Whole-Body SPECT/CT: Protocol Variation and Technical Consideration—A Narrative Review

Introducing a hybrid imaging approach, such as single-photon emission computerized tomography with X-ray computed tomography (SPECT)/CT, improves diagnostic accuracy and patient management. The ongoing advancement of SPECT hardware and software has resulted in the clinical application of novel approaches. For example, whole-body SPECT/CT (WB-SPECT/CT) studies cover multiple consecutive bed positions, similar to positron emission tomography-computed tomography (PET/CT). WB-SPECT/CT proves to be a helpful tool for evaluating bone metastases (BM), reducing equivocal findings, and enhancing user confidence, displaying effective performance in contrast to planar bone scintigraphy (PBS). Consequently, it is increasingly utilized and might substitute PBS, which leads to new questions and issues concerning the acquisition protocol, patient imaging time, and workflow process. Therefore, this review highlights various aspects of WB-SPECT/CT acquisition protocols that need to be considered to help understand WB-SPECT/CT workflow processes and optimize imaging protocols.


Introduction
The last two decades have witnessed the emergence and rapid clinical adoption of hybrid positron emission tomography with computed tomography (PET/CT) over PETonly devices.The primary driver of this adoption is the ability of PET/CT to generate functional and morphologic correlative images that can enhance diagnostic accuracy [1].Similar advancements are also being observed for single-photon emission tomography with CT (SPECT/CT), an approach that is fast transitioning from a rather underutilized technological option to a recognized position for enhancing the diagnostic competencies of single-photon imaging, with possible implications for the management of patients [2].The usage of hybrid imaging techniques, including PET/CT, PETs with magnetic resonance imaging (PET/MR) and SPECT/CT are rapidly growing.They provide three-dimensional (3D) functional and anatomical information, which improves sensitivity and specificity compared to planar bone scintigraphy (PBS) [3].In addition, the use of transmissionderived anatomical data enables the modeling and correction of certain radiation transport events, such as the attenuation of photons, to be incorporated into the reconstruction algorithm for improvements in both qualitative and quantitative data.Advances in imaging technologies are aimed at improving diagnostic accuracy and confidence in clinical practice.For example, total-body (TB) PET, with its large axial field of view, enables the imaging of the entire human body simultaneously.Also, using time of flight (TOF) PET improves lesion detection and image quantification, and reduces scanning time [4].Furthermore, SPECT quantification is becoming practicable in multi-center and multi-vendor [5,6].
Recently, the novel approach of whole-body (WB) SPECT/CT has emerged, encompassing numerous successive bed positions equivalent to the acquisition of PET/CT [2].
Clinical evidence suggests that WB-SPECT/CT is more effective than planar bone scintigraphy (PBS) to evaluate bone metastasis (BM) and investigate suspicious lesions [7][8][9].Consequently, WB-SPECT/CT may substitute PBS scintigraphy.Nevertheless, its lengthy scan time and the non-uniform protocols used in bone procedures are considered new challenges and concerns [2,10].At the same time, the majority of commercial vendors offer improved reconstruction software that speeds up SPECT acquisition while keeping similar image quality, allowing for multi-field of view (FOV) SPECT/CT acquisitions [7,11].Therefore, this article aims to investigate the acquisition protocols and types of reconstruction utilized for WB-SPECT/CT imaging, which can then inform the possibility of optimizing acquisition protocols to reduce scanning time while maintaining image quality.

