-439

https://doi.org/10.1016/j.nuclcard.2004.04.009 Get rights and content

Abstract

Background

Septal penetration of high-energy photons may degrade the quality of single photon emission computed tomography (SPECT) of the heart with iodine 123–labeled tracers. We investigated the impact of collimator choice on cardiac SPECT with I-123.

Methods and results

SPECT of a thoracic phantom containing I-123 solution was performed with a low-energy high-resolution (LEHR) collimator, special LEHR (SLEHR) collimator, and medium-energy (ME) collimator, and the cavity-to-myocardium contrast, wall thickness, and defect contrast were compared among the collimators. For all indices, use of the SLEHR collimator yielded the best results. Comparison between the LEHR and ME collimators revealed that the cavity-to-myocardium contrast and contrast for large defects were better with the ME collimator, whereas wall thickness and contrast for small defects were similar. Scatter correction by the triple-energy window method improved the indices examined; however, the superiority of the SLEHR collimator was still observed after correction.

Conclusions

Collimator choice substantially influences the quality of cardiac SPECT with I-123–labeled agents, and an appropriate collimator needs to be selected in consideration of septal penetration and spatial resolution.

Section snippets

Equipment

A dual-headed gamma camera with a 5/8-inch-thick sodium iodide crystal (E.CAM+; Siemens, Erlangen, Germany) was used in this study. Three parallel-hole collimators were used: low-energy high-resolution (LEHR), special LEHR (SLEHR), and ME collimators. The LEHR and ME collimators were provided by Siemens and the SLEHR collimator by Siemens-Asahi Medical Technologies (Tokyo, Japan). The physical characteristics of the three collimators are presented in Table 1. 11 The SLEHR collimator has a

Results

Cavity/myocardium ratios were highest with the LEHR collimator, followed by the ME collimator and SLEHR collimator, suggesting that the lowest (poorest) contrast and highest (best) contrast were obtained with the LEHR collimator and SLEHR collimator, respectively (Figure 2). The addition of I-123 solutions into compartments other than the myocardium decreased the difference between the SLEHR and ME collimators and increased the difference between the LEHR and ME collimators. Scatter correction

Discussion

In this study we assessed the quality of cardiac SPECT with I-123, in relation to collimator choice, using a thoracic phantom. In addition to estimation of the cardiac sympathetic system with I-123 MIBG, cardiac fatty acid metabolism may be assessed with I-123–labeled tracers such as I-123 beta-methyl-iodophenyl-pentadecanoic acid (BMIPP).14 Both I-123 MIBG and I-123 BMIPP are accumulated intensely in the liver. Whereas pulmonary accumulation is low for I-123 BMIPP, it varies widely for I-123

Acknowledgements

The authors have indicated they have no financial conflicts of interest.

