Adaptive scan duration in SPECT: Evaluation for radioembolization

Purpose: It may be challenging to select the optimal scan duration for single-photon emission computed tomography (SPECT) protocols because the activity distribution characteristics can differ in every scan. Using simulations and experiments, we investigated whether the scan duration can be optimized for every scan separately by evaluating the activity distribution during scanning. We refer to this as adaptive scanning. Methods: The feasibility of adaptive scanning was evaluated for the detection of extrahepatic depositions in the pretreatment procedure of radioembolization, in which 99mTc-labeled macroaggregated albumin ( 99mTc-MAA) is injected into the liver. We simulated fast 1-min detector rotations and updated the reconstruction with the newly collected counts after every rotation. The scan was terminated when one of the two criteria was met: (a) when the mask difference of the detected extrahepatic deposition between two consecutive rotations was lower than 5%; or (b) when the reconstructed extrahepatic activity was negligible with respect to the total reconstructed activity (<0.075%). The performance of adaptive scanning was evaluated using a digital phantom with various activity distributions, a physical phantom experiment, and simulations based on 129 patient activity distributions. Results: The digital phantom data showed that the scan termination times substantially depended on the activity distribution characteristics. The experimental phantom data showed the feasibility of adaptive scanning with physical scanner measurements and illustrated that fast detector motion was not limiting the adaptive scanning performance. The patient data showed a large spread in the scan terminations times. By adaptive scanning, the mean scan duration of the patient distributions was shortened from 20 min (current clinical protocol) to 4.8 ± 0.2 min. The detection accuracy of extrahepatic depositions was unaffected and the mean difference in the extrahepatic deposition masks (compared with the 20-min scan) was only 7.0 ± 1.0%. Conclusion: Our study suggests that the SPECT scan duration can be personalized by assessing the activity distribution characteristics during scanning for the detection of extrahepatic depositions in the pretreatment procedure of radioembolization. The adaptive scanning approach might also be of benefit for other SPECT protocols, as long as a measure of interest is available for optimization

Monte Carlo-based scatter correction for the SMARTZOOM collimator

BACKGROUND: Myocardial perfusion imaging is a commonly performed SPECT protocol and hence it would be beneficial if its scan duration could be shortened. For traditional gamma cameras, two developments have separately shown to allow for a shortened scan duration: (i) reconstructing with Monte Carlo-based scatter correction instead of dual-energy window scatter correction and (ii) acquiring projections with the SMARTZOOM collimator instead of a parallel-hole collimator. This study investigates which reduction in scan duration can be achieved when both methods are combined in a single system. RESULTS: The SMARTZOOM collimator was implemented in a Monte Carlo-based reconstruction package and the implementation was validated through image quality phantom experiments. The potential for scan duration reduction was evaluated with a phantom configuration that is realistic for myocardial perfusion imaging. The original reconstruction quality was achieved in 76 ± 8% of the original scan duration when switching from dual-energy window scatter correction to Monte Carlo-based scatter correction. The original reconstruction quality was achieved in 56 ± 13% of the original scan duration when switching from the parallel-hole to the SMARTZOOM collimator. After combining both methods in a single system, the original reconstruction quality was achieved in 34 ± 7% of the original scan duration. CONCLUSIONS: Monte Carlo-based scatter correction combined with the SMARTZOOM collimator can further decrease the scan duration in myocardial perfusion imaging

Interventional respiratory motion compensation by simultaneous fluoroscopic and nuclear imaging: a phantom study

Purpose A compact and mobile hybrid c-arm scanner, capable of simultaneously acquiring nuclear and fluoroscopic projections and SPECT/CBCT, was developed to aid fluoroscopy-guided interventional procedures involving the administration of radionuclides (e.g. hepatic radioembolization). However, as in conventional SPECT/CT, the acquired nuclear images may be deteriorated by patient respiratory motion. We propose to perform compensation for respiratory motion by extracting the motion signal from fluoroscopic projections so that the nuclear counts can be gated into motion bins. The purpose of this study is to quantify the performance of this motion compensation technique with phantom experiments. Methods Anthropomorphic phantom configurations that are representative of distributions obtained during the pre-treatment procedure of hepatic radioembolization were placed on a stage that translated with three different motion patterns. Fluoroscopic projections and nuclear counts were simultaneously acquired under planar and SPECT/CBCT imaging. The planar projections were visually assessed. The SPECT reconstructions were visually assessed and quantitatively assessed by calculating the activity recovery of the spherical inserts in the phantom. Results The planar nuclear projections of the translating anthropomorphic phantom were blurry when no motion compensation was applied. With motion compensation, the nuclear projections became representative of the stationary phantom nuclear projection. Similar behavior was observed for the visual quality of SPECT reconstructions. The mean error of the activity recovery in the uncompensated SPECT reconstructions was 15.8±0.9% for stable motion, 11.9±0.9% for small variations, and 11.0±0.9% for large variations. When applying motion compensation, the mean error decreased to 1.8±1.6% for stable motion, 2.2±1.5% for small variations, and 5.2±2.5% for large variations. Conclusion A compact and mobile hybrid c-arm scanner, capable of simultaneously acquiring nuclear and fluoroscopic projections, can perform compensation for respiratory motion. Such motion compensation results in sharper planar nuclear projections and increases the quantitative accuracy of the SPECT reconstructions

