Image distortion and correction in single photon emission CT

Feasibility of using counts-per-volume approach with a new SPECT phantom to optimize the relationship between administered dose and acquisition time

We developed a phantom for single-photon emission computed tomography (SPECT), with the objective of assessing image quality to optimize administered dose and acquisition time. We investigated whether the concept of counts-per-volume (CPV), which is used as a predictor of visual image quality in positron emission tomography, can be used to estimate the acquisition time required for each SPECT image. QIRE phantoms for the head (QIRE-h) and torso (QIRE-t) were developed to measure four physical indicators of image quality in a single scan: uniformity, contrast of both hot and defective lesions with respect to the background, and linearity between radioactivity concentration and count density. The target organ's CPV (TCPV), sharpness index (SI), and contrast-to-noise ratio (CNR) were measured for QIRE-h and QIRE-t phantoms, and for anthropomorphic brain and torso phantoms. The SPECT image quality of the four phantoms was visually assessed on a 5-point scale. The acquisition time and TCPV were correlated for all four phantoms. The SI and CNR values were nearly identical for the QIRE and anthropomorphic phantoms with comparable TCPV. The agreement between the visual scores of QIRE-h and brain phantoms, as well as QIRE-t and torso phantoms, was moderate and substantial, respectively. Comparison of SPECT image quality between QIRE and anthropomorphic phantoms revealed close agreement in terms of physical indicators and visual assessments. Therefore, the TCPV concept can also be applied to SPECT images of QIRE phantoms, and optimization of imaging parameters for nuclear medicine examinations may be possible using QIRE phantoms alone.

Optimization of cross-calibration factor for quantitative bone SPECT without attenuation and scatter correction in the lumbar spine: head-to-head comparison with attenuation and scatter correction

OBJECTIVES

Quantitative single-photon emission computed tomography (SPECT) with computed tomography (SPECT/CT) is known to improve diagnostic performance. Although SPECT-alone systems are used widely, accurate quantitative SPECT using these systems is challenging. This study aimed to improve the accuracy of quantitative bone SPECT of the lumbar spine with the SPECT-alone system.

METHODS

The cross-calibration factor (CCF) was measured using three kinds of phantoms and the optimal values were determined. The recovery coefficient with and without attenuation and scatter correction (ACSC) were compared. Bone SPECT/CT was performed on 93 consecutive patients with prostate cancer, and the standardized uptake values (SUVs) were compared using the respective CCFs. The first 60 patients were classified according to body weight, and the correlation coefficient between SUVs with and without ACSC were calculated; the slopes were defined as body weight-based coefficients (BWCs). In the remaining 33 patients, the SUV was adjusted according to BWC, and the accuracy of the adjustment was verified.

RESULTS

The quantitative SPECT values obtained from the CCF using SIM2 bone phantom showed nearly accurate radioactivity concentrations, even without ACSC. The recovery coefficients with and without ACSC were similar. Unadjusted SUVs with and without ACSC were strongly correlated; however, SUVs without ACSC were significantly higher than those with ACSC (P < 0.0001). The mean difference between the SUVs with and without ACSC disappeared when the SUVs without ACSC were adjusted by BWC (P = 0.9814).

CONCLUSIONS

Our cross-calibration method for quantitative bone SPECT enables interpretation with a harmonized SUV even in SPECT-alone systems.

Small-angle Compton Scatter Artifact in Tc-99m-IDA Hepatobiliary Scintigraphy Resulting in the Breast Overlying the Liver in Planar Dynamic Imaging

Compton scatter photons are generally considered problematic in nuclear medicine imaging. Therefore, efforts are being made to minimize the involvement of these photons by employing some special strategies in daily practice. Basically, photons scattering at a small angle and traveling in the proper direction stand a chance of getting recorded and thereby participate in the image formation. These photons may create artifactual hot spots in the vicinity of a region with high concentration of radioactivity. The present study focuses on the negative impact of such photons during routine imaging in clinical setting, through an artifact detected in technetium-99m-IDA hepatobiliary scintigraphy, with the purpose of highlighting this issue to the nuclear medicine practitioners.