Advantages and Pitfalls
SPECT/CT has several advantages over PBS images, the most significant of which are enhanced attenuation correction, increased specificity, localized metabolic abnormalities to anatomical structures, and potential impacts on adjacent tissues.Moreover, when using SPECT/CT, it has been shown that the location and characteristics of endocrine and neuroendocrine tumors can be accurately determined, as well as solitary pulmonary nodules, lung cancers, lymphomas, brain tumors, and prostate cancers, as well as benign bone lesions, malignant bone lesions, and localized infection [12,13].
The widespread adoption and application of SPECT/CT in clinical practice is a further advantage of its use.However, the time required to obtain the entire axial skeleton to be imaged is a significant limitation that must be considered.This can cause significant patient discomfort and reduce departmental output.Long scan times can increase patient movement, which is especially problematic when acquiring multiple bed positions [10].In addition, the required scan range will cause the CT radiation dosage to rise.
However, several studies show that using low-dose CT methods can reduce radiation exposure to an acceptable level, particularly when incorporating vendor software for automated exposure control.A study by Löfgren et al. [14] stated that the approximate radiation exposure is 4-5 mSv from a low-dose CT incorporated in the WB-SPECT/CT examination from the head to the mid-thigh.Other studies reported that the estimated radiation exposure for CT from the skull to the proximal femurs is 4.2 ± 1.0 mSv using ImPACT CT Patients Dosimetry Calculator (version 1.0.4,Imaging Performance Assessment on Computed tomography, www.impactscan.org,accessed on 21 June 2024)) [15,16].Meanwhile, Fleury et al. [8] suggest that irradiation caused by morphologically modulated, low-dose CT performed for WB-SPECT/CT can be considered acceptable with an effective dose of around 6 mSv.

