Gopinath Gnanasegaran, MD

Multimodality imaging is rapidly becoming an essential tool, particularly in oncology, where many publications have focused on a role for PET/CT. Dedicated multislice SPECT/CT is a new addition to the diagnostic armamentarium in radionuclide imaging.

Several early reports on fusion imaging addressed the benefits of combining SPECT with CT. These reports related to data acquired on separate cameras at different time points, brought together with software-based fusion. As a concept, software fusion was fascinating, but in practice the process was cumbersome and time-consuming, prone to error, and not user-friendly.

A number of dedicated SPECT/CT systems incorporating multislice CT are now commercially available. This relatively new imaging modality allows both sets of data to be acquired with the patient in the same position. Data on distribution and accumulation of the radiotracer used in the SPECT scan can be fused directly with the anatomical information shown in the corresponding area on CT.

SPECT/CT is becoming increasingly popular, spurred on by publications showing the diagnostic benefits of this modality.

 The addition of SPECT to planar imaging increases sensitivity and specificity, though accurate localization and characterization are often difficult. Radionuclide imaging can, however, demonstrate pathology, such as fractures or malignant lesions, at an early stage, before the abnormalities can be seen on anatomical images. Importantly, a single SPECT/CT scan will provide both functional and anatomical information.

SPECT/CT examinations generally focus on a specific area or region of interest and are usually performed after conventional whole-body radionuclide scintigraphy. The patient should remain in the same position for both the SPECT and CT scans. Any change in position, orientation, or physiological status can lead to problems with misregistration. Specialized software reconstructs the CT and SPECT data sets, adjusts for differences in format and scanner geometry, and then fuses the two results into a single image (Figure 1).

Usually a colored SPECT image is overlaid on a monochrome CT. Merging anatomical information from CT with functional or molecular information from SPECT makes it easier to identify the position/location of any functional lesions. The CT data may also be used to correct for photon attenuation within the patient, allowing the appearance of radionuclide uptake seen deep within the patient to be on a par with any uptake seen near the skin's surface. CT images can be considered very high photon flux transmission scans and represent a highly detailed map of tissue attenuation. The high photon flux from the CT transmission scan means that the additional contribution of detected photons from the SPECT radionuclide is negligible. The high photon flux from the CT also leads to a high total detected photon count, which reduces the statistical noise compared with the low photon count rate from the more traditional methods of transmission scanning that use radionuclides, such as gadolinium- 153, for attenuation correction.

Adding multislice CT to a SPECT study leads to a substantial increase in the scan time from SPECT alone or SPECT with traditional (Gd-153) transmission scans, due to patient preparation and positioning. The radiation dose to patients will also be higher than that delivered by a standard nuclear medicine scan. This is a major consideration that should not be overlooked.

Radiation dose for medical purposes is a major contributor to the population's exposure to artificial sources of ionizing radiation. It is therefore essential that practices involving medical exposures are fully optimized. The European Medical Exposures Directive 97/43/Euratom outlines basic measures for the protection from ionizing radiation of persons undergoing medical examination or treatment. In the U.K., the Ionising Radiation (Medical Exposure) Regulations 2000, (IR[ME]R 2000) came into force to implement this directive, and facilities have a duty to ensure that the radiation dose received by patients is kept as low as is reasonably achievable and still be consistent with answering the diagnostic question.

Although radiation doses and diagnostic reference levels are becoming reasonably well established in conventional CT, the same is not yet true for SPECT/CT. Radiologists and nuclear medicine physicians need to understand that in some clinical situations, the lower image quality obtained from a low-dose CT scan may still be good enough for diagnosis. This is often the case when CT is being used in an adjunctive role in fusion imaging. An acceptable level of image quality should be chosen for each diagnostic task to minimize patients' exposure to ionizing radiation.