Comparison of the Biograph Vision and Biograph mCT for quantitative Y-90 PET/CT imaging for radioembolisation

BACKGROUND: New digital PET scanners with improved time of flight timing and extended axial field of view such as the Siemens Biograph Vision have come on the market and are expected to replace current generation photomultiplier tube (PMT)-based systems such as the Siemens Biograph mCT. These replacements warrant a direct comparison between the systems, so that a smooth transition in clinical practice and research is guaranteed, especially when quantitative values are used for dosimetry-based treatment guidance. The new generation digital PET scanners offer increased sensitivity. This could particularly benefit 90Y imaging, which tends to be very noisy owing to the small positron branching ratio and high random fraction of 90Y. This study aims to determine the ideal reconstruction settings for the digital Vision for quantitative 90Y imaging and to evaluate the image quality and quantification of the digital Vision in comparison with its predecessor, the PMT-based mCT, for 90Y imaging in radioembolisation procedures. METHODS: The NEMA image quality phantom was scanned to determine the ideal reconstruction settings for the Vision. In addition, an anthropomorphic phantom was scanned with both the Vision and the mCT, mimicking a radioembolisation patient with lung, liver, tumour, and extrahepatic deposition inserts. Image quantification of the anthropomorphic phantom was assessed by the lung shunt fraction, the tumour to non-tumour ratio, the parenchymal dose, and the contrast to noise ratio of extrahepatic depositions. RESULTS: For the Vision, a reconstruction with 3 iterations, 5 subsets, and no post-reconstruction filter is recommended for quantitative 90Y imaging, based on the convergence of the recovery coefficient. Comparing both systems showed that the noise level of the Vision is significantly lower than that of the mCT (background variability of 14% for the Vision and 25% for the mCT at 2.5·103 MBq for the 37 mm sphere size). For quantitative 90Y measures, such as needed in radioembolisation, both systems perform similarly. CONCLUSIONS: We recommend to reconstruct 90Y images acquired on the Vision with 3 iterations, 5 subsets, and no post-reconstruction filter for quantitative imaging. The Vision provides a reduced noise level, but similar quantitative accuracy as compared with its predecessor the mCT

A compact and mobile hybrid C-arm scanner for simultaneous nuclear and fluoroscopic image guidance

Purpose: This study evaluates the performance of a mobile and compact hybrid C-arm scanner (referred to as IXSI) that is capable of simultaneous acquisition of 2D fluoroscopic and nuclear projections and 3D image reconstruction in the intervention room. Results: The impact of slightly misaligning the IXSI modalities (in an off-focus geometry) was investigated for the reduction of the fluoroscopic and nuclear interference. The 2D and 3D nuclear image quality of IXSI was compared with a clinical SPECT/CT scanner by determining the spatial resolution and sensitivity of point sources and by performing a quantitative analysis of the reconstructed NEMA image quality phantom. The 2D and 3D fluoroscopic image of IXSI was compared with a clinical CBCT scanner by visualizing the Fluorad A+D image quality phantom and by visualizing a reconstructed liver nodule phantom. Finally, the feasibility of dynamic simultaneous nuclear and fluoroscopic imaging was demonstrated by injecting an anthropomorphic phantom with a mixture of iodinated contrast and 99mTc. Conclusion: Due to the divergent innovative hybrid design of IXSI, concessions were made to the nuclear and fluoroscopic image qualities. Nevertheless, IXSI realizes unique image guidance that may be beneficial for several types of procedures. Key Points: • IXSI can perform time-resolved planar (2D) simultaneous fluoroscopic and nuclear imaging. • IXSI can perform SPECT/CBCT imaging (3D) inside the intervention room

Feasibility of imaging 90Y microspheres at diagnostic activity levels for hepatic radioembolization treatment planning