WB-SPECT/CT Acquisition Protocols and Technical Considerations
Acquisition protocols and reconstruction parameters of the WB-SPECT/CT studies published in the literature are summarized and listed in Table 1, including the scanner type, radiotracer, administration dose, and body range scanned.The SPECT/CT scanners used in the studies varied in model, including thirteen Siemens (Symbia T2, T6, T16, E-CAM and Intevo 6), nine GE Healthcare (Hawkeye 4 Infinia or Varicam and Discovery NM/CT 670) and two Precedence models (Philips Healthcare).There were nine studies that reported using hydroxy diphosphonate/methylene (HDP/HMDP) and seven using MDP, with one exceptional study that used diphosphono-1,2-propanodicarboxylic acid (DPD) [17].The administration dose range was 523-1000 MBq with a mean ± standard deviation value of 735.5 ± 120.A recent study stated that the radiation effective dose caused by the 99mTc-HMDP scintigraphy is 3.8 mSv [18].This number is close to the diagnostic reference levels DRLs (3.82 mSv) considering the mean injected activity is below the ICRP limit (750 MBq) [19].Nonetheless, the effective dose of this dose is largely determined by the patient's weight and the type of radiopharmaceutical administered.SPECT/CT imaging protocols included two or more axial FOV tomographic acquisitions that covered the skull vertex to the mid-thigh or the cervical region to the proximal femur.The gamma cameras employed in many studies had an axial FOV of 38.7 cm, which corresponds to a total axial FOV of 112 cm for three SPECT acquisitions, including a 2 cm overlap for sequential bed positions [14][15][16].Many studies suggested that three bed positions from the vertex to the mid femur with around 112 cm would be sufficient unless otherwise clinically required in specific cases.Indeed, WB-SPECT/CT significantly increased the number of observed metastases that could have been missed on a PBS [7,8,20,21,24].Moreover, as shown in Figure 1, the study found that lesion detection using SPECT/CT was improved for lesions situated in the lower thoracic and lumbar vertebral column [31][32][33].Additionally, it is argued by Even-Sapir et al. [7] that the performance of WB-SPECT/CT on the whole axial skeleton enables metastases to be detected in other areas, including the skull, rib cage, upper vertebral column, long bones, and pelvis (see Figure 2).
overlap for sequential bed positions [14][15][16].Many studies suggested that three bed posi tions from the vertex to the mid femur with around 112 cm would be sufficient unles otherwise clinically required in specific cases.Indeed, WB-SPECT/CT significantly in creased the number of observed metastases that could have been missed on a PBS [7,8,20,21,24].Moreover, as shown in Figure 1, the study found that lesion detection using SPECT/CT was improved for lesions situated in the lower thoracic and lumbar vertebra column [31][32][33].Additionally, it is argued by Even-Sapir et al. [7] that the performance o WB-SPECT/CT on the whole axial skeleton enables metastases to be detected in other ar eas, including the skull, rib cage, upper vertebral column, long bones, and pelvis (see Fig ure 2).
When interpreting WB-SPECT/CT, reporting physicians must be mindful of the pos sibility of artefacts.Multi-FOV SPECT can sometimes overlap to smooth the transition and this overlap is automatically pasted together by image reconstruction software.Some times a slight zip line can be seen at the fusion site.Usually, this does not affect interpre tation so long as the patient remains still.However, if the patient moves, this can be inad vertently overlooked (note the overlooked rib fracture in the WB-SPECT study presented in Figure 3 [22].It is also better if the injection site is outside the FOV, as this can eliminat the reconstruction artefacts caused by interstitial injections.When interpreting WB-SPECT/CT, reporting physicians must be mindful of the possibility of artefacts.Multi-FOV SPECT can sometimes overlap to smooth the transition, and this overlap is automatically pasted together by image reconstruction software.Sometimes a slight zip line can be seen at the fusion site.Usually, this does not affect interpretation so long as the patient remains still.However, if the patient moves, this can be inadvertently overlooked (note the overlooked rib fracture in the WB-SPECT study presented in Figure 3 [22].It is also better if the injection site is outside the FOV, as this can eliminate the reconstruction artefacts caused by interstitial injections.The acquisition times for WB-SPECT/CT protocol for the GE healthcare system, Siemens Medical Solution, and the Philips Healthcare system ranged from 22 to 75 min for total examination time incorporating the use of low-energy high-resolution (LEHR) collimators.However, the number of projection angles were varied, as demonstrated in Table 1.The majority of the articles used ordered subset expectation maximization (OSEM) with distance-dependent resolution modeling and RR for the image reconstructions (Table 2).The acquisition times for WB-SPECT/CT protocol for the GE healthcare system, Siemens Medical Solution, and the Philips Healthcare system ranged from 22 to 75 min for total examination time incorporating the use of low-energy high-resolution (LEHR) collimators.However, the number of projection angles were varied, as demonstrated in Table 1.The majority of the articles used ordered subset expectation maximization (OSEM) with distance-dependent resolution modeling and RR for the image reconstructions (Table 2).The acquisition times for WB-SPECT/CT protocol for the GE healthcare system, Siemens Medical Solution, and the Philips Healthcare system ranged from 22 to 75 min for total examination time incorporating the use of low-energy high-resolution (LEHR) collimators.However, the number of projection angles were varied, as demonstrated in Table 1.The majority of the articles used ordered subset expectation maximization (OSEM) with distance-dependent resolution modeling and RR for the image reconstructions (Table 2).The OSEM algorithm is a modified version of the maximum likelihood expectation maximization (MLEM) strategy that partitions the collected projection data into subsets to minimize computational load [34].Even though different effects can be modeled using MLEM (i.e., scatter elimination, system response, and photon attenuation correction), the primary issue with the MLEM algorithm is that as the number of iterations increases, image noise increases, which is even more prominent in low-count acquisitions [35].Meanwhile, in resolution recovery (RR) methods, the three-dimensional collimator response in image production is taken into account, which reduces the impact of the point-spread function (PSF) on image resolution.This method can thus be extremely useful when noise reduction is essential for ensuring image quality in low-count studies.However, such correction shaves been known to introduce Gibbs, or ringing, artefacts in the reconstructed data around areas of high contrast.Notably, the RR OSEM reconstruction approach applies collimator detector response (CDR) to reconstructed SPECT images by incorporating intrinsic response, geometric penetration, septal scatter, and penetration components [36].
Image noise in CT, which degrades CT image quality, is inversely related to the Xray beam energy and increases as tube current or tube voltage decreases.The CT X-ray tube current-time product, as reported in the previously quoted WB-SPECT/CT articles, ranged from 2.5 to 100 mAs.Nonetheless, the literature evidence suggests that low-tube current-time products ranging between 2.5 to 30 mAs can be used to decrease radiation dose while maintaining adequate CT image quality for attenuation correction purposes and basic anatomical localization [37].Palmedo et al. [21] reported that using a low currenttime product of 2.5 mAs resulted in an effective dose of 0.4 mSv and caused a significant improvement in diagnostic precision and patient management without significantly increasing the overall radiation dosage to the patient.In addition, to minimize radiation exposure, Rager et al. [15] recommend reducing the tube current to 40 mAs and activating the intensity modulation tool (CARE Dose 4D, Siemens Healthcare).

