Quantitative terbium-161 SPECT/CT imaging: demonstrating the feasibility of image-based dosimetry and highlighting pitfalls

Collimator and energy window optimization for practical imaging protocol and quantification of Yttrium-90 bremsstrahlung spect/ct: A phantom study

Introduction:The quality of images obtained from the nuclear medicine imaging systems depends on different factors. One of the most important of these factors is the geometrical and physical characteristics of collimator used for imaging with a given radioisotope.Aims and Objectives:The aim of this study is to investigate the contribution of different components of collimator response for determining the most suitable parallel-hole collimator for the different radioisotope energies used in nuclear medicine imaging.Materials and Methods:In this study, the SIMIND Monte Carlo simulation program is used to determine the contribution of geometrical, penetrating and scattered response components of four hexagonal parallel-hole collimators including low-energy high-resolution (LEHR), low-energy general-purpose (LEGP), medium-energy general-purpose (MEGP), and high-energy general-purpose (HEGP) collimators, for 12 different energies used in nuclear medicine imaging.Results:According to the simulation results, the use of both the LEHR and LEGP collimators leads to a geometrical component above 60% for energies between 69 and 171 keV. On the other hand, for energies between 185 and 245 keV, the MEGP collimator and for energy of 364 keV, the HEGP collimator gives the geometrical components above 70% and 60%, respectively, while for energy of 511 keV, the geometrical response of all four collimators is below 20%.Conclusion:The results of this study show that for two low-energy single-photopeak radioisotopes, Tc-99m and I-123, the LEHR and LEGP collimators, and for high-energy single-photopeak radioisotope, I-131, the HEGP collimator are most suitable collimators. For dual-photopeak In-111 radioisotope and triple-photopeak Ga-67 radioisotope, the MEGP and HEGP collimators and for triple-photopeak Tl-201 radioisotopes, the LEHR and LEGP collimators are proposed as most suitable collimators.

Background. Bremsstrahlung (BS) imaging during radioembolization (RE) confirms the deposition of radiotracer in hepatic/extrahepatic tumors. The aim of this study is to demonstrate 32P images and to optimize the imaging parameters. Materials and Methods. Thirty-nine patients with variable types of hepatic tumors, treated with the intra-arterial injection of 32P, were included. All patients underwent BS SPECT imaging 24–72 h after tracer administration, using low energy high resolution (LEHR) (18 patients) or medium energy general purpose (MEGP) (21 patients) collimators. A grading scale from 1 to 4 was used to express the compatibility of the 32P images with those obtained from CT/MRI. Results. Although the image quality obtained with the MEGP collimator was visually and quantitatively better than with the LEHR (76% concordance score versus 71%, resp.), there was no statistically significant difference between them. Conclusion. The MEGP collimator is the first choice for BS SPECT imaging. However, if the collimator change is time consuming (as in a busy center) or an MEGP collimator is not available, the LEHR collimator could be practical with acceptable images, especially in a SPECT study. In addition, BS imaging is a useful method to confirm the proper distribution of radiotherapeutic agents and has good correlation with anatomical findings.

One of the main problems in quantification of single photon emission computer tomography imaging is scatter. In iodine-123 (I-123) imaging, both the primary 159 keV photons and photons of higher energies are scattered. In this experimental study, different scatter correction methods, based on energy window subtraction, have been compared with each other. Iodine-123 single photon emission computed tomography images of a phantom with a known intensity ratio between background and hollow spheres were acquired for three different collimators (low energy high resolution, low energy general purpose, and medium energy general purpose). The hollow spheres were filled with a higher activity concentration than the uniform background activity concentration, resulting in hot spots. Counts were collected in different energy windows, and scatter correction was performed by applying different methods such as effective scatter source estimation, triple and dual energy window (TEW and DEW), double peak window (DPW) and downscatter correction. The intensity ratio between the spheres and the background was used to compare the performance of the different methods. The results revealed that the efficiency of the scatter correction techniques vary depending on the collimator used. For the low energy high resolution collimator, all correction methods except the effective scatter source estimation and the DPW perform well. For the medium energy general purpose collimator, even without scatter correction, the calculated ratio is close to the real ratio. The DEW and DPW methods tend to overestimate the ratio. For the low energy general purpose collimator, only the DEW and the combined DEW and downscatter correction methods perform well. The only correction method that provides a ratio that differs by less than 5% from the real ratio for all the collimators is the combined DEW and downscatter correction method.

