Monte Carlo simulation of PET and SPECT imaging of 90Y

Monte Carlo simulation study to explore optimum conditions for Astatine-211 SPECT

²¹¹At is a promising nuclide for targeted radioisotope therapy. Direct imaging of this nuclide is important for in vivo evaluation of its distribution. We investigated suitable conditions for single-photon emission computed tomography (SPECT) imaging of ²¹¹At and assessed their feasibility using a homemade Monte Carlo simulation code, MCEP-SPECT. Radioactivity concentrations of 5, 10, or 20 kBq/mL were distributed in six spheres in a National Electrical Manufactures Association (NEMA) body phantom with a background of 1 kBq/mL. The energy window, projection number, and acquisition time were 71-88 keV, 60, and 60 s, respectively, per projection. A medium-energy collimator and three low-energy collimators were tested. SPECT images were reconstructed using the ordered subset expectation maximization (OSEM) method with attenuation correction (Chang method) and scatter correction (triple-energy-windows method). Image quality was evaluated using the contrast-to-noise ratio (CNR) for detectability and the contrast recovery coefficient (CRC) for quantitavity. The low-energy, high-sensitivity collimator exhibited the best detectability among the four types of collimators, with a maximum CNR value of 43. In contrast, the low-energy, high-resolution collimator exhibited excellent quantitavity, with a maximum CRC value of 102%. Scatter correction improved the image quality. In particular, the CRC value almost doubled after scatter correction. The detection of spheres smaller than 20 mm in diameter was difficult. In summary, low-energy collimators were suitable for the SPECT imaging of ²¹¹At. In addition, scatter correction was extremely effective in improving the image quality. The feasibility of ²¹¹At SPECT was demonstrated for lesions larger than 20 mm.

Post Yttrium-90 Imaging

Transarterial radioembolization with yttrium-90 ( ⁹⁰ Y) is a mainstay for the treatment of liver cancer. Imaging the distribution following delivery is a concept that dates back to the 1960s. As β particles are created during ⁹⁰ Y decay, bremsstrahlung radiation is created as the particles interact with tissues, allowing for imaging with a gamma camera. Inherent qualities of bremsstrahlung radiation make its imaging difficult. SPECT and SPECT/CT can be used but suffer from limitations related to low signal-to-noise bremsstrahlung radiation. However, with optimized imaging protocols, clinically adequate images can still be obtained. A finite but detectable number of positrons are also emitted during ⁹⁰ Y decay, and many studies have demonstrated the ability of commercial PET/CT and PET/MR scanners to image these positrons to understand ⁹⁰ Y distribution and help quantify dose. PET imaging has been proven to be superior to SPECT for quantitative imaging, and therefore will play an important role going forward as we try and better understand dose/response and dose/toxicity relationships to optimize personalized dosimetry. The availability of PET imaging will likely remain the biggest barrier to its use in routine post- ⁹⁰ Y imaging; thus, SPECT/CT imaging with optimized protocols should be sufficient for most posttherapy subjective imaging.

Monte Carlo simulation of the acquisition conditions for 177Lu molecular imaging of hepatic tumors

OBJECTIVE

To examine the impact of acquisition time on Lutetium-177 (177Lu) single-photon emission computed tomography (SPECT) images using Monte Carlo simulation.

METHODS

A gamma camera simulation based on the Monte Carlo method was performed to produce SPECT images. The phantom was modeled on a NEMA IEC BODY phantom including six spheres as tumors. After the administration of 7.4 GBq of 177Lu, radioactivity concentrations of the tumor/liver at 6, 24, and 72 h after administration were set to 1.85/0.201, 2.12/0.156, and 1.95/0.117 MBq/mL, respectively. In addition, the radioactivity concentrations of the tumor at 72 h after administration varied by 1/2, 1/4, and 1/8 when comparison was made. Acquisition times examined were 1.2, 1.5, 2, 3, 6, and 12 min. To assess the impact of collimators, SPECT data acquired at 72 h after the administration using six collimators of low-energy high-resolution (LEHR), extended low-energy general-purpose (ELEGP), medium-energy, and general-purpose (MEGP-1, MEGP-2, and MEGP-3) and high-energy general-purpose (HEGP) were examined. After prefiltering using a Butterworth filter, projection images were reconstructed using ordered subset expectation maximization. The detected photons were classified into direct rays, scattered rays, penetrating rays, and characteristic X-rays from lead. The image quality was evaluated through visual assessment, and physical assessment of contrast recovery coefficient (CRC) and contrast-to-noise ratio (CNR). In this study, the CNR threshold for detectability was assumed to be 5.0.