PURPOSE: Prior to 90 Y hepatic radioembolization, a dosage of 99m Tc-macroaggregated albumin ( 99m Tc-MAA) is administered to simulate the distribution of the 90 Y-loaded microspheres. This pretreatment procedure enables lung shunt estimation, detection of potential extrahepatic depositions, and estimation of the intrahepatic dose distribution. However, the predictive accuracy of the MAA particle distribution is often limited. Ideally, 90 Y microspheres would also be used for the pretreatment procedure. Based on previous research, the pretreatment activity should be limited to the estimated safety threshold of 100 MBq, making imaging challenging. The purpose of this study was to evaluate the quality of intra- and extrahepatic imaging of 90 Y-based pretreatment positron emission tomography/computed tomography (PET/CT) and quantitative single photon emission computed tomography (SPECT)/CT scans, by means of phantom experiments and a patient study. METHODS: An anthropomorphic phantom with three extrahepatic depositions was filled with 90 Y chloride to simulate a lung shunt fraction (LSF) of 5.3% and a tumor to nontumor ratio (T/N) of 7.9. PET /CT (Siemens Biograph mCT) and Bremsstrahlung SPECT/CT (Siemens Symbia T16) images were acquired at activities ranging from 1999 MBq down to 24 MBq, representing post- and pretreatment activities. PET/CT images were reconstructed with the clinical protocol and SPECT/CT images were reconstructed with a quantitative Monte Carlo-based reconstruction protocol. Estimated LSF, T/N, contrast to noise ratio of all extrahepatic depositions, and liver parenchymal and tumor dose were compared with the phantom ground truth. A clinically reconstructed SPECT/CT of 150 MBq 99m Tc represented the current clinical standard. In addition, a 90 Y pretreatment scan was simulated for a patient by acquiring posttreatment PET/CT and SPECT/CT data with shortened acquisition times. RESULTS: At an activity of 100 MBq 90 Y, PET/CT overestimated LSF [+10 percentage point (pp)], underestimated liver parenchymal dose (-3 Gy/GBq), and could not detect the extrahepatic depositions. SPECT/CT more accurately estimated LSF (-0.7 pp), parenchymal dose (-0.3 Gy/GBq) and could detect all three extrahepatic depositions. 99m Tc SPECT/CT showed similar accuracy as 90 Y SPECT/CT (LSF: +0.2 pp, parenchymal dose: +0.4 Gy/GBq, all extrahepatic depositions visible), although the noise level in the liver compartment was considerably lower for 99m Tc SPECT/CT compared to 90 Y SPECT/CT. The patient's SPECT/CT simulating a pretreatment 90 Y procedure accurately represented the posttreatment 90 Y microsphere distribution. CONCLUSIONS: Quantitative SPECT/CT of 100 MBq 90 Y could accurately estimate LSF, T/N, parenchymal and tumor dose, and visualize extrahepatic depositions

Nuclear imaging for improved radioembolization treatment planning

Radioembolization is a minimally invasive treatment for liver cancer. During a radioembolization procedure small radioactive spheres (microspheres) are administered in the vasculature of the liver. These microspheres lodge (mostly) in the tumorous tissue, where they locally irradiate liver tumours, whilst (mostly) sparing the healthy liver tissue. Prior to the treatment procedure, a pre-treatment safety procedure is performed in which surrogate particles, technetium-99m labelled macroaggregated albumin (99mTc-MAA), are administered to simulate the distribution of the microspheres. These 99mTc-MAA particles are visualised using nuclear imaging. Based on the amount of 99mTc-MAA shunting to the lungs (lung shunt fraction), the unwanted accumulation of 99mTc-MAA elsewhere outside the liver (extrahepatic depositions), and the distribution of 99mTc-MAA within the liver (intrahepatic dose distribution), a nuclear medicine physician decides whether or not it is safe to proceed with the treatment procedure. When considered safe, the treatment will usually take place 1-2 weeks later. The microspheres that are administered during the radioembolization treatment have a certain level of radioactivity, measured in becquerel (Bq). Deciding where and how much activity needs to be administered for treatment is called treatment planning. The calculation of the amount of activity that needs to be administered is based on physical models, but these are generally not tailored to individual patients. This thesis introduced several ways to improve radioembolization treatment planning using nuclear imaging

Accelerated SPECT image reconstruction with FBP and an image enhancement convolutional neural network

BACKGROUND: Monte Carlo-based iterative reconstruction to correct for photon scatter and collimator effects has been proven to be superior over analytical correction schemes in single-photon emission computed tomography (SPECT/CT), but it is currently not commonly used in daily clinical practice due to the long associated reconstruction times. We propose to use a convolutional neural network (CNN) to upgrade fast filtered back projection (FBP) image quality so that reconstructions comparable in quality to the Monte Carlo-based reconstruction can be obtained within seconds. RESULTS: A total of 128 technetium-99m macroaggregated albumin pre-treatment SPECT/CT scans used to guide hepatic radioembolization were available. Four reconstruction methods were compared: FBP, clinical reconstruction, Monte Carlo-based reconstruction, and the neural network approach. The CNN generated reconstructions in 5 sec, whereas clinical reconstruction took 5 min and the Monte Carlo-based reconstruction took 19 min. The mean squared error of the neural network approach in the validation set was between that of the Monte Carlo-based and clinical reconstruction, and the lung shunting fraction difference was lower than 2 percent point. A phantom experiment showed that quantitative measures required in radioembolization were accurately retrieved from the CNN-generated reconstructions. CONCLUSIONS: FBP with an image enhancement neural network provides SPECT reconstructions with quality close to that obtained with Monte Carlo-based reconstruction within seconds