The collimator in the SPECT imaging system is a critical component that uniquely influences image quality. Collimator selection for the imaging of the specific isotope is of the utmost importance. This study used Monte Carlo simulations to evaluate the response of different collimators for commonly used radionuclides in SPECT imaging. Methods: The Simulating Medical Imaging Nuclear Detectors Monte Carlo program was used to simulate the Discovery NM/CT 670 Pro SPECT system equipped for a collimator-radionuclide pair to optimize the selection of the collimator for SPECT imaging. Low-energy high-resolution (LEHR), medium-energy general-purpose (MEGP), and high-energy general-purpose (HEGP) collimators were simulated with 99mTc, 177Lu, and 131I point sources (1 MBq) to evaluate spatial resolution, sensitivity, scatter fraction, and septal penetration. The results were analyzed for the optimization of the collimator-radionuclide pair. Results: For 99mTc (γ-energy, 140 keV), the resolution (full width at half maximum), sensitivity, scatter fraction, and septal penetration for LEHR, MEGP, and HEGP were 7.03 mm, 189 counts per minute (cpm)/μCi, 3.50%, and 2.65%; 9.3 mm, 184 cpm/μCi, 2.32%, and 1.35%; and 11.3 mm, 224 cpm/μCi, 2.05%, and 1.27%, respectively. For 177Lu (γ-energy, 113 and 208 keV), the respective values were 7.5 mm, 62.52 cpm/μCi, 22.22%, and 18.56%; 9.6 mm, 20 cpm/μCi, 3.36%, and 2.19%; and 12.03 mm, 25 cpm/μCi, 2.88%, and 1.89%. For 131I (γ-energy, 364 keV), the respective values were 11.5 mm, 6,027 cpm/μCi, 28.80%, and 49.78%; 11.3 mm, 152 cpm/μCi, 43.49%, and 32.89%; and 14.08 mm, 86 cpm/μCi, 23.85%, and 17.96%. Conclusion: The study highlighted the need to understand collimator characteristics as a function of photon energy, where quantitative evaluation is the main aspect. The study suggests that the collimators that had optimal characteristics for imaging with 99mTc, 177Lu, and 131I were the LEHR, MEGP, and HEGP collimators, respectively.

BackgroundImage-based measurement of absorbed dose of Ra-223 dichloride may be useful in predicting therapeutic outcome in patients with castration-resistant prostate cancer (CRPC). In general, SPECT has been found to be more accurate than planar imaging in terms of lesion-based analysis. The aims of this study were to assess the feasibility and clinical usefulness of Ra-223 SPECT.The energy spectrum of Ra-223 and SPECT images of a cylindrical phantom with a hot rod were obtained to determine the collimator candidates and energy window settings suitable for clinical Ra-223 SPECT (basic study A). Another phantom with a tube-shaped chamber and two spheres simulating bowel activity and metastatic lesions in the lumbar spine was scanned with medium-energy general-purpose (MEGP) and high-energy general-purpose (HEGP) collimators (basic study B). Ten patients with CRPC underwent SPECT imaging 2 h after Ra-223 injection successively with MEGP and HEGP collimators in random order for 30 min each. Lesion detectability and semi-quantitative analyses of bone metastasis (i.e. lesion-to-background ratio (LBR)) were performed compared to Tc-99m HMDP SPECT.ResultsBasic study A revealed that an 84-keV photopeak ± 20% using the HEGP collimator offers better SPECT image quality than the other imaging conditions. Basic study B showed that uptake in one of the spheres was overestimated by overlapped activity of the tube-shaped chamber in planar imaging whereas the spheres had similar counts and significantly higher sphere-to-background ratio in SPECT. On both planar and SPECT images, HEGP gave higher image contrast than MEGP (p < 0.01). In the clinical study, Ra-223 SPECT at 84 keV ± 20% depicted more lesions with the HEGP than with the MEGP collimator (51 vs 36, p = 0.013). There was a positive correlation between LBR in Tc-99m SPECT and in Ra-223 SPECT (r = 0.67 with the MEGP and 0.69 with the HEGP collimator, p < 0.01). LBRs were significantly higher with the HEGP than with the MEGP collimator (p < 0.01).ConclusionsWe recommended the use of the HEGP collimator at 84 keV ± 20% for Ra-223 SPECT imaging. Lesion-based semi-quantitative analysis in the human study revealed a good correlation between Ra-223 and Tc-99m HMDP SPECT in the early phase (2–3 h post injection).

