SPECT/CT technology

Technological Advances in SPECT and SPECT/CT Imaging

Single photon emission tomography/computed tomography (SPECT/CT) is a mature imaging technology with a dynamic role in the diagnosis and monitoring of a wide array of diseases. This paper reviews the technological advances, clinical impact, and future directions of SPECT and SPECT/CT imaging. The focus of this review is on signal amplifier devices, detector materials, camera head and collimator designs, image reconstruction techniques, and quantitative methods. Bulky photomultiplier tubes (PMTs) are being replaced by position-sensitive PMTs (PSPMTs), avalanche photodiodes (APDs), and silicon PMs to achieve higher detection efficiency and improved energy resolution and spatial resolution. Most recently, new SPECT cameras have been designed for cardiac imaging. The new design involves using specialised collimators in conjunction with conventional sodium iodide detectors (NaI(Tl)) or an L-shaped camera head, which utilises semiconductor detector materials such as CdZnTe (CZT: cadmium-zinc-telluride). The clinical benefits of the new design include shorter scanning times, improved image quality, enhanced patient comfort, reduced claustrophobic effects, and decreased overall size, particularly in specialised clinical centres. These noticeable improvements are also attributed to the implementation of resolution-recovery iterative reconstructions. Immense efforts have been made to establish SPECT and SPECT/CT imaging as quantitative tools by incorporating camera-specific modelling. Moreover, this review includes clinical examples in oncology, neurology, cardiology, musculoskeletal, and infection, demonstrating the impact of these advancements on clinical practice in radiology and molecular imaging departments.

The Diagnostic Accuracy of SPECT Imaging in Patients With Suspected Pulmonary Embolism: Systematic Review and Network Meta-analysis

BACKGROUND

This meta-analysis and systematic review assessed the diagnostic accuracy of lung SPECT compared with lung planar imaging in patients with suspected acute pulmonary embolism (PE) or chronic thromboembolic pulmonary hypertension.

PATIENTS AND METHODS

A search of Medline, Embase, and Cochrane databases identified suitable articles published before October 2023. Meta-analyses were performed to determine the diagnostic accuracy of SPECT imaging modalities, including perfusion (Q) SPECT, ventilation (V)/Q SPECT, Q SPECT/CT, and V/Q SPECT/CT. Network meta-analyses were performed to compare the diagnostic accuracy of SPECT and planar imaging in paired-design studies.

RESULTS

Twenty-four articles (total n = 6576) were included in the analysis. For suspected acute PE, the respective sensitivity and specificity of SPECT imaging modalities were as follows: Q SPECT, 0.93 (95% confidence interval [CI], 0.87-0.99; I2 = 49%) and 0.72 (95% CI, 0.54-0.95; I2 = 94%); V/Q SPECT, 0.96 (95% CI, 0.94-0.98; I2 = 51%) and 0.95 (95% CI, 0.92-0.98; I2 = 80%); Q SPECT/CT, 0.93 (95% CI, 0.87-0.98; I2 = 66%) and 0.82 (95% CI, 0.70-0.96; I2 = 87%); and V/Q SPECT/CT, 0.97 (95% CI, 0.93-1.00; I2 = 7%) and 0.98 (95% CI, 0.97-1.00; I2 = 31%). The relative sensitivity and specificity of SPECT compared with planar imaging were 1.17 (95% CI, 1.06-1.30; P < 0.001) and 1.14 (95% CI, 1.00-1.29; P = 0.05), respectively. For suspected chronic thromboembolic pulmonary hypertension, the pooled sensitivity and specificity of SPECT imaging were 0.97 (95% CI, 0.95-1.00; I2 = 0%) and 0.91 (95% CI, 0.87-0.94; I2 = 0%), respectively.

CONCLUSIONS

SPECT exhibited superior diagnostic performance for PE. V/Q SPECT/CT was the most accurate modality.