RESULTS

To compare collimators, the highest sensitivity was observed with ELEGP, followed by LEHR and MEGP-1. The highest ratio of direct ray was also observed in ELEGP followed by MEGP-1. In comparison of the radioactivity concentration ratios of tumor/liver, CRC and CNR were significantly decreased with smaller radioactivity concentration ratios. This effect was greater with larger spheres. According to the visual assessment, the acquisition time of 6, 6, and 3 min or longer was required using ELEGP collimator at 6, 24, and 72 h after administration, respectively. Physical assessment based on CNR and CRC also suggested that 6, 6, and 3 min or longer acquisition time was necessary at 6, 24, and 72 h after administration.

CONCLUSION

177Lu-SPECT images generated via the Monte Carlo simulation suggested that the recommended acquisition time was 6 min or longer at 6 and 24 h and 3 min or longer at 72 h after administration.

SPECT performance evaluation on image of Yttrium 90 - Bremsstrahlung using Monte Carlo simulation

Yttrium-90 (⁹⁰Y) is one of the most widely used radionuclides in Nuclear Medicine practice. However, characteristic energy of this beta emitter constitutes a difficulty for dose planning using SPECT imaging. This work aimed to study bremsstrahlung X-rays effects produced by ⁹⁰Y beta particles during SPECT image acquisition using Monte Carlo code MCNPX. Several simulations were carried out to evaluate different aspects that could affect SPECT image quality, such as: collimator type, source-collimator distance and composition of each interacting material. Two configurations of ⁹⁰Y sources were simulated: a point source in several spheres of different materials (soft tissue, water, articular cartilage, and bone) and dimensions with radius ranging from 1 to 20 mm; and a uniformly distributed source in a Lucite cylindrical phantom filled with water. It was evaluated the bremsstrahlung photon emission generated inside different materials; for this was considered the number photons that passing through every different sphere's surface for each radii and material. In case of cylindrical phantom filled with water, in order to obtain the energy deposited over NaI (Tl) crystal detector; there was considered Median Energy General Purpose (MEGP) and Low Energy High Resolution (LEHR) collimators. Moreover, using TMESH routine available in the MCNPX Monte Carlo code, energy distribution images according to the collimator type and the source-collimator distance were obtained. The simulation was validated by comparing with the spectral distribution of the ⁹⁰Y bremsstrahlung X-rays obtained experimentally from an acrylic cylindrical phantom. Results corroborated the importance of Monte Carlo simulation method to evaluate a performance of SPECT image acquisition with ⁹⁰Y. The best resolution was obtained with MEGP collimator independent of source-collimator distance.

Cerenkov luminescence and PET imaging of 90Y: capabilities and limitations in small animal applications

The in vivo sensitivity limits and quantification performance of Cerenkov luminescence imaging have been studied using a tissue-like mouse phantom and ⁹⁰Y. For a small, 9 mm deep target in the phantom, with no background activity present, the Cerenkov luminescence ⁹⁰Y detection limit determined from contrast-to-noise ratios is 10 nCi for a 2 min exposure with a sensitive CCD camera and no filters. For quantitative performance, the values extracted from regions of interest on the images are linear within 5% of a straight line fit versus target activity for target activity of 70 nCi and above. The small branching ratio to decay with positron emission for ⁹⁰Y also permits low-statistics PET imaging of the radionuclide. For PET imaging of the same phantom, with a small animal LSO detector-based scanner, the ⁹⁰Y detection limit is approximately 3 orders of magnitude higher at 10 µCi.