Evidence on the Role of WB-SPECT/CT in Imaging of Bone Metastases
There is published literature evaluating WB-SPECT/CT's role in detecting BM on a per-patient or per-lesion basis compared to PBS.WB-SPECT/CT had a superior accuracy in BM detection and more reliable performance with less equivocal results (2.8% vs. 20%).(See the review for more details) [9].Abikhzer et al. [22], in a retrospective study, demonstrated the clinical utility of WB-SPECT compared to PBS for the BM detectability in breast cancer patients.In this study, WB-SPECT lesion detection (100%) was not statistically different from PBS (97%) performance in a patient-based analysis.However, statistically significant differences were seen in a lesion-based analysis with PBS detection (80%), falling well below WB-SPECT (100%) outcomes (p < 0.001).Furthermore, confidence levels were higher for WB-SPECT in comparison with PBS for both benign (p < 0.01) and malignant (p < 0.05) lesions.
Higher diagnostic confidence is gained with WB-SPECT/CT when compared to PBS.The diagnostic confidence of WB-SPECT/CT in breast and prostate cancer patients was supported by the reported significant reduction in suspicious findings characterization (2.5%) compared to PBS (28.7%) [21].Moreover, the anatomical site of an equivocal lesion was not truly indicated on PBS and/or SPECT images but was on WB-SPECT/CT images; through WB-SPECT/CT, new tumor sites were identified and false-positive lesions were identified as true-negatives on PBS and/or SPECT images.The most commonly identified lesions were located in the spine (448) and pelvis (211).However, a significant number of lesions were found in other skeletal areas including the ribs, shoulders, sternum, femora, and cranium (180) [21].Furthermore, among 80 patients with various primary cancers, 151 indeterminate lesions on PBS were characterized in WB-SPECT/CT.The improvement was mostly (135 lesions) located in the axial skeletal [26].Therefore, these outcomes demonstrate the usefulness of WB-SPECT/CT imaging in the detection of axial skeletal lesions.
In another work by Guezennec et al. [17], the authors reached an agreement with a previous published study by Palmedo et al. [21], concluding that the testing of the acquisition protocol of WB-SPECT/CT for the initial staging of patients with cancer had limited incremental diagnostic value compared to the commonly used PBS and targeted SPECT/CT strategy in regard to specificity.However, the tested protocol showed slightly enhanced sensitivity in unseen lesions on PBS, which could be advantageous in initial staging activities, thereby supporting the clinical utility of the addition of SPECT/CT to PBS.
Furthermore, research involving 194 patients with prostate carcinoma showed that, in patient-based analysis, the diagnostic accuracy for the WB-SPECT/CT examinations were higher than for PBS, particularly in patients who had serum PSA levels under 40 ng/mL.Nonetheless, there was no difference in the sensitivity of PBS and WB-SPECT/CT in patients with PSA levels higher than 40 ng/mL.Thus, carrying out multi-FOV SPECT/CT acquisitions on prostate carcinoma patients with PSA levels under 40 ng/mL could allow a reduction in the gamma camera workload whilst maintaining the diagnostic advantages of multi-FOV SPECT/CT in challenging patients [20].Additionally, SPECT/CT will be useful in cases where CT is insufficient, especially in distinguishing between BM and early avascular necrosis [38].