Preliminary study on diagnostic reference levels of whole-body PET-CT examinations in Jiangsu Province, China

To survey the whole-body positron emission tomography/computed tomography (PET-CT) examinations in Jiangsu Province and establish the diagnostic reference levels (DRL). A total of nine hospitals in southern, central and northern Jiangsu Provinces were selected. Activity duration product (ADP), activity per body weight (AW), dose-length product (DLP) and total effective dose (ET) were choosen to establish DRLs for whole-body PET-CT examinations. DRL and achievable dose (AD) were calculated according to International Commission on Radiological Protection report. The conversion coefficient method was used to calculate the ET induced by PET-CT whole-body imaging. DRLs of whole-body PET-CT examinations in Jiangsu Province were set to be 922 mGy·cm (DLP), 4453 MBq.min (ADP), 5.22 MBq.kg-1(AW), 19.8 mSv (ET). The ET for whole-body PET-CT examinations is higher than other countries. The effective dose from radionuclides (EF) in female patients is lower than that in male patients. Effective doses from CT(ECT), ET and ECT/ET in female patients are higher than that in male patients (P < 0.01). Provincial DRLs are established for four radiation metrics in whole-body PET-CT examinations. DRL of AW in whole-body PET-CT examinations is at the medium level, and DLP is higher than that reported in most literature, which has the potential to be further reduced.

Whole-body single photon emission computed tomography/computed tomography for assessment of oncological bone disease - is an extended field of view (from vertex to toes) of clinical value?

OBJECTIVES

Whole-body single photon emission computed tomography/computed tomography (WB-SPECT/CT) is useful for diagnosing bone metastases. When performed on a dual-headed gamma camera, this may cover from clavicles to proximal femurs due to time constraints. In contrast, the novel 360 o cadmium-zinc-telluride scanner can perform WB-SPECT/CT (from vertex to toes) in approximately 20 min. The aim was to assess the prevalence of additional findings above the clavicles and below the lesser trochanters and the prevalence of incidental findings in the CT component.

METHODS

Retrospective study of 117 WB-SPECT/CT scans for oncological bone assessment over a 4-month-period. Scan analysis was performed by two independent experienced radionuclide radiologists.

RESULTS

The male:female ratio was 71:46 and the mean patient age was 68 years. The primary malignancies were predominantly prostate 65/117 (55.6%) and breast 40/117 (34.2%). There were additional findings of malignancy above the clavicles in 16/116 scans (13.8%) and below the lesser trochanters in 16/117 scans (13.7%). Two cases in the 'above the clavicles' group were suspected solitary metastases, whereas four cases in the 'below lesser trochanters' group were bone metastases at risk of pathological fracture. Incidental findings of clinical significance included suspected new malignancy in 11/117 (9.4%).

CONCLUSION

A WB-SPECT/CT (from vertex to toes) oncological bone protocol is useful for the detection of additional findings of clinical significance above the clavicles and below the lesser trochanters. Reviewing and reporting the CT findings in SPECT/CT is important.

Impact of CT tube-voltage and bone density on the quantitative assessment of tracer uptake in Tc-99m bone SPECT/CT: A phantom study

PURPOSE

To evaluate the effect of computed tomography (CT) tube voltage and CT density for CT-based attenuation correction (CTAC) on quantification of tracer uptake in single photon emission computed tomography (SPECT)/CT.

METHODS

A cylindrical phantom contained 7 cylinders with diameter of 30 mm. The central cylinder and background part were filled with 17 kBq/ml of ^99mTc-pertechnetate solution. Of the remaining 6 cylinders, one cylinder was filled with water and 5 cylinders were filled with each own different concentration of K₂HPO₄ solution (120, 275, 450, 666, and 960 mg/cm³) to simulate different bone densities. The 6 cylinders also contained ^99mTc-pertechnetate solution with the same radioactivity concentration (207 kBq/ml). CT scans were performed with 4 different tube voltages of 80, 100, 120, and 140 kVp for CTAC. The radioactivity concentration in the 6 cylinders were measured on the SPECT images processed with 4 different attenuation coefficient maps derived from each tube voltage of CT images.

RESULTS

Compared with the water cylinder without K₂HPO₄ solution, the measured radioactivity of the highest density cylinder (K₂HPO₄ solution concentration: 960 mg/cm³) was found to be overestimated by 3.3 % and 4.3 %, respectively, when the tube voltage was 120 kVp and 140 kVp (p = 0.022). The use of low-tube voltage, such as 80 kVp, has improved the quantitative accuracy of bone SPECT/CT.

CONCLUSIONS

SPECT quantitative evaluation of tracers in high-density objects tends to overestimate as tube voltage for CTAC increases. However, the overestimation in quantitative SPECT/CT evaluation in simulated bone area is less than 5% at most.