PurposePatient-specific dosimetry of lutetium-177 (177Lu)-DOTATATE treatment in neuroendocrine tumours is important, because uptake differs across patients. Single photon emission computer tomography (SPECT)-based dosimetry requires a conversion factor between the obtained counts and the activity, which depends on the collimator type, the utilized energy windows and the applied scatter correction techniques. In this study, energy window subtraction-based scatter correction methods are compared experimentally and quantitatively.Materials and methods177Lu SPECT images of a phantom with known activity concentration ratio between the uniform background and filled hollow spheres were acquired for three different collimators: low-energy high resolution (LEHR), low-energy general purpose (LEGP) and medium-energy general purpose (MEGP). Counts were collected in several energy windows, and scatter correction was performed by applying different methods such as effective scatter source estimation (ESSE), triple-energy and dual-energy window, double-photopeak window and downscatter correction. The intensity ratio between the spheres and the background was measured and corrected for the partial volume effect and used to compare the performance of the methods.ResultsLow-energy collimators combined with 208 keV energy windows give rise to artefacts. For the 113 keV energy window, large differences were observed in the ratios for the spheres. For MEGP collimators with the ESSE correction technique, the measured ratio was close to the real ratio, and the differences between spheres were small.ConclusionFor quantitative 177Lu imaging MEGP collimators are advised. Both energy peaks can be utilized when the ESSE correction technique is applied. The difference between the calculated and the real ratio is less than 10% for both energy windows.

Purpose: To determine optimal collimator and gamma camera combination for Ioflupane I-123 (DaTscan) striatal SPECT. Methods: Anthropomorphic basal ganglia phantom was used. The striatal chambers (caudate and putamen chambers) and the large chamber simulating nonspecific background activity in the remainder of the brain were filled with I-123 with the specific activity ratio 7.7. SPECT data were acquired using triple-head gamma camera (THGC) with fan-beam low-energy ultra high-resolution (LEUR) collimators, with dual-head gamma camera (DHGC) with parallel-beam lowenergy high-resolution (LEHR) collimators and medium-energy general-purpose (MEGP) collimators. Data were acquired at 159 keV with a 20% window, with I-123 and Tc-99m flood table for THGC and DHGC, respectively. The images were reconstructed using the OSEM algorithm with resolution modeling and uniform attenuation correction, and Butterworth postfilter, 5th order and 0.64, 0.78 and 1.0 Ny for LEUR, LEHR, and MEGP, respectively. The filter parameters were chosen to optimize the balance between image noise and spatial resolution. Results: The best image quality in terms of spatial resolution and contrast was obtained with fan-beam LEUR/THGC. The MEGP/DHBC produced images with better contrast-to-noise ratio than LEHR/DHGC. The measured ratio of mean activity in striatal chambers to the remainder of the brain was comparable for all three collimator/camera combinations. Conclusions: Based on phantom DaTscan striatal SPECT, the THGC with fan-beam LEUR collimators is preferable. If DHGC is used MEGP collimators provide better image quality, as compared to LEHR. More studies including patient studies are needed to confirm best collimator/camera combination.

Introduction/AimTerbium-161 (161Tb) has emerged as a promising therapeutic radionuclide, yet standardized imaging guidelines are lacking. This study aimed to characterize a SPECT/CT system, currently used in an ongoing clinical trial (BETA PLUS; NCT05359146), focusing on sensitivity, septal penetration, and dead-time effects.MethodsMeasurements were conducted on a Siemens Symbia Intevo system using two collimators: low-energy high-resolution (LEHR) and medium-energy low-penetration (MELP). Two energy windows were evaluated: 75 keV ± 10% and 48 keV ± 20%. Planar sensitivity and penetration were assessed using a 161Tb-filled Petri dish. Penetration fractions were determined as a function of distance for each collimator-window combination. Dead time was measured intrinsically for each detector using a set of 161Tb point sources. SPECT measurements of a homogenous cylinder phantom were performed to assess count rate performance and predict activity levels at which dead-time effects could occur. To evaluate the potential impact of dead time in patient imaging, SPECT projection data from patients treated with 1 GBq of [161Tb]Tb-DOTA-LM3 (n = 8) was analyzed.ResultsSensitivity was comparable for both collimators at 75 keV (LEHR: 15.7 cps/MBq, MELP: 18.5 cps/MBq) and increased at 48 keV (LEHR: 44.4 cps/MBq, MELP: 67.9 cps/MBq). Maximum penetration occurred at 75 keV with the LEHR collimator (7.5% at 10 cm). In acquired spectra, more than half of the detected counts (51.6%) appeared above the 75 keV window with LEHR, compared to only 12.2% with MELP. Dead-time analyses revealed non-linear detector responses at wide-spectrum count rates exceeding 93 kcps, corresponding to in-field activities of 1.4–2.0 GBq for LEHR and 1.7–2.2 GBq for MELP. The dead-time constant was determined to 0.42 µs for both detector heads, however, the maximum recorded count rate differed significantly (384 kcps vs. 546 kcps). The median and maximum wide-spectrum count rate for patients treated with [161Tb]Tb-DOTA-LM3 was estimated to ~ 20 and ~ 40 kcps per GBq 3 h p.i., respectively, when imaged with LEHR, corresponding to a maximum estimated dead-time loss of 1.7%.ConclusionsWhile high-quality 161Tb SPECT imaging is feasible, careful consideration is essential; the wide range of photons emitted will produce a higher wide-spectrum count rate as compared to 177Lu. The use of low-energy collimators increases penetration and scatter, impairing quantitative accuracy and elevating the wide-spectrum count rate, which may intensify dead-time effects. At therapeutic activity levels (e.g., 7.4 GBq), dead time should be closely monitored to ensure reliable quantification.Supplementary InformationThe online version contains supplementary material available at 10.1186/s40658-025-00792-x.