WB-SPECT/CT vs. PET/CT
PET/CT has been used with different tracers for the detection of breast and prostate cancer metastases, such as Fluorine-18 fluorodeoxyglucose (18F-FDG) and sodium fluoride (18F-NaF).In particular, 18F-NaF PET/CT is superior to the SPECT/CT [24] but, with regard to 18F-FDG PET/CT, a study by Rager et al. [16] on breast cancer patients demonstrated, in a lesion-based analysis, a higher diagnostic accuracy of WB-SPECT/CT (p = 0.004) during follow-up (n = 21 months).In contrast, WB-SPECT/CT should be used in conjunction with 18F-FDG PET/CT to identify non-bone extra-axillary nodal involvement and distant metastases.
In the study by Dyrberg et al. [39], a prospective study of BM detection in prostate cancer patients, all three nuclear medicine methods (WB-SPECT/CT, NaF-PET/CT, and choline-PET/CT) and WB-MRI performed similarly in all patient-based diagnostic measures with WB-SPECT/CT sensitivity (100%), specificity (91%), and accuracy (94%) reporting higher than WB-MRI at 89%, 90%, and 90%, respectively.Like Jambor et al. [24], Dyrberg et al.'s [39] protocol used 99mTc-HDP in a similar coverage area but at a slightly higher average dose (717 MBq) and in a different cancer type target.In the study by Dyrberg et al. [39], total acquisition time (75 min) was more than double that of Jambor et al. [24] (30 min).Also, WB-SPECT/CT specificity (88%) fell by 6% in patients receiving androgendeprivation therapy (ADT), which was 12% lower than SPECT/CT specificity performance in Fonager et al.'s study [25],which may be explained by study sample differences, although both sensitivity and specificity were statistically similar in both studies.
Whilst PET has superior accuracy and spatial resolution over SPECT, which may play a role in the detection of small lesions, cost-effectiveness and availability are major considerations in modality choice when comparing both modalities.The effective dose to the patient from 18F-NaF PET/CT varies with the administered activity.Current guidelines suggest a dose range of 185-370 MBq [41].An effective dose of 10 mSv is associated with 370 MBq of 18F-NaF, which is higher than the 5.3 mSv dose from 925 MBq of 99mTc-MDP [42].However, the CT dose in PET/CT and SPECT/CT systems is approximately equivalent over the same axial range [14].Thus, the difference in radiation dose is attributed solely to the radiopharmaceuticals used.However, given the high bone-to-background ratio (i.e., high signal-to-noise ratio) of 18F-NaF, it is possible to reduce the injected activity (e.g., by approximately 50%) without significantly compromising image quality.This reduction would lower the effective dose to a level comparable to that of 99mTc-MDP [43].