Transition to Fast Whole-Body SPECT/CT Bone Imaging: An Assessment of Image Quality

OBJECTIVE

To investigate the impact of reduced SPECT acquisition time on reconstructed image quality for diagnostic purposes.

METHOD

Data from five patients referred for a routine bone SPECT/CT using the standard multi-bed SPECT/CT protocol were reviewed. The acquisition time was 900 s using gating technique; SPECT date was resampled into reduced data sets of 480 s, 450 s, 360 s and 180 s acquisition duration per bed position. Each acquisition time was reconstructed using a fixed number of subsets (8 subsets) and 4, 8, 12, and 16 iterations, followed by a post-reconstruction 3D Gaussian filter of 8 mm FWHM. Two Nuclear Medicine physicians analysed all images independently to score image quality, noise and diagnostic confidence based on a pre-defined 4-point scale.

RESULTS

Our result showed that the most frequently selected categories for 480 s and 450 s images were good image quality, average noise and fair confidence, particularly at lower iteration numbers 4 and 8. For the shortened acquisition time of 360 s and 180 s, statistical significance was observed in most reconstructed images compared with 900 s.

CONCLUSION

The SPECT/CT can significantly shorten the acquisition time with maintained image quality, noise and diagnostic confidence. Therefore, acquiring data over 480 s and 450 s is feasible for WB-SPECT/CT bone scans to provide an optimal balance between acquisition time and image quality.

Development of attenuation correction methods using deep learning in brain-perfusion single-photon emission computed tomography

PURPOSE

Computed tomography (CT)-based attenuation correction (CTAC) in single-photon emission computed tomography (SPECT) is highly accurate, but it requires hybrid SPECT/CT instruments and additional radiation exposure. To obtain attenuation correction (AC) without the need for additional CT images, a deep learning method was used to generate pseudo-CT images has previously been reported, but it is limited because of cross-modality transformation, resulting in misalignment and modality-specific artifacts. This study aimed to develop a deep learning-based approach using non-attenuation-corrected (NAC) images and CTAC-based images for training to yield AC images in brain-perfusion SPECT. This study also investigated whether the proposed approach is superior to conventional Chang's AC (ChangAC).

METHODS

In total, 236 patients who underwent brain-perfusion SPECT were randomly divided into two groups: the training group (189 patients; 80%) and the test group (47 patients; 20%). Two models were constructed using Autoencoder (AutoencoderAC) and U-Net (U-NetAC), respectively. ChangAC, AutoencoderAC, and U-NetAC approaches were compared with CTAC using qualitative analysis (visual evaluation) and quantitative analysis (normalized mean squared error [NMSE] and the percentage error in each brain region). Statistical analyses were performed using the Wilcoxon signed-rank sum test and Bland-Altman analysis.

RESULTS

U-NetAC had the highest visual evaluation score. The NMSE results for the U-NetAC were the lowest, followed by AutoencoderAC and ChangAC (P < 0.001). Bland-Altman analysis showed a fixed bias for ChangAC and AutoencoderAC and a proportional bias for ChangAC. ChangAC underestimated counts by 30-40% in all brain regions. AutoencoderAC and U-NetAC produced mean errors of <1% and maximum errors of 3%, respectively.

CONCLUSION

New deep learning-based AC methods for AutoencoderAC and U-NetAC were developed. Their accuracy was higher than that obtained by ChangAC. U-NetAC exhibited higher qualitative and quantitative accuracy than AutoencoderAC. We generated highly accurate AC images directly from NAC images without the need for intermediate pseudo-CT images. To verify our models' generalizability, external validation is required.