Microfluidic radiobioassays: a radiometric detection tool for understanding cellular physiology and pharmacokinetics

The investigation of molecular uptake and its kinetics in cells is valuable for understanding the cellular physiological status, the observation of drug interventions, and the development of imaging agents and pharmaceuticals. Microfluidic radiobioassays, or microfluidic radiometric bioassays, constitute a radiometric imaging-on-a-chip technology for the assay of biological samples using radiotracers. From 2006 to date, microfluidic radiobioassays have shown advantages in many applications, including radiotracer characterization, enzyme activity radiobioassays, fast drug evaluation, single-cell imaging, facilitation of dynamic positron emission tomography (PET) imaging, and cellular pharmacokinetics (PK)/pharmacodynamics (PD) studies. These advantages lie in the minimized and integrated detection scheme, allowing real-time tracking of dynamic uptake, high sensitivity radiotracer imaging, and quantitative interpretation of imaging results. In this review, the basics of radiotracers, various radiometric detection methods, and applications of microfluidic radiobioassays will be introduced and summarized, and the potential applications and future directions of microfluidic radiobioassays will be forecasted.

The origin and reduction of spurious extrahepatic counts observed in 90Y non-TOF PET imaging post radioembolization

Our literature survey revealed a physical effect unknown to the nuclear medicine community, i.e. internal bremsstrahlung emission, and also the existence of long energy resolution tails in crystal scintillation. None of these effects has ever been modelled in PET Monte Carlo (MC) simulations. This study investigates whether these two effects could be at the origin of two unexplained observations in ⁹⁰Y imaging by PET: the increasing tails in the radial profile of true coincidences, and the presence of spurious extrahepatic counts post radioembolization in non-TOF PET and their absence in TOF PET. These spurious extrahepatic counts hamper the microsphere delivery check in liver radioembolization. An acquisition of a ³²P vial was performed on a GSO PET system. This is the ideal setup to study the impact of bremsstrahlung x-rays on the true coincidence rate when no positron emission and no crystal radioactivity are present. A MC simulation of the acquisition was performed using Gate-Geant4. MC simulations of non-TOF PET and TOF-PET imaging of a synthetic ⁹⁰Y human liver radioembolization phantom were also performed. Internal bremsstrahlung and long energy resolution tails inclusion in MC simulations quantitatively predict the increasing tails in the radial profile. In addition, internal bremsstrahlung explains the discrepancy previously observed in bremsstrahlung SPECT between the measure of the ⁹⁰Y bremsstrahlung spectrum and its simulation with Gate-Geant4. However the spurious extrahepatic counts in non-TOF PET mainly result from the failure of conventional random correction methods in such low count rate studies and poor robustness versus emission-transmission inconsistency. A novel proposed random correction method succeeds in cleaning the spurious extrahepatic counts in non-TOF PET. Two physical effects not considered up to now in nuclear medicine were identified to be at the origin of the unusual ⁹⁰Y true coincidences radial profile. TOF reconstruction removing of the spurious extrahepatic counts was theoretically explained by a better robustness against emission-transmission inconsistency. A novel random correction method was proposed to overcome the issue in non-TOF PET. Further studies are needed to assess the novel random correction method robustness.

Fast GPU-based Monte Carlo code for SPECT/CT reconstructions generates improved 177Lu images

Background

Full Monte Carlo (MC)-based SPECT reconstructions have a strong potential for correcting for image degrading factors, but the reconstruction times are long. The objective of this study was to develop a highly parallel Monte Carlo code for fast, ordered subset expectation maximum (OSEM) reconstructions of SPECT/CT images. The MC code was written in the Compute Unified Device Architecture language for a computer with four graphics processing units (GPUs) (GeForce GTX Titan X, Nvidia, USA). This enabled simulations of parallel photon emissions from the voxels matrix (128³ or 256³). Each computed tomography (CT) number was converted to attenuation coefficients for photo absorption, coherent scattering, and incoherent scattering. For photon scattering, the deflection angle was determined by the differential scattering cross sections. An angular response function was developed and used to model the accepted angles for photon interaction with the crystal, and a detector scattering kernel was used for modeling the photon scattering in the detector. Predefined energy and spatial resolution kernels for the crystal were used. The MC code was implemented in the OSEM reconstruction of clinical and phantom ¹⁷⁷Lu SPECT/CT images. The Jaszczak image quality phantom was used to evaluate the performance of the MC reconstruction in comparison with attenuated corrected (AC) OSEM reconstructions and attenuated corrected OSEM reconstructions with resolution recovery corrections (RRC).

