The effect of energy and source location on gamma camera intrinsic and extrinsic spatial resolution: an experimental and Monte Carlo study

Gamma camera imaging characteristics of 166Ho and 99mTc used in Selective Internal Radiation Therapy

BACKGROUND

The administration of a 166Ho scout dose is available as an alternative to 99mTc particles for pre-treatment imaging in Selective Internal Radiation Therapy (SIRT). It has been reported that the 166Ho scout dose may be more accurate for the prediction of microsphere distribution and the associated therapy planning. The aim of the current study is to compare the scintigraphic imaging characteristics of both isotopes, considering the objectives of the pre-treatment imaging using clinically geared phantoms.

METHODS

Planar and SPECT/CT images were obtained using a NEMA image quality phantom in different phantom setups and another body-shaped phantom with several inserts. The influence of collimator type, count statistics, dead time effects, isotope properties and patient obesity on spatial resolution, contrast recovery and the detectability of small activity accumulations was investigated. Furthermore, the effects of the imaging characteristics on personalized dosimetry are discussed.

RESULTS

The images with 99mTc showed up to 3 mm better spatial resolution, up to two times higher contrast recovery and significantly lower image noise than those with 166Ho. The contrast-to-noise ratio was up to five times higher for 99mTc than for 166Ho. Only when using 99mTc all activity-filled spheres could be distinguished from the activity-filled background. The measurements mimicking an obese patient resulted in a degraded image quality for both isotopes.

CONCLUSIONS

Our measurements demonstrate better scintigraphic imaging properties for 99mTc compared to 166Ho in terms of spatial resolution, contrast recovery, image noise, and lesion detectability. While the 166Ho scout dose promises better prediction of the microsphere distribution, it is important to consider the inferior imaging characteristics of 166Ho, which may affect individualized treatment planning in SIRT.

Quantitative validation of Monte Carlo SPECT simulation: application to a Mediso AnyScan GATE simulation

BACKGROUND

Monte Carlo (MC) simulations are used in nuclear medicine imaging as they provide unparalleled insight into processes that are not directly experimentally measurable, such as scatter and attenuation in an acquisition. Whilst MC is often used to provide a 'ground-truth', this is only the case if the simulation is fully validated against experimental data. This work presents a quantitative validation for a MC simulation of a single-photon emission computed tomography (SPECT) system.

METHODS

An MC simulation model of the Mediso AnyScan SCP SPECT system installed at the UK National Physical Laboratory was developed in the GATE (Geant4 Application for Tomographic Emission) toolkit. Components of the detector head and two collimator configurations were modelled according to technical specifications and physical measurements. Experimental detection efficiency measurements were collected for a range of energies, permitting an energy-dependent intrinsic camera efficiency correction function to be determined and applied to the simulation on an event-by-event basis. Experimental data were collected in a range of geometries with [Formula: see text]Tc for comparison to simulation. The procedure was then repeated with [Formula: see text]Lu to determine how the validation extended to another isotope and set of collimators.

RESULTS

The simulation's spatial resolution, sensitivity, energy spectra and the projection images were compared with experimental measurements. The simulation and experimental uncertainties were determined and propagated to all calculations, permitting the quantitative agreement between simulated and experimental SPECT acquisitions to be determined. Statistical agreement was seen in sinograms and projection images of both [Formula: see text]Tc and [Formula: see text]Lu data. Average simulated and experimental sensitivity ratios of ([Formula: see text]) were seen for emission and scatter windows of [Formula: see text]Tc, and ([Formula: see text]) and ([Formula: see text]) for the 113 and 208 keV emissions of [Formula: see text]Lu, respectively.

CONCLUSIONS

MC simulations will always be an approximation of a physical system and the level of agreement should be assessed. A validation method is presented to quantify the level of agreement between a simulation model and a physical SPECT system.

Capabilities of the Monte Carlo Simulation Codes for Modeling of a Small Animal SPECT Camera

Purpose

This study aims to compare Monte Carlo-based codes' characteristics in the determination of the basic parameters of a high-resolution single photon emission computed tomography (HiReSPECT) scanner.

Methods

The geometry of this dual-head gamma camera equipped with a pixelated CsI(Na) scintillator and lead hexagonal hole collimator were accurately described in the GEANT4 Application for the Tomographic Emission (GATE), Monte Carlo N-particle extended (MCNP-X), and simulation of imaging nuclear detectors (SIMIND) codes. We implemented simulation procedures similar to the experimental test for calculation of the energy spectra, spatial resolution, and sensitivity of HiReSPECT by using ^99mTc sources.

Results

The energy resolutions simulated by SIMIND, MCNP-X, and GATE were 17.53, 19.24, and 18.26%, respectively, while it was calculated at 19.15% in experimental test. The average spatial resolutions of the HiReSPECT camera at 2.5 cm from the collimator surface simulated by SIMIND, MCNP-X, and GATE were 3.18, 2.9, and 2.62 mm, respectively, while this parameter was reported at 2.82 mm in the experiment test. The sensitivities simulated by SIMIND, MCNP-X, and GATE were 1.44, 1.27, and 1.38 cps/μCi, respectively, on the collimator surface.

Conclusions

Comparison between simulation and experimental results showed that among these MC codes, GATE enabled to accurately model realistic SPECT system and electromagnetic physical processes, but it required more time and hardware facilities to run simulations. SIMIND was the most flexible and user-friendly code to simulate a SPECT camera, but it had limitations in defining the non-conventional imaging device. The most important characteristics like time and speed of simulation, preciseness of results, and user-friendliness should be considered during simulations.