Quantitative SPECT/CT-Technique and Clinical Applications

The continuous development of SPECT over the past 50 years has led to improved image quality and increased diagnostic confidence. The most influential developments include the realization of hybrid SPECT/CT devices, as well as the implementation of attenuation correction and iterative image reconstruction techniques. These developments have led to a preference for SPECT/CT devices over SPECT-only systems and to the widespread adoption of the former, strengthening the role of SPECT/CT as the workhorse of Nuclear Medicine imaging. New trends in the ongoing development of SPECT/CT are diverse. For example, whole-body SPECT/CT images, consisting of acquisitions from multiple consecutive bed positions in the manner of PET/CT, are increasingly performed. Additionally, in recent years, some interesting approaches in detector technology have found their way into commercial products. For example, some SPECT cameras dedicated to specific organs employ semiconductor detectors made of cadmium telluride or cadmium zinc telluride, which have been shown to increase the obtainable image quality by offering a higher sensitivity and energy resolution. However, the advent of quantitative SPECT/CT which, like PET, can quantify the amount of tracer in terms of Bq/mL or as a standardized uptake value could be regarded as most important development. It is a major innovation that will lead to increased diagnostic accuracy and confidence, especially in longitudinal studies and in the monitoring of treatment response. The current work comprises two main aspects. At first, physical and technical fundamentals of SPECT image formation are described and necessary prerequisites of quantitative SPECT/CT are reviewed. Additionally, the typically achievable quantitative accuracy based on reports from the literature is given. Second, an extensive list of studies reporting on clinical applications of quantitative SPECT/CT is provided and reviewed.

(Hybrid) SPECT and PET Technologies

SPECT and PET are nuclear tomographic imaging modalities that visualize functional information based on the accumulation of radioactive tracer molecules. However, SPECT and PET lack anatomical information, which has motivated their combination with an anatomical imaging modality such as CT or MRI. This chapter begins with an overview over the fundamental physics of SPECT and PET followed by a presentation of the respective detector technologies, including detection requirements, principles and different detector concepts. The reader is subsequently provided with an introduction into hybrid imaging concepts, before a dedicated section presents the challenges that arise when hybridizing SPECT or PET with MRI, namely, mutual distortions of the different electromagnetic fields in MRI on the nuclear imaging system and vice versa. The chapter closes with an overview about current hybrid imaging systems of both clinical and preclinical kind. Finally, future developments in hybrid SPECT and PET technology are discussed.

Performance of a dual-layer scanner for hybrid SPECT/CBCT

Fluoroscopic procedures involving radionuclides would benefit from interventional nuclear imaging by obtaining real-time feedback on the activity distribution. We have previously proposed a dual-layer detector that offers such procedural guidance by simultaneous fluoroscopic and nuclear planar imaging. Acquisition of single photon computed tomography (SPECT) and cone beam computed tomography (CBCT) could provide additional information on the activity distribution. This study investigates the feasibility and the image quality of simultaneous SPECT/CBCT, by means of phantom experiments and simulations. Simulations were performed to study the obtained reconstruction quality for (i) clinical SPECT/CT, (ii) a dual-layer scanner configured with optimized hardware, and (iii) our (non-optimized) dual-layer prototype. Experiments on an image quality phantom and an anthropomorphic phantom (including extrahepatic depositions with volumes and activities close to the median values encountered in hepatic radioembolization) were performed with a clinical SPECT/CT scanner and with our dual-layer prototype. Nuclear images were visually and quantitatively evaluated by measuring the tumor/non-tumor (T/N) ratio and contrast-to-noise ratio (CNR). The simulations showed that the maximum obtained CNR was 38.8  ±  0.8 for the clinical scanner, 30.2  ±  0.9 for the optimized dual-layer scanner, and 20.8  ±  0.4 for the prototype scanner. T/N ratio showed a similar decline. The phantom experiments showed that performing simultaneous SPECT/CBCT is feasible. The CNR obtained from the SPECT reconstruction of largest sphere in the image quality phantom was 43.1 for the clinical scanner and 28.6 for the developed prototype scanner. The anthropomorphic phantom showed that the extrahepatic depositions were detected with both scanners. A dual-layer detector is able to simultaneously acquire SPECT and CBCT. Both CNR and T/N ratio are worse than that of a clinical system, but the phantom experiments showed that extrahepatic depositions with volumes and activities close to the median values encountered in hepatic radioembolization could be distinguished.