Developments in WB-SPECT/CT Protocol
Iterative reconstruction methods using correction for attenuation, scatter, and CDR can shorten bone SPECT acquisition times without negatively affecting image quality or diagnostic accuracy.Study findings support the use of a WB-SPECT-based protocol to improve existing bone scan best practices, including the possible justified elimination of initial PBS [22].Another study by Even-Sapir et al. [7] also reported similar advantages of using CDR.They reported that the total acquisition time of multi-FOV SPECT imaging is within 24-32 min using CDR, by which a quantitative model of the CDR function of the acquisition system is integrated into the reconstruction algorithm in order to minimize collimator detection blur effects and therefore enhance image resolution and signal-tonoise characteristics.Furthermore, iterative reconstruction techniques integrating RR have recently been introduced, which have the potential to minimize imaging times while maintaining image quality.Reconstruction with RR attempts to correct characteristics of the imaging system that act to degrade the final image (primarily collimator geometry and detector response), thereby producing reconstructed images that show improved image quality compared with conventional SPECT [44,45].
Due to recent advances in technology, new SPECT/CT machines have been developed that can sequentially cover and accurately merge multiple bed positions.It was noted that studies used gamma camera systems, and specially-designed WB-SPECT software covering all of the SPECT fields was employed to scan the whole SPECT/CT area from the cervical region to the proximal femora [20,21].In addition, many new methods are being continuously developed for SPECT but have not yet been integrated into standard clinical practice.Examples include cadmium-zinc-telluride (CZT) detectors, list-mode SPECT acquisitions, multi-pinhole collimators, novel non-parallel hole collimators, and radiopharmaceutical quantification.It is anticipated that deep learning techniques will be utilized in nuclear medicine imaging in the future [46].Efforts should be increased to standardize acquisition protocols.
While multiple protocols exist for SPECT/CT, including both targeted SPECT/CT and WB-SPECT/CT, the literature synthesis indicates the importance of WB-SPECT/CT to augment PBS, as it can reduce equivocal findings and improve user confidence [2,21,26,28].In addition, since many routine bone procedures involve planar plus one or two bed SPECT/CT over the area of interest, the application of fast WB-SPECT/CT is likely to enhance patient comfort, increase throughput, and decrease motion artefact [2].Therefore, the following question must be asked: can fast WB-SPECT/CT replace PBS in cancer patients?Recently, manufacturers such as GE and Spectrum Dynamics introduced a new SPECT-only system design that can enable WB imaging in a short acquisition time with enhanced tomographic spatial resolution [47].However, the current conventional gamma camera system will continue to be the "workhorse" for some years, and we need WB-SPECT even though not all departments will have one of these new scanners.Therefore, optimizing the acquisition protocol to reduce acquisition time while maintaining image quality is required.
Molecular imaging in oncology has generally shifted away from the utilization of SPECT/CT toward PET/CT due to the better resolution, sensitivity, quantification, and lesion contrast [42,[48][49][50].However, imaging with single-photon emitting nuclides continues to be widely used, providing affordable and clinically relevant information in oncologic and non-oncologic indications [51][52][53][54][55].In addition, continuous advancements in gamma camera technology, reconstruction, and detector system modeling have also enabled quantitative SPECT imaging [56].A recent article "Quantitative SPECT/CT: SPECT joins PET as a quantitative imaging modality" by Bailey et al. [57] highlights the potential for clinical applications of quantitative measurements utilizing SPECT/CT following the widespread adoption of SPECT/CT systems.The authors have demonstrated the advantages of quantitative SPECT over PET, including the ability to perform multi-tracer studies simultaneously using various radionuclides, and the longer physical half-lives of radionuclides which allow for the imaging of extended biological processes.Dickson et al. [5] revealed that quantitative SPECT provides meaningful metrics for longitudinal assessments of disease progression and post-radionuclide therapy dosimetry, but its inter-patient use cases require further evaluation to determine clinical value.Another review on quantitative SPECT, "Does quantification have a role to play in the future of bone SPECT"?authored by Ross et al. [58] stated that the availability of WB-SPECT/CT for quantitative assessments might provide additional insight into extent of disease before and after radionuclide therapy.As a result, SPECT-based quantitative imaging has gained increased significance in clinical practice.
Recent advancements in detector technology, particularly the introduction of CZT detectors, are enabling more widespread WB-SPECT imaging [59].CZT technology offers several advantages over traditional sodium iodide (NaI) scintillation crystals.Firstly, CZT detectors provide superior energy resolution, typically in the range of 5-6%, compared to 9-10% for NaI crystals.This enhancement allows for better discrimination between primary photons and scattered radiation, resulting in improved image contrast and reduced noise.Secondly, CZT detectors offer higher sensitivity through the direct conversion of gamma rays to electrical signals, eliminating the need for photomultiplier tubes.This improvement leads to increased detection efficiency and sensitivity, potentially enabling the use of lower injected doses or shorter acquisition times.The pixelated nature of CZT detectors also contributes to improved intrinsic spatial resolution, further enhancing image quality [60,61].
SPECT systems utilizing CZT have demonstrated the ability to perform WB-SPECT/CT in under 20 min, as reported by Gregoire et al. [62] and Arvola et al. [63].The enhanced count rate capabilities of CZT detectors make dynamic SPECT imaging more feasible, opening new possibilities for functional imaging studies [64].Moreover, the superior energy resolution of CZT detectors allows for better separation of different isotopes, facilitating more accurate simultaneous multi-isotope imaging [65].Finally, CZT technology, when combined with advanced reconstruction algorithms, improves the quantitative accuracy of SPECT imaging, bringing it closer to PET in terms of quantitative capabilities.
As CZT technology becomes more widely available, it is expected to have a significant impact on clinical protocols.The potential for faster acquisitions could improve patient comfort and increase throughput, while the ability to reduce injected doses aligns well with the as low as reasonably achievable (ALARA) principle in radiation protection.Future research should focus on optimizing acquisition protocols for CZT-based WB-SPECT/CT systems and evaluating their impact on diagnostic accuracy and patient outcomes in various clinical scenarios.
The generation of synthetic planar images from WB-SPECT data may increase the adoption of WB-SPECT and facilitate the transition from PBS.This approach enables the creation of 2D images without requiring additional time for planar acquisition and allows for multiple planar views through a full 360 • rotation, offering a more accurate representation of SPECT images compared to the current maximum intensity projection (MIP) method [66].Many studies have investigated the use of synthetic planar images from SPECT data, particularly in lung imaging.These investigations have focused on novel algorithmic methods [67] or new camera types, such as CZT detectors [59].In a prospective study, synthetic images were generated from resampled SPECT data using list mode.Angular-summed and reprojected planar images were produced by summing projections over a 12 • angular range for multiple perspectives and constructing a 3D synthesized attenuation map of the lungs and torso.This attenuation map was used to generate views corresponding to traditional planar imaging, with in-house software resampling and formatting of the images to match the pixel size of true planar scans [68].The results demonstrated that R-PBS significantly enhances the reading of SPECT/CT, making this technique valuable in providing 2D images to aid clinicians, especially during the transition to 3D-only data due to their historical familiarity with 2D images.
Based on a thorough literature review and considering the balance between image quality and acquisition time, we recommend the following for an optimal WB-SPECT/CT protocol: The optimization of reconstruction parameters requires a careful trade-off between noise levels and acquisition time.Our recent study suggests that rapid scanning, approaching 8 s per bed position with a low number of iterations (4-8) and 8 subsets, could be feasible and provides an optimal balance between acquisition time and image quality for WB bone SPECT/CT [69].