Investigation of attenuation correction for small-animal single photon emission computed tomography

The quantitative accuracy of SPECT is limited by photon attenuation and scatter effect when photons interact with atoms. In this study, we developed a new attenuation correction (AC) method, CT-based mean attenuation correction (CTMAC) method, and compared it with various methods that were often used currently to assess the AC phenomenon by using the small-animal SPECT/CT data that were acquired from various physical phantoms and a rat. The physical phantoms and an SD rat, which were injected with (99m)Tc, were scanned by a parallel-hole small-animal SPECT, and then they were imaged by the 80 kVp micro-CT. Scatter was estimated and corrected by the triple-energy window (TEW) method. Absolute quantification was derived from a known activity point source scan. In the physical-phantom studies, we compared the images with original, scatter correction (SC) only, and the scatter-corrected images with AC performed by using Chang's method, CT-based attenuation correction (CTAC), CT-based iterative attenuation compensation during reconstruction (CTIACR), and the CTMAC. From the correction results, we find out that the errors of the previous six configurations are mostly quite similar. The CTMAC needs the shortest correction time while obtaining good AC results.

Simultaneous dual-isotope imaging based on an artificial neural network for evaluating myocardial perfusion and fatty acid metabolism

BACKGROUND

We contrived a scatter correction method based on an artificial neural network (ANN) and applied it to the simultaneous evaluation of myocardial perfusion and fatty acid metabolism in single-photon emission computed tomography (SPECT).

METHODS

The count data of three energy windows were used as inputs of the ANN. The count ratios of the estimated primary-to-total photons for (99m)Tc and (123)I, which were used to reconstruct (99m)Tc and (123)I images, were calculated using the ANN. In a phantom study, single- and dual-isotope imaging with (99m)Tc/(123)I and (201)Tl/(123)I was performed by means of a cardiac phantom simulating patients with and without obesity. In a human study, five normal volunteers and ten patients with myocardial infarction underwent myocardial perfusion and fatty acid metabolism imaging with single and dual SPECT with combinations of (99m)Tc-methoxyisobutylisonitrile/(123)I-beta-methyl(p-iodophenyl)pentadecanoic acid (BMIPP) and (201)Tl/(123)I-BMIPP as tracers.

RESULTS

Technetium-99m yielded more homogeneous images than (201)Tl because of the lower degree of photon attenuation, especially in the condition of obese patients, resulting in clearer visualization of the perfusion-metabolism mismatch. Dual (99m)Tc/(123)I SPECT offered comparable images with single SPECT in assessing myocardial damage.

CONCLUSIONS

The method effectively separated (99m)Tc and (123)I primary photons and proved applicable to (99m)Tc/(123)I dual-isotope myocardial SPECT.

[Novel phantom for evaluating contrast resolution in single photon emission computed tomography (SPECT)]

OBJECTIVES

We have designed a phantom to evaluate acquisition and reconstruction parameters using contrast transfer function (CTF). The goal of this study was to evaluate the accuracy of the phantom for contrast resolution.

METHODS

The phantom consisted of spaced (0-14 mm, 1 mm intervals) pairs of cubic containers (5 mm wide, 20 mm long, and 50 mm high). The phantom's accuracy was examined by comparing a real value of a measured count profile using the phantom with a theoretical value obtained by the line spread function (LSF) using a line source. A SPECT acquisition was 256×256 matrix (pixel size: 0.9×0.9 mm(2)), 360 degrees at 30 s/view. The radius of rotation was set to 15 cm, and the types of collimators were low energy high resolution (LEHR) and low middle energy general purpose (LMEGP). Reconstructions were performed with filtered backprojection and ordered subsets expectation maximization method (10 iterations, 10 subsets) with collimator-detector response correction.

RESULTS

The actual measured count profile and CTF accorded closely with the theoretical one.

DISCUSSION

The line pair (LP) phantom, obtained with smaller pixels, was really accurate. The size of cubic containers poses as a minimal problem for accurately evaluating the contrast resolution and plotted counts profile.

CONCLUSIONS

This phantom could be a useful method for evaluating acquisition and reconstruction parameters in SPECT.