Radiation dose in nuclear medicine: the hybrid imaging

Hybrid imaging procedures such as single-photon emission computed tomography/computed tomography (SPECT/CT) and positron emission tomography/computed tomography (PET/CT) showed a rapid diffusion in recent years because of their high sensitivity, specificity, and accuracy, due to a more accurate localization and definition of scintigraphic findings. However, hybrid systems inevitably lead to an increase in patient radiation exposure because of the added CT component. Effective doses due to the radiopharmaceuticals can be estimated by multiplying the administered activities by the effective dose coefficients, while for the CT component the dose-length product can be multiplied by a conversion coefficient k. However, the effective dose value is subject to a high degree of uncertainty and must be interpreted as a broad, generic estimate of biologic risk. Although the effective dose can be used to estimate and compare the risk of radiation exposure across multiple imaging techniques, clinicians should be aware that it represents a generic evaluation of the risk derived from a given procedure to a generic model of the human body. It cannot be applied to a single individual and should not be used for epidemiologic studies or the estimation of population risks due to the inherent uncertainties and oversimplifications involved. Practical ways to reduce radiation dose to patients eligible for hybrid imaging involve adjustments to both the planning phase and throughout the execution of the study. These methods include individual justification of radiation exposure, radiopharmaceutical choice, adherence to diagnostic reference levels (DLR), patient hydration and bladder voiding, adoption of new technical devices (sensitive detectors or collimators) with new reconstruction algorithms, and implementation of appropriate CT protocols and exposure parameters.

The role of hybrid bone SPECT/CT imaging in the work-up of the limping patient: a symptom-based and joint-oriented review

A vast spectrum of lower limb bone and joint disorders (hip, knee, ankle, foot) present with a common clinical presentation: limping. Too often this symptom generates an inefficient cascade of imaging studies. This review attempts to optimise the diagnostic effectiveness of bone scintigraphy using the hybrid SPECT/CT technique in relation to the diagnostic clues provided by other imaging modalities, discusses the appropriate clinical indications, optimal scintigraphic procedures and illustrates updated image pattern-oriented reporting. Frequent lower limb bone and joint pathologies that can now be reliably diagnosed using hybrid bone SPECT/CT imaging will be reviewed. Bone SPECT/CT can be an effective problem-solving tool in patients with persistent limping when careful history taking, clinical examination, and first-line imaging modalities fail to identify the underlying cause.

A Prospective Study Comparing 99mTc-Hydroxyethylene-Diphosphonate Planar Bone Scintigraphy and Whole-Body SPECT/CT with 18F-Fluoride PET/CT and 18F-Fluoride PET/MRI for Diagnosing Bone Metastases

We prospectively evaluated and compared the diagnostic performance of ^99mTc-hydroxyethylene-diphosphonate (^99mTc-HDP) planar bone scintigraphy (pBS), ^99mTc-HDP SPECT/CT, ¹⁸F-NaF PET/CT, and ¹⁸F-NaF PET/MRI for the detection of bone metastases. Methods: One hundred seventeen patients with histologically proven malignancy referred for clinical pBS were prospectively enrolled. pBS and whole-body SPECT/CT were performed followed by ¹⁸F-NaF PET/CT within 9 d. ¹⁸F-NaF PET/MRI was also performed in 46 patients. Results: Bone metastases were confirmed in 16 patients and excluded in 101, which was lower than expected. The number of equivocal scans was significantly higher for pBS than for SPECT/CT and PET/CT (18 vs. 5 and 6, respectively; P = 0.004 and 0.01, respectively). When equivocal readings were excluded, no statistically significant difference in sensitivity, specificity, positive predictive value, negative predictive value, or overall accuracy were found when comparing the different imaging techniques. In the per-patient analysis, equivocal scans were either assumed positive for metastases ("pessimistic analysis") or assumed negative for metastases ("optimistic analysis"). The percentages of misdiagnosed patients for the pessimistic analysis were 21%, 15%, 9%, and 7% for pBS, SPECT/CT, PET/CT, and PET/MRI, respectively. Corresponding figures for the optimistic analysis were 9%, 12%, 5%, and 7%. In those patients identified as having bone metastases according to the reference standard, SPECT/CT, ¹⁸F-NaF PET/CT, and PET/MRI detected additional lesions compared with pBS in 31%, 63%, and 71%, respectively. Conclusion:¹⁸F-NaF PET/CT and whole-body SPECT/CT resulted in a significant reduction of equivocal readings compared with pBS, which implies an improved diagnostic confidence. However, the clinical benefit of using, for example, ¹⁸F-NaF PET/CT or PET/MRI as compared with SPECT/CT and pBS in this patient population with a relatively low prevalence of bone metastases (14%) is likely limited. This conclusion is influenced by the low prevalence of patients with osseous metastases. There may well be significant differences in the sensitivity of SPECT/CT, PET/CT, and PET/MRI compared with pBS, but a larger patient population or a patient population with a higher prevalence of bone metastases would have to be studied to demonstrate this.