Result

The performance of the MC code was 3200 million photons/s. The required number of photons emitted per voxel to obtain a sufficiently low noise level in the simulated image was 200 for a 128³ voxel matrix. With this number of emitted photons/voxel, the MC-based OSEM reconstruction with ten subsets was performed within 20 s/iteration. The images converged after around six iterations. Therefore, the reconstruction time was around 3 min. The activity recovery for the spheres in the Jaszczak phantom was clearly improved with MC-based OSEM reconstruction, e.g., the activity recovery was 88% for the largest sphere, while it was 66% for AC-OSEM and 79% for RRC-OSEM.

Conclusion

The GPU-based MC code generated an MC-based SPECT/CT reconstruction within a few minutes, and reconstructed patient images of ¹⁷⁷Lu-DOTATATE treatments revealed clearly improved resolution and contrast.

A Monte Carlo study on (223)Ra imaging for unsealed radionuclide therapy

PURPOSE

Radium-223 ((223)Ra), an α-emitting radionuclide, is used in unsealed radionuclide therapy for metastatic bone tumors. The demand for qualitative (223)Ra imaging is growing to optimize dosimetry. The authors simulated (223)Ra imaging using an in-house Monte Carlo simulation code and investigated the feasibility and utility of (223)Ra imaging.

METHODS

The Monte Carlo code comprises two modules, hexagon and nai. The hexagon code simulates the photon and electron interactions in the tissues and collimator, and the nai code simulates the response of the NaI detector system. A 3D numeric phantom created using computed tomography images of a chest phantom was installed in the hexagon code. (223)Ra accumulated in a part of the spine, and three x-rays and 19 γ rays between 80 and 450 keV were selected as the emitted photons. To evaluate the quality of the (223)Ra imaging, the authors also simulated technetium-99m ((99m)Tc) imaging under the same conditions and compared the results.

RESULTS

The sensitivities of the three photopeaks were 147 counts per unit of source activity (cps MBq(-1); photopeak: 84 keV, full width of energy window: 20%), 166 cps MBq(-1) (154 keV, 15%), and 158 cps MBq(-1) (270 keV, 10%) for a low-energy general-purpose (LEGP) collimator, and those for the medium-energy general-purpose (MEGP) collimator were 33, 13, and 8.0 cps MBq(-1), respectively. In the case of (99m)Tc, the sensitivity was 55 cps MBq(-1) (141 keV, 20%) for LEGP and 52 cps MBq(-1) for MEGP. The fractions of unscattered photons of the total photons reflecting the image quality were 0.09 (84 keV), 0.03 (154 keV), and 0.02 (270 keV) for the LEGP collimator and 0.41, 0.25, and 0.50 for the MEGP collimator, respectively. Conversely, this fraction was approximately 0.65 for the simulated (99m)Tc imaging. The sensitivity with the LEGP collimator appeared very high. However, almost all of the counts were because of photons that penetrated or were scattered in the collimator; therefore, the proportions of unscattered photons were small.

CONCLUSIONS

Their simulation study revealed that the most promising scheme for (223)Ra imaging is an 84-keV window using an MEGP collimator. The sensitivity of the photopeaks above 100 keV is too low for (223)Ra imaging. A comparison of the fractions of unscattered photons reveals that the sensitivity and image quality are approximately two-thirds of those for (99m)Tc imaging.

Analysis of the influence of 111In on 90Y-bremsstrahlung SPECT based on Monte Carlo simulation

OBJECTIVE

⁹⁰Y-ibritumomab tiuxetan (Zevalin) which is used for the treatment of malignant lymphomas can be used for SPECT imaging based on bremsstrahlung from ⁹⁰Y beta particles. However, gamma rays emitted by ¹¹¹In, which is administered to evaluate the indication for the treatment, contaminate the ⁹⁰Y bremsstrahlung images. Our objective is to investigate the influence of ¹¹¹In on the ⁹⁰Y SPECT images using Monte Carlo simulation.