Geometrical parameters and scattered radiation effects on the extrinsic sensitivity and counting efficiency of a rectangular gamma camera

A point source is used to investigate the effect of water phantom thickness and source-to-detector distance (SDD) on the sensitivity and counting efficiency of a rectangular detector gamma camera. The increase in water thickness resulted in an increase in scatter fraction, a decrease in sensitivity, and counting efficiency. The increase in SDD resulted in a decrease in sensitivity and an increase in counting efficiency. An SDD of 0.79±0.02m is found to provide a good compromise for acceptable sensitivity and reasonable counting efficiency.

Scattered radiation effects on the extrinsic sensitivity and counting efficiency of a gamma camera

An evaluation of the effect of scattered radiation on the performance of a gamma camera is carried out using a specially designed home-made homogeneous circular planar flood source filled with 0.2±0.01 GBq of (99m)Tc solution as a scattering medium. The scatter effects are assessed by analyzing the energy spectrum of (99m)Tc for the scatter fraction and calculating the camera's extrinsic counting efficiency and sensitivity, for five flood source thicknesses (12, 50, 100, 150 and 200 mm ) and three source-to-detector distances (0.7, 0.9 and 1.1 m). Results showed an increase in the scatter fraction from 0.29 to 22.96 as the source thickness increased. This increase was associated with a decrease in the extrinsic sensitivity from 121.36 to 49.58 counts/s GBq, and a decrease in counting efficiency as from 3.78 to 1.55%. With the increase in source-to-detector distance, the extrinsic sensitivity decreases from 121.36 to 118.77 counts/s GBq, while the counting efficiency increases from 3.78 to 11.66%. It was found that a source-to-detector distance of 0.96±9×10(-3) m is a good compromise for an acceptable extrinsic sensitivity and a reasonable counting efficiency.

Performance characteristics of a new pixelated portable gamma camera

PURPOSE

To evaluate and characterize the performance of a new commercially available pixelated portable gamma camera Ergo (Digirad, Poway, CA).

METHODS

The authors evaluated a pixelated portable gamma camera system, Ergo, that consists of 11 520 elements of 3 × 3 mm(2) CsI(Tl) crystals that are 6-mm thick and are coupled to silicon photodiodes. The detector element has a size of 3.31 × 3.24 mm(2). The gamma camera performance was evaluated for both low-energy all-purpose (LEAP) and low-energy high-resolution (LEHR) collimators. The flood-field uniformity for (99m)Tc and (201)Tl was assessed using fillable uniform flood phantoms. Energy spectra were acquired for (99m)Tc, (111)In, (201)Tl, and (67)Ga to evaluate energy linearity and energy resolution. Spectral fits were performed to calculate the photopeak energies and resolutions. The pixel size and multiwindow spatial registration (MWSR) was evaluated by measuring mixed (99m)Tc and (201)Tl point sources placed at known distances apart. The system's sensitivity was measured according to the National Electrical Manufacturer's Association (NEMA) NU1-2007 standards for both LEAP and LEHR collimators as a function of distance from the collimator surface (5, 10, 15, 20, 25, 30, and 40 cm). The system resolution without scatter was measured for both LEAP and LEHR using (99m)Tc-filled capillary tubes located at 0, 2, 4, 6, 10, and 12 cm away from the surface of the collimator. As a measure of the spatial resolution, the full width at half maximum (FWHM) at a given distance was calculated from the presampling line spread function (LSF), constructed from the line profiles of the capillary tubes at the same distance. As a comparison, the FWHM at 10 cm away from LEHR and LEAP collimators was also calculated from linear interpolation as described by NEMA NU-1 2007 and from fitting the profiles to a Gaussian-plus-constant model.

RESULTS

All isotope-collimator pairs demonstrated good flood-field uniformity with an integral uniformity of ≤5% and a differential uniformity of ≤3%. The system demonstrated excellent energy linearity with maximum discrepancy of measured keV from true keV of <1%. The energy resolution of the (99m)Tc 140-keV photopeak was 7.4%. The image pixel size was measured as 3.23 × 3.18 mm(2), and the MWSR was within 0.3 mm (or ~10% of the nominal pixel size). The system sensitivity at 10 cm was 112.6 cps/MBq (249.9 cpm/μCi) for LEAP and 63.1 cps/MBq (140.1 cpm/μCi) for LEHR. The system spatial resolution varied linearly with distance from the collimator and the FWHM were measured to be 7.2 and 8.9 mm at 10 cm for LEHR and LEAP, respectively.

CONCLUSIONS

Herein, the authors describe detailed performance evaluation procedures of a new pixelated portable gamma camera system, which can also be applied to evaluate other pixelated gamma camera system. Spatial resolution assessment in near-field imaging condition offers a unique challenge where the measured FWHM is highly dependent on relative position between the capillary tube and the detector element. The evaluations of the Ergo gamma camera suggest suitable clinical imaging performance. This portable gamma camera has a high (LEAP) planar sensitivity, high energy and spatial resolutions that are comparable to other available gamma cameras, and it exhibits superior count rate performance that is linear up to tens of millions count per second. The Ergo imaging performance, however, can still be improved, for example, by optimizing collimator design for near field imaging.