Performance of 3DOSEM and MAP algorithms for reconstructing low count SPECT acquisitions

PURPOSE

Low count single photon emission computed tomography (SPECT) is becoming more important in view of whole body SPECT and reduction of radiation dose. In this study, we investigated the performance of several 3D ordered subset expectation maximization (3DOSEM) and maximum a posteriori (MAP) algorithms for reconstructing low count SPECT images.

MATERIALS AND METHODS

Phantom experiments were conducted using the National Electrical Manufacturers Association (NEMA) NU2 image quality (IQ) phantom. The background compartment of the phantom was filled with varying concentrations of pertechnetate and indiumchloride, simulating various clinical imaging conditions. Images were acquired using a hybrid SPECT/CT scanner and reconstructed with 3DOSEM and MAP reconstruction algorithms implemented in Siemens Syngo MI.SPECT (Flash3D) and Hermes Hybrid Recon Oncology (Hyrid Recon 3DOSEM and MAP). Image analysis was performed by calculating the contrast recovery coefficient (CRC),percentage background variability (N%), and contrast-to-noise ratio (CNR), defined as the ratio between CRC and N%. Furthermore, image distortion is characterized by calculating the aspect ratio (AR) of ellipses fitted to the hot spheres. Additionally, the performance of these algorithms to reconstruct clinical images was investigated.

RESULTS

Images reconstructed with 3DOSEM algorithms demonstrated superior image quality in terms of contrast and resolution recovery when compared to images reconstructed with filtered-back-projection (FBP), OSEM and 2DOSEM. However, occurrence of correlated noise patterns and image distortions significantly deteriorated the quality of 3DOSEM reconstructed images. The mean AR for the 37, 28, 22, and 17mm spheres was 1.3, 1.3, 1.6, and 1.7 respectively. The mean N% increase in high and low count Flash3D and Hybrid Recon 3DOSEM from 5.9% and 4.0% to 11.1% and 9.0%, respectively. Similarly, the mean CNR decreased in high and low count Flash3D and Hybrid Recon 3DOSEM from 8.7 and 8.8 to 3.6 and 4.2, respectively. Regularization with smoothing priors could suppress these noise patterns at the cost of reduced image contrast. The mean N% was 6.4% and 6.8% for low count QSP and MRP MAP reconstructed images. Alternatively, regularization with an anatomical Bowhser prior resulted in sharp images with high contrast, limited image distortion, and low N% of 8.3% in low count images, although some image artifacts did occur. Analysis of clinical images suggested that the same effects occur in clinical imaging.

CONCLUSION

Image quality of low count SPECT acquisitions reconstructed with modern 3DOSEM algorithms is deteriorated by the occurrence of correlated noise patterns and image distortions. The artifacts observed in the phantom experiments can also occur in clinical imaging.

Synthesis and Evaluation of a Radioiodinated Tracer with Specificity for Poly(ADP-ribose) Polymerase-1 (PARP-1) in Vivo

Interest in nuclear imaging of poly(ADP-ribose) polymerase-1 (PARP-1) has grown in recent years due to the ability of PARP-1 to act as a biomarker for glioblastoma and increased clinical use of PARP-1 inhibitors. This study reports the identification of a lead iodinated analog 5 of the clinical PARP-1 inhibitor olaparib as a potential single-photon emission computed tomography (SPECT) imaging agent. Compound 5 was shown to be a potent PARP-1 inhibitor in cell-free and cellular assays, and it exhibited mouse plasma stability but approximately 3-fold greater intrinsic clearance when compared to olaparib. An (123)I-labeled version of 5 was generated using solid state halogen exchange methodology. Ex vivo biodistribution studies of [(123)I]5 in mice bearing subcutaneous glioblastoma xenografts revealed that the tracer had the ability to be retained in tumor tissue and bind to PARP-1 with specificity. These findings support further investigations of [(123)I]5 as a noninvasive PARP-1 SPECT imaging agent.