Improved scatter correction with factor analysis for planar and SPECT imaging

Evaluation of quantitative accuracy among different scatter corrections for quantitative bone SPECT/CT imaging

Although scatter correction improves SPECT image contrast and thus image quality, the effects of quantitation accuracy under various conditions remain unclear. The present study aimed to empirically define the conditions for the optimal scatter correction of quantitative bone SPECT/CT images. Scatter correction was performed by applying dual and triple energy windows (DEW and TEW) with different sub-energy window widths, and effective scatter source estimation (ESSE) to CT-based scatter correction. Scattered radiation was corrected on images acquired using a triple line source (TLSP) phantom and an uniform cylinder phantom. The TLSP consisted of a line source containing 74.0 MBq of 99mTc in the middle, and a background component containing air, water or a K2HPO4 solution with a density equivalent to that of bone. The sum of all pixels in air, water and the K2HPO4 solution was measured on SPECT images. Scatter fraction (SF) and normalized mean square error (NMSE) based on counts from the air background as a reference were then calculated to assess quantitative errors due to scatter correction. The uniform cylinder phantom contained the same K2HPO4 solution and 222.0 MBq of 99mTc. The coefficient of variation (CV) was calculated from the count profile of this phantom to assess the uniformity of SPECT images across scatter correction under various conditions. Both SF and NMSE in SPECT images of phantoms containing water in the background were lower at a TEW sub-window of 3% (TEW3%), than in other scatter corrections, whereas those in K2HPO4 were lower at a DEW sub-window of 20% (DEW20%). Larger DEW and smaller TEW sub-energy windows allowed more effective correction. The CV of the uniform cylinder phantom, DEW20%, was inferior to all other tested scatter corrections. The quantitative accuracy of bone SPECT images substantially differed according to the method of scatter correction. The optimal scatter correction for quantitative bone SPECT was DEW20% (k = 1), but at the cost of slightly decreased image uniformity.

Verification of phantom accuracy using a Monte Carlo simulation: bone scintigraphy chest phantom

We aimed to compare the measurement and simulation data of bone scintigraphy of a chest phantom using a Monte Carlo simulation to verify the accuracy of the simulated data. The SIM² bone phantom was enclosed using 300 kBq/mL of technetium-99 m (^99mTc) to represent the bone tumor and 50 kBq/mL of ^99mTc to represent normal bone. Projection data were obtained using single-photon emission computed tomography (SPECT). Simulated projection data were constructed based on CT data. The contrast ratio, recovery coefficient (RC), % coefficient variation (CV), and power spectrum density (PSD) of each part were calculated from the reconstructed data. The contrast ratio and RC were equal between the actual and simulated data. Higher % CV values were noted for soft tissue than for normal bone. The PSD was equal for all frequency band ranges. Our results prove the utility of the Monte Carlo simulation for verifying various data using phantoms.

Variation of Contrast Values for Myocardial Perfusion Imaging in Single-photon Emission Computed Tomography/Computed Tomography Hybrid Systems with Different Correction Methods

Objectives:

Single-photon emission computed tomography/computed tomography (SPECT/CT) hybrid systems have the advantage of performing various scans using the same imaging setting. Absorption and scattering of the gamma rays by the patient’s body significantly affect images obtained from scintigraphy, especially in myocardial perfusion imaging. An important parameter for image quality in SPECT is image contrast which is defined as the difference in density between regions of the image corresponding to different levels of radioactive uptake in the patient. The objective of the study was to evaluate the influence of applying different correction methods on image contrast of myocardial SPECT/CT images.

Material and Methods:

A total of 114 patients, 43 females and 71 males, patient’s raw data were processed and analyzed using attenuation correction (AC), scatter correction (SC), both attenuation and scatter correction together (ACSC), and no correction (NC). The short axis (coronal) slices resulted from the raw data reconstruction were chosen to perform the processing for hot and cold spheres for contrast values measurement. Statistical analysis was made for the measured contrast values for AC, SC, ACSC, and NC to determine the best image contrast.

Results:

When applying SC alone, it yields better contrast value (0.834), compared to AC (0.677) and ACSC (0.739). Both ACSC and AC had better image contrast compared to NC (0.592).

Conclusion:

The intercomparison study between the correction conditions indicates that the counts in SPECT/ CT are highly affected by all correction methods. The image contrast has been significantly improved by using SC, AC, and ACSC when compared with the NC image. Furthermore, SC is superior in the image contrast than the other correction conditions in the reconstruction of SPECT/CT MPI.

Assessment of Four Scatter Correction Methods in In-111 SPECT Imaging: A Simulation Study

Introduction:

Detection of compton scattered photons is one of the most important factors affecting the quality of single-photon emission computed tomography (SPECT) images. In most cases, the multiple-energy window acquisition methods are used for estimation of the scatter contribution into the main energy window(s) used in imaging.

Aims and Objectives:

The purpose of this study is to evaluate and compare the performance of four different scatter correction methods in In-111 SPECT imaging. Due to the lack of sufficient studies in this field, it can be useful to perform a more detailed and comparative study.

Materials and Methods:

Four approximations for scatter correction of In-111 SPECT images are evaluated by using the Monte Carlo simulation. These methods are firstly applied on each of photopeak windows, separately. Then, the effect of the correction methods is investigated by considering both the photopeak windows. The images obtained from a simulated multiple-spheres phantom are used for the evaluation of the correction methods by using three assessment criteria, including the image contrast, relative noise, and the recovery coefficient.

Results:

The results of this study show that the correction methods, when using the single photopeak windows, result in increase in image contrast with a significant level of noise. In return, when both the photopeak energy windows are used for imaging, it is possible to achieve the better image characteristics.

Conclusion:

The use of the proposed correction methods, by considering both the photopeak windows, leads to improve the image contrast with a reasonable level of image noise.

Quality control in dual head gamma-cameras: Comparison between methods and software used for image analysis

Patient radiation dose and image quality are primary issues in the conduct of nuclear medicine (NM) procedures. A range of protocols are currently used in image acquisition and analysis of quality control (QC) tests, with National Electrical Manufacturers Association (NEMA) methods and protocols widely accepted in providing an accurate description, measurement and report of γ-camera performance parameters. However, no standard software is available for image analysis. Present study compares vendor QC software analysis and three types of software freely downloadable from the internet: NMQC, NM Toolkit and ImageJ-NM Toolkit software. These were used for image analysis of QC tests of γ-cameras based on NEMA protocols including non-uniformity evaluation. Ten non-uniformity QC images were obtained using a dual head γ-camera installed in Trieste General Hospital and then analyzed. Excel analysis was used as the baseline calculation for the non-uniformity test according to NEMA procedures. The results of non-uniformity analysis showed good agreement between the independent types of software and Excel calculations (the average differences were 0.3%, 2.9%, 1.3% and 1.6% for the Useful Field of View (UFOV) integral, UFOV differential, Central Field of View (CFOV) integral and CFOV differential, respectively), while significant differences were detected following analysis using the company QC software when compared with Excel analysis (the average differences were 14.6%, 20.7%, 25.7% and 31.9% for the UFOV integral, UFOV differential, CFOV integral and CFOV differential, respectively). Compared to use of Excel calculations use of NMQC software was found to be in close accord. Variation in results obtained using the three types of software and γ-camera QC software was due to the use of different pixel sizes. It is important to conduct independent analyses tests in addition to using the vendor QC software in order to determine the differences between values.