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However, the majority of these papers have focused only on a specific radionuclide. Other efforts have been limited to optimizing collimators for simultaneous or sequential dual-isotope imaging [14,15]. There has been no initiative so far to actually get rid of the energy dependency of SPECT nuclides on collimators by exploiting the superior energy resolution and high stopping power of CZT or CdTe detectors to avoid collimator changes.
In single photon emission computed tomography, it is a challenging task to maintain reasonable performance using only one specific collimator for radiotracers over a broad spectrum of diagnostic photon energies, since photon scatter and penetration in a collimator differ with the photon energy. Frequent collimator exchanges are inevitable in daily clinical SPECT imaging, which hinders throughput while subjecting the camera to operational errors and damage. Our objective is to design a collimator, which is independent of the photon energy, performs reasonably well for commonly used radiotracers with low- to medium-energy levels of gamma emissions. Using the Geant4 simulation toolkit, we simulated and evaluated a parallel-hole collimator mounted to a CZT detector. With the pixel-geometry-matching collimation, the pitch of the collimator hole was fixed to match the pixel size of the CZT detector throughout this work. Four variables, hole shape, hole length, hole radius/width and the source-to-collimator distance were carefully studied. Scatter and penetration of the collimator, sensitivity and spatial resolution of the system were assessed for four radionuclides including ⁵⁷Co, ^99mTc, ¹²³I and ¹¹¹In, with respect to the aforementioned four variables. An optimal collimator was then decided upon such that it maximized the total relative sensitivity (TRS) for the four considered radionuclides while other performance parameters, such as scatter, penetration and spatial resolution, were benchmarked to prevalent commercial scanners and collimators. Digital phantom studies were also performed to validate the system with the optimal square-hole collimator (23 mm hole length, 1.28 mm hole width, and 0.32 mm septal thickness) in terms of contrast, contrast-to-noise ratio and recovery ratio. This study demonstrates promise of our proposed energy-optimized collimator to be used in a CZT-based gamma camera, with comparable or even better imaging performance versus commercial collimators such as low-energy high resolution (LEHR) and medium energy general purpose (MEGP) collimators.
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To compare the performance of low-energy (LE) and medium-energy (ME) collimators and the use of scatter correction on ¹²³I SPECT images as assessed by visual inspection and semi-quantitative analysis.
Both brain and heart/thorax phantoms were used to evaluate the effects of collimator selection and correction methods on brain and cardiac ¹²³I imaging. Experiments were performed to measure the target-to-background ratio for calculation of object contrast. The projection data were corrected using X-ray-based attenuation maps for attenuation correction (AC) and dual-energy window method for scatter correction (SC). Reconstructed images using different methods were compared, which included FBP (filtered backprojection), and OSEM (ordered subsets expectation maximization) without any correction, with AC, with SC, and with AC and SC.
In both phantom studies, the image contrasts provided by the ME collimator were better than that of the LE collimator. The OSEM with simple dual-energy window SC method improved image contrast and visual image quality as compared to the FBP method. The use of AC improved the visual image quality but did not improve image contrast, if SC method was not implemented at the same time.
This study demonstrates the importance of SC and ME collimator in ¹²³I SPECT imaging. If image contrast is the figure of merit, then SC should be performed and the use of the ME collimator appears to be preferable in semi-quantitative ¹²³I SEPCT studies.
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Inoue et al. studied the collimator selection for a 123I planar study, and found that using the ME collimator provided high quantitative accuracy, and may enhance the reliability of an evaluation.9 The same group also investigated the use of a special LEHR collimator in addition to LEHR and ME collimators.10 For SPECT, the ME collimator was comparable or superior to the LEHR collimator, but this depended on the condition of the MIBG defect, the cavity/myocardial ratio, and lung activity.
To overcome differences in the choice of collimator for an iodine-123 (¹²³I)-labeled meta-iodobenzylguanidine (MIBG) heart-to-mediastinum (H/M) ratio, we examined multi-window correction methods with ¹²³I dual-window (IDW) and triple-energy window (TEW) acquisition.
Standard phantoms, which consisted of the heart, mediastinum, lung, and liver, were generated. Three correction methods were compared: TEW and two IDW methods (IDW₀ and IDW₁). Low-energy high-resolution (LEHR), medium-energy (ME), and ¹²³I-specific low-medium-energy high-resolution (LMEHR) collimators were used. Clinical studies were performed in 10 patients. In the phantom study, the H/M ratio was significantly underestimated without correction, with both the LEHR and ME collimators (70% and 88% of the true value). When H/M with the LEHR collimator was divided by uncorrected H/M with the ME collimator, the ratio (mean ± SD) was 80% ± 5%, 98% ± 5%, 104% ± 7%, and 98% ± 5% for the no-correction, TEW, IDW₀, and IDW₁ methods, respectively. Clinical studies with the LEHR collimator after TEW and IDW correction (uncorrected average H/M ratio, 1.86 ± 0.23; TEW, 2.47 ± 0.46, P = .0015; IDW, 2.46 ± 0.46, P = .0017) provided comparable values to the uncorrected ME collimator (2.56 ± 0.46, P = NS vs TEW and IDW).
The H/M ratio with the ME collimator, after application of the TEW or IDW methods, was close to the theoretical value in the phantom study. However, the corrected H/M ratios with the LEHR collimator provided comparable H/M ratios to the uncorrected ME data in phantom and clinical studies.
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Deconvolution of septal penetration (DSP) has been developed to improve quantification so as to allow the use of low-energy high-resolution collimators for iodine 123 cardiac single photon emission computed tomography (SPECT) imaging. The purpose of this study is to optimize its acquisition and processing protocols.
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Original articleCollimator choice in cardiac SPECT with I-123–labeled tracers

Abstract

Background

Methods and results

Conclusions

Section snippets

Equipment

Results

Discussion

Acknowledgements

J Nucl Cardiol

J Nucl Cardiol

J Nucl Cardiol

Influence of high-energy photons on the spectrum of iodine-123 with low- and medium-energy collimatorsconsequences for imaging with 123I-labelled compounds in clinical practice

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J Nucl Med

Optimal collimator choice for sequential iodine-123 and technetium-99m imaging

Eur J Nucl Med

Influence of collimator characteristics on quantification in SPECT

J Nucl Med

Influence of methodology on the presence and extent of mismatching between 99mTc-MIBI and 123I-BMIPP in myocardial viability studies

J Nucl Med

Imaging 123I with a scintillation camera. A study of detection performance and quality factor concepts

Phys Med Biol

MIBG imaging

J Nucl Cardiol

Metaiodobenzylguanidine scintigraphy of the heartwhat have we learnt clinically?

Eur J Nucl Med

Compton scatter compensation using the triple-energy window method for single- and dual-isotope SPECT

J Nucl Med

Original article
Collimator choice in cardiac SPECT with I-123–labeled tracers

Influence of high-energy photons on the spectrum of iodine-123 with low- and medium-energy collimatorsconsequences for imaging with ¹²³I-labelled compounds in clinical practice

Influence of methodology on the presence and extent of mismatching between ^99mTc-MIBI and ¹²³I-BMIPP in myocardial viability studies

Imaging ¹²³I with a scintillation camera. A study of detection performance and quality factor concepts