Future Work
WB-SPECT/CT can reduce equivocal findings, enhance user confidence, and potentially improve patient management.However, longer scan times negatively affects patient throughput and results in greater patient motion during the study due to reduced patient comfort and compliance.Nevertheless, there is a variation in recommendations and findings because there is a lack of standardized guidance for WB-SPECT/CT bone procedures.Hence, it is suggested that future research should focus on WB-SPECT/CT to assess acquisition and reconstructions by specialist review and comparison.This could be achieved by using a gated dataset along with an ECG trigger or list mode and proportion of the time-bin reconstruction for the clinical optimization of WB-SPECT/CT acquisition and achievement of equilibrium between image quality and time.Such an approach could permit the data and generation of acquisitions that have any percentage of counts that belong to the full-time dataset without increasing the number of necessary scans.Additionally, ensuring the reproducibility of these examinations across different SPECT/CT devices would be an interesting exercise to confirm that the proposed approach applies to any SPECT/CT scanner.

Conclusions
WB-SPECT/CT appears to be a promising imaging method for BM detection.Standardization of protocols and additional research are required to optimize the impact of such technology in clinical practice.

Figure 1 .
Figure 1.Patient with PBS Sc 3 and WB-SPECT/CT Sc 1 proved as a true positive by follow-up.In 79-year-old female with newly diagnosed breast cancer with involved lymph node (T1N1), PBS in ventral (A) and dorsal (B) projections did not reveal any suspicious bone lesion.Right iliac aisl uptake (red arrow) was seen on (C) trunk SPECT/CT MIP (maximum intensity projection) and (D fusion SPECT/CT with a sclerosis lesion observed on CT (E).Bone metastasis was confirmed usin MRI and fat-saturated T1 and T1 gadolinium (F,G) and clinical follow-up [8].

Figure 1 .
Figure 1.Patient with PBS Sc 3 and WB-SPECT/CT Sc 1 proved as a true positive by follow-up.In a 79-year-old female with newly diagnosed breast cancer with involved lymph node (T1N1), PBS in ventral (A) and dorsal (B) projections did not reveal any suspicious bone lesion.Right iliac aisle uptake (red arrow) was seen on (C) trunk SPECT/CT MIP (maximum intensity projection) and (D) fusion SPECT/CT with a sclerosis lesion observed on CT (E).Bone metastasis was confirmed using MRI and fat-saturated T1 and T1 gadolinium (F,G) and clinical follow-up [8].