Introduction: In Yttrium-90 SPECT imaging, the energy window and collimator used during acquisition can have a major effect on image quality. In this work, we used a new and independent method to verify the prevoius results suggest different energy ranges, but all centered at 130 keV. Materials and method: We used Siemens Symbia SPECT-CT system fitted with High Energy General Purpose (HEGP), Medium Energy General Purpose (MEGP) and Low Energy High Resolution (LEHR) to acquire data from NEMA IEC PET Body Phantom with its 6 different spheres of 3.7, 2.8, 2.2, 1.7, 1.3, 1.0 cm diameter. Results:HEGP collimator is the most suitable for acquisitions of 90Y bremsstrahlung radiation from the point of view of the correct volume reproduction. For the bigger sphere’s study, the optimum ISO-counting curves is related to the energy range centred in 130 keV.Conclusion: The results obtained are consistent with previous studies. The Iso Counting Curves method can help to improve image quality.

Nonuniform patient attenuation maps can be acquired using an axially moving point source of a high energy isotope that emits a fanbeam of photons. We simulated the Beacon attenuation correction tool attached to multiheaded Single Photon Emission Computed Tomography (SPECT) cameras which uses this approach. We investigated the scatter order of the photons reaching the detector, and the scatter contributions from the different detector components were evaluated for different energy windows. In case of simultaneous emission and transmission scanning the spatial signals are electronically windowed to allow differentiation between photons from the attenuation and from the emission source. However, an additional correction needs to be applied for cross-contamination. When applying the Beacon device in hybrid mode [gammacamera-Positron Emission Tomography (PET)] there are no degrees of freedom for crystal and collimator. The inter-window contamination was thus examined in detail to derive possible protocol optimizations for that dedicated setup. For the case of applying Beacon-SPECT, we performed multiple types of simulations including different crystal thicknesses and different collimators to evaluate the inter-window contamination. The main conclusion of this work is that a thick crystal detector coupled to a Low Energy High Resolution (LEHR) collimator is the best solution for acquiring attenuation maps in low energy applications. For medium energy studies attenuation maps have to be rescaled to account for the low sensitivity near the center of the patient. Fully Monte Carlo simulating the system matrix for medium energy studies on low energy collimators in order to replace the Medium Energy General Purpose (MEGP) collimators by the LEHR variants appeared to be a more valuable approach. This last method is penalized by a computational burden but results in an improved image quality after reconstruction.

Radium-223 (223Ra) is used in unsealed radionuclide therapy for metastatic bone tumors. The aim of this study is to apply a computational model observer to 223Ra planar images, and to assess the performance of collimators in 223Ra imaging. The 223Ra planar images were created via an in-house Monte Carlo simulation code using HEXAGON and NAI modules. The phantom was a National Electrical Manufacturers Association body phantom with a hot sphere. The concentration of the background was 55Bq/mL, and the sphere was approximately 1.5-20 times that of the background concentration. The acquisition time was 10min. The photopeaks (and the energy window) were 84 (full width of energy window: 20%), 154 (15%), and 270keV (10%). Each 40 images, with and without hot concentration, were applied to a three-channel difference-of-Gaussian channelized Hotelling observer (CHO), and the signal-to-noise ratio (SNR) of the hot region was calculated. The images were examined using five different collimators: two low-energy general-purpose (LEGP), two medium-energy general-purpose (MEGP), and one high-energy general-purpose (HEGP) collimators. The SNR value was linearly proportional to the contrast of the hot region for all collimators and energy windows. The images of the 84-keV energy window with the MEGP collimator that have thicker septa and larger holes produced the highest SNR value. The SNR values of two LEGP collimators were approximately half of the MEGP collimators. The HEGP collimator was halfway between the MEGP and LEGP. Similar characteristics were observed for other energy windows (154, 270keV). The SNR value of images captured via the 270-keV energy window was larger than 154-keV, although the sensitivity of the 270-keV energy window is lower than 154-keV. The results suggested a positive correlation between the SNR value and the fraction of unscattered photons. The SNR value of CHO reflected the performance of collimators and was available to assess and quantitatively evaluate the collimator performance in 223Ra imaging. The SNR value depends on the magnitudes of unscattered photon count and the fraction of unscattered photon count. Consequently, in this study, MEGP collimators performed better than LEGP and HEGP collimators for 223Ra imaging.