METHODS

We used an in-house developed simulation code for the Monte Carlo simulation of electrons and photons (MCEP). Two hot spheres with diameters of 40 mm were put in an elliptical phantom. Both spheres ("sphere 1" and "sphere 2") were filled with ⁹⁰Y and ¹¹¹In mixed solutions. The activities of ⁹⁰Y in sphere 1 and sphere 2 were 241 and 394 kBq/mL, respectively, and the ones of ¹¹¹In were 8.14 and 13.3 kBq/mL, respectively. The background activity of ⁹⁰Y was 38.6 kBq/mL, whereas that of ¹¹¹In was 1.30 kBq/mL; moreover, the acquisition time was 30 min. Two energy windows were used: one is 90-190 keV included the ¹¹¹In photopeak; the other is 90-160 keV. To evaluate the quality of the SPECT images, the contrast recovery coefficient (CRC) and the constant noise ratio (CNR) of the SPECT images were derived.

RESULTS

For the energy window between 90 and 160 keV, the ¹¹¹In count was 74 % of the total. In that case, the CRC values were 30.1 and 30.7 % for "sphere 1" and "sphere 2", respectively, whereas the CNR values were 6.8 and 12.1, respectively. For the energy window between 90 and 190 keV, the ¹¹¹In count reached 85 % of the total count. The CRC and CNR values were 38.6 and 40.0 % and 10.6 and 19.4, respectively.

CONCLUSIONS

Our simulation study revealed that the cross talk between ¹¹¹In and ⁹⁰Y in SPECT imaging is rather serious. Even for the energy window excluding the ¹¹¹In photopeak, the count ratio of ⁹⁰Y was less than 30 % of the total. However, the influence of ¹¹¹In on ⁹⁰Y-SPECT imaging cannot be ignored, and the count ratio because of ¹¹¹In is important to estimate the density of ⁹⁰Y.

A descriptive analysis of remnant activity during (90)Y resin microspheres radioembolization of hepatic tumors: technical factors and dosimetric implications

OBJECTIVE

Activity planning for (90)Y radioembolization aims to maximize the effect of the treatment while keeping toxicity acceptably low. Our aim was to describe the amount of residual activity in post-treatment v-vials and tubing and analyze the possible factors affecting it (total activity administered, number of split activity injection(s), previous treatments, administration artery and microcatheter size), as these may influence dosimetric planning and treatment.

METHODS

This was a retrospective review using case records of patients who received (90)Y-radioembolization for hepatic tumors at a single tertiary center. From August 2013 to September 2015, seventy-seven out of one hundred and fifty patients who received radioembolization with (90)Y resin microspheres due to inoperable Hepatocellular Carcinoma (HCC) or liver metastases were included. The rest were mainly excluded due to incomplete data sets. The number of split activities (injections) for the radioembolization could be: one single injection, two or three. The remnant activity in post-treatment v-vials and tubing were measured for every patient. The administration arteries evaluated were: proper hepatic artery (PHA), right hepatic artery (RHA), middle hepatic artery (MHA), left hepatic artery (LHA) and small caliber branch arteries. The sizes of the microcatheters (2.2 or 2.7 Fr) used to administer the dose were also evaluated.

RESULTS

In total, 77 out of 150 patients were included in the final analysis. There were 59 men of median age 64.0 years old. The total median dose loss was 0.10 GBq. The total dose loss increased 0.244 GBq [95 % CI = (0.169, 0.318)] when three split activities were given compared to single activity injection. Activity loss for each injection increased 0.0297 GBq [95 % CI = (0.0151, 0.0443)] for every 1.0 GBq increase of split activity injection. There were no significant statistical differences in the rest of patient characteristics.

CONCLUSIONS

There is significant loss of activity observed during radioembolization, which can have a major dosimetric impact. The total administered activity and the number of split injections during radioembolization are the main influencing factors. Further prospective studies as well as measures of clinical outcome are warranted.