Figure 2 .
Figure 2. Early metastatic dissemination that has been on planar BS for a 57-year-old patient with prostate cancer.Image (A) shows the posterior and anterior planar BS, (B) shows the multi-FOV SPECT.(C) PBS was found to be negative on SPECT (osteoblastic rib metastasis).(D) Osteoblastic metastasis in skull on SPECT (metastasis marked by arrowhead) [7].

Figure 3 .
Figure 3. Planar BS image showing benign rib has been missed on WB-SPECT due to the presence of a reconstruction artefact (a).Posterior PBS highlights focal tracer uptake in the right 6th rib and spinous process of L4 fractures (arrowheads).This image is of a breast cancer and renal osteodystrophy patient.The rib finding was classified as an artefact on WB-SPECT ((b) MIP, (c) selected sagittal, and (d) coronal slices, arrowheads) due to SPECT reconstruction artefact at the overlap of the first and second SPECT FOV (arrows) caused by slight patient movement with subsequent ill-defined activity at the level of the zip artefact that could not be prospectively characterized as a true lesion.BS, bone scintigraphy; FOV, field of view; MIP, maximum intensity projection; WB-SPECT, wholebody single-photon emission computed tomography [22].

Figure 2 .Figure 2 .
Figure 2. Early metastatic dissemination that has been on planar BS for a 57-year-old patient with prostate cancer.Image (A) shows the posterior and anterior planar BS, (B) shows the multi-FOV SPECT.(C) PBS was found to be negative on SPECT (osteoblastic rib metastasis).(D) Osteoblastic metastasis in skull on SPECT (metastasis marked by arrowhead) [7].

Figure 3 .
Figure 3. Planar BS image showing benign rib has been missed on WB-SPECT due to the presence of a reconstruction artefact (a).Posterior PBS highlights focal tracer uptake in the right 6th rib and spinous process of L4 fractures (arrowheads).This image is of a breast cancer and renal osteodystrophy patient.The rib finding was classified as an artefact on WB-SPECT ((b) MIP, (c) selected sagittal, and (d) coronal slices, arrowheads) due to SPECT reconstruction artefact at the overlap of the first and second SPECT FOV (arrows) caused by slight patient movement with subsequent ill-defined activity at the level of the zip artefact that could not be prospectively characterized as a true lesion.BS, bone scintigraphy; FOV, field of view; MIP, maximum intensity projection; WB-SPECT, wholebody single-photon emission computed tomography [22].

Figure 3 .
Figure 3. Planar BS image showing benign rib has been missed on WB-SPECT due to the presence of a reconstruction artefact (a).Posterior PBS highlights focal tracer uptake in the right 6th rib and spinous process of L4 fractures (arrowheads).This image is of a breast cancer and renal osteodystrophy patient.The rib finding was classified as an artefact on WB-SPECT ((b) MIP, (c) selected sagittal, and (d) coronal slices, arrowheads) due to SPECT reconstruction artefact at the overlap of the first and second SPECT FOV (arrows) caused by slight patient movement with subsequent ill-defined activity at the level of the zip artefact that could not be prospectively characterized as a true lesion.BS, bone scintigraphy; FOV, field of view; MIP, maximum intensity projection; WB-SPECT, whole-body single-photon emission computed tomography [22].

Table 1 .
Summary of acquisition protocol of WB-SPECT/CT.
The purpose was to obtain quantitative measures (SUVs) for both WB-SPECT/CT and 18F-NaF PET/CT.The strong correlation between SUVs of 99m Tc-HDP WB-SPECT/CT and 18F-NaF PET/CT indicates that WB-SPECT is an effective tool for clinical quantification of patients with breast or prostate cancer.PSA, prostate specific antigen; SUV; standard uptake value; matrix size applied for all studies is 128 × 128.

Table 2 .
A summary of the recommended optimal acquisition protocol for WB-SPECT/CT.