BackgroundIt has been proposed, and preclinically demonstrated, that 161Tb is a better alternative to 177Lu for the treatment of small prostate cancer lesions due to its high emission of low-energy electrons. 161Tb also emits photons suitable for single-photon emission computed tomography (SPECT) imaging. This study aims to establish a SPECT protocol for 161Tb imaging in the clinic.Materials and methodsOptimal settings using various γ-camera collimators and energy windows were explored by imaging a Jaszczak phantom, including hollow-sphere inserts, filled with 161Tb. The collimators examined were extended low-energy general purpose (ELEGP), medium-energy general purpose (MEGP), and low-energy high resolution (LEHR), respectively. In addition, three ordered subset expectation maximization (OSEM) algorithms were investigated: attenuation-corrected OSEM (A-OSEM); attenuation and dual- or triple-energy window scatter-corrected OSEM (AS-OSEM); and attenuation, scatter, and collimator-detector response-corrected OSEM (ASC-OSEM), where the latter utilized Monte Carlo-based reconstruction. Uniformity corrections, using intrinsic and extrinsic correction maps, were also investigated. Image quality was assessed by estimated recovery coefficients (RC), noise, and signal-to-noise ratio (SNR). Sensitivity was determined using a circular flat phantom.ResultsThe best RC and SNR were obtained at an energy window between 67.1 and 82.1 keV. Ring artifacts, caused by non-uniformity, were removed with extrinsic uniformity correction for the energy window between 67.1 and 82.1 keV, but not with intrinsic correction. Analyzing the lower energy window between 48.9 and 62.9 keV, the ring artifacts remained after uniformity corrections. The recovery was similar for the different collimators when using a specific OSEM reconstruction. Recovery and SNR were highest for ASC-OSEM, followed by AS-OSEM and A-OSEM. When using the optimized parameter setting, the resolution of 161Tb was higher than for 177Lu (8.4 ± 0.7 vs. 10.4 ± 0.6 mm, respectively). The sensitivities for 161Tb and 177Lu were 7.41 and 8.46 cps/MBq, respectively.ConclusionSPECT with high resolution is feasible with 161Tb; however, extrinsic uniformity correction is recommended to avoid ring artifacts. The LEHR collimator was the best choice of the three tested to obtain a high-resolution image. Due to the complex emission spectrum of low-energy photons, window-based scatter correction had a minor impact on the image quality compared to using attenuation correction only. On the other hand, performing attenuation, scatter, and collimator-detector correction clearly improved image quality. Based on these data, SPECT-based dosimetry for 161Tb-labeled radiopharmaceuticals is feasible.

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. 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. 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. 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.

PURPOSE The use of the low-energy high-resolution (LEHR) collimator for lymphoscintigraphy causes the appearance of star-shaped artifacts at injection sites. The aim of this study was to confirm whether the lower resolution of the low- to medium-energy general-purpose (LMEGP) collimator is compensated by decrease in the degree of septal penetration and the reduction in star-shaped artifacts. METHODS A total of 106 female patients with breast cancer, diagnosed by biopsy, were enrolled in this study. Tc phytate (37 MBq, 1 mCi) was injected around the tumor, and planar and SPECT/CT images were obtained after 3 to 4 hours. When sentinel lymph nodes (SLNs) could not be identified from planar and SPECT/CT images by using the LEHR collimator, we repeated the study with the LMEGP collimator. RESULTS Planar imaging performed using the LEHR and LEHR + LMEGP collimators positively identified SLNs in 96.2% (102/106) and 99.1% (105/106) of the patients, respectively. Using combination of planar and SPECT/CT imaging with the LEHR and LEHR + LMEGP collimators, SLNs were positively identified in 97.2% (103/106) and 100% (106/106) of the patients, respectively. CONCLUSIONS The LMEGP collimator provided better results than the LEHR collimator because of the lower degree of septal penetration. The use of the LMEGP collimator improved SLN detection.