Scatter correction based on an artificial neural network for 99mTc and 123I dual-isotope SPECT in myocardial and brain imaging

The aim of this study was to elucidate the clinical usefulness of scatter correction with an artificial neural network (ANN) in 99mTc and 123I dual-isotope SPECT.

METHODS

Two algorithms for ANN scatter correction were tested: ANN-10 and ANN-3 employing 10 and 3 energy windows for data acquisition, respectively. Three patients underwent myocardial or brain SPECT with one of the following combinations of radiopharmaceuticals administered: 99mTc-tetrofosmin and 123I-metaiodobenzylguanidine (MIBG), 99mTc-methoxyisobutylisonitrile (MIBI) and 123I-beta-methyl-paraiodophenyl-pentadecanoic acid (BMIPP), or 99mTc-ethyl-cistainate dimmer (ECD) and 123I-iomazenil. The patients were also referred for single-isotope imaging incorporating conventional triple-energy window (TEW) scatter correction. Crosstalk- and scatter-corrected 99mTc- and 123I-SPECT images in dual-isotope acquisition with ANN were compared with those in single-isotope acquisition.

RESULTS

The ANN method well separated 123I and 99mTc primary photons. Although ANN-10 yielded images of poor quality, ANN-3 offered comparable image quality with the single-isotope scan without significant increase of acquisition time.

CONCLUSION

The proposed method is clinically useful because it provides various combinations of information without anatomical misregistration with one acquisition.

Physiological imaging of the lung: single-photon-emission computed tomography (SPECT)

Emission tomography provides three-dimensional, quantitative images of the distribution of radiotracers used to mark physiological, metabolic, or pathological processes. Quantitative single photon emission computed tomography (SPECT) requires correction for the image-degrading effects due to photon attenuation and scatter. Phantom experiments have shown that radioactive concentrations can be assessed within some percentage of the true value when relevant corrections are applied. SPECT is widely spread, and radiotracers are available that are easy to use and comparably inexpensive. Compared with other methods, SPECT suffers from a lower spatial resolution, and the time required for image acquisition is longer than for some alternative methods. In contrast to some other methods, SPECT allows simultaneous imaging of more than one process, e.g., both regional blood flow and ventilation, for the whole lung. SPECT has been used to explore the influence of posture and clinical interventions on the spatial distribution of lung blood flow and ventilation. Lung blood flow is typically imaged using macroaggregates of albumin. Both radioactive gases and particulate aerosols labeled with radioactivity have been used for imaging of regional ventilation. However, all radiotracers are not equally suited for quantitative measurements; all have specific advantages and limitations. With SPECT, both blood flow and ventilation can be marked with radiotracers that remain fixed in the lung tissue, which allows tracer administration during conditions different from those at image registration. All SPECT methods have specific features that result from the used radiotracer, the manner in which it is administered, and how images are registered and analyzed.

Recent developments and future trends in nuclear medicine instrumentation

Molecular imaging using high-resolution single-photon emission computed tomography (SPECT) and positron emission tomography (PET) has advanced elegantly and has steadily gained importance in the clinical and research arenas. Continuous efforts to integrate recent research findings for the design of different geometries and various detector technologies of SPECT and PET cameras have become the goal of both the academic comcameras have become the goal of both the academic community and nuclear medicine industry. As PET has recently become of more interest for clinical practice, several different design trends seem to have developed. Systems are being designed for "low cost" clinical applications, very high-resolution research applications (including small-animal imaging), and just about everywhere in-between. The development of dual-modality imaging systems has revolutionized the practice of nuclear medicine. The major advantage being that SPECT/PET data are intrinsically aligned to anatomical information from the X-ray computed tomography (CT), without the use of external markers or internal landmarks. On the other hand, combining PET with Magnetic Resonance Imaging (MRI) technology is scientifically more challenging owing to the strong magnetic fields. Nevertheless, significant progress has been made resulting in the design of a prototype small animal PET scanner coupled to three multichannel photomultipliers via optical fibers, so that the PET detector can be operated within a conventional MR system. Thus, many different design paths are being pursued--which ones are likely to be the main stream of future commercial systems? It will be interesting, indeed, to see which technologies become the most popular in the future. This paper briefly summarizes state-of-the art developments in nuclear medicine instrumentation. Future prospects will also be discussed.