Early detection or exclusion of bone metastases is extremely important to the clinical management of cancer patients. Bone scintigraphy with technetium- 99m-labeled phosphates, such as Tc-99m methylene dipho-sphonate (MDP), has a high sensitivity to osseous metastases. The specificity of radionuclide bone scintigraphy is nonetheless quite low, making it difficult to differentiate accurately between malignant and benign lesions. Additional nonspecific tracer uptake may also require further correlative imaging tests before a definitive diagnosis can be made.

SPECT/CT is reported to be useful in classifying lesions rated “equivocal” or “indeterminate” on radionuclide bone scans.

 One group found that SPECT/ CT improved the accuracy of bone scintigraphy by classifying equivocal lesions correctly (Figures 2 and 3).

 Another has described how SPECT-guided CT clarified more than 90% of SPECT findings classified as indeterminate.

 A separate, retrospective study has shown how diagnostic confidence increases when using fused SPECT/CT images to differentiate malignant and benign bone lesions, instead of separate sets of scintigraphic and CT images.

 Patients may also appreciate the one-stop-imaging aspect of SPECT/CT, and the reduced need for additional, confirmatory, imaging examinations.

Another potential role for SPECT/ CT is the localization of sentinel nodes in patients with melanoma and breast cancer.

 Current evidence favors SPECT/CT for preoperative sentinel node localization in patients with head and neck melanoma. The lymphatic drainage system in the head and neck is complex. Traditional dynamic and planar imaging with markers may not be reliable enough to locate the precise site/level of the sentinel node.

Studies show that SPECT/CT can help localize lymph nodes and bone metastases in patients with differentiated thyroid cancer.

 One study comparing the clinical utility of iodine-131 SPECT/ CT with an analysis of separate CT and radioiodine scans found that fusion provided additional diagnostic information in 88% of cases.20 Other studies have reported that combined SPECT/CT produced additional diagnostic information in 44% to 57% of cases.

 SPECT/CT can also be used to differentiate remnant thyroid tissue from the physiological uptake of the radiotracer.

The inability to accurately localize lesions in patients with neuroendocrine tumors is a major limitation of both indium-111 pentetreotide (octreoscan) and I-123/131 meta-iodobenzylguanidine (MIBG) whole-body scintigraphy and SPECT. SPECT/CT is reported to be useful in the localization and characterization of lesions in patients with neuroendocrine tumors (Figure 4).

 Other applications for which the value of SPECT/CT has been reported include the evaluation of solitary pulmonary nodules,

 and dosimetry estimations for radionuclide therapy.

The benefits of SPECT/CT in the assessment of benign musculoskeletal pathology are increasingly being recognized. One prospective study looked at lowdose SPECT/CT in nononcology patients who had nonspecific findings on planar bone scans.25 SPECT/CT reached a final diagnosis in 58% of lesions (49/85) in 59% of patients (45/76), obviating additional imaging.

 The SPECT/CT results guided additional imaging in another 30% of patients (23/76).

SPECT/CT is particularly useful when assessing foot and wrist pathologies, where localization is difficult on planar images owing to the structures' complex anatomy. SPECT/CT is especially useful in assessing foot and ankle pathology following surgery. In general, metal artifacts can limit the value of MRI in this role.

In a retrospective study, we found multislice SPECT/CT of the ankles and feet provided additional diagnostic information in 81% of patients (26/31) who had unexplained foot pain or had undergone surgery for a range of complaints including osteochondral defect, fracture or postsurgical nonunion, stress fracture, impingement syndromes of the ankle, and plantar fasciitis. Imaging with the combined modality potentially changed management in 62% of patients (Figure 5).

Several conventional radiologic and radionuclide techniques are used in the evaluation of wrist pain. Plain radiography is often the first-line imaging modality. Planar Tc-99m MDP bone scintigraphy (two- or three-phase) has a high sensitivity but relatively low specificity, and it is often difficult to localize increased tracer uptake accurately. Coregistration of Tc-99m MDP bone scans with plain radiography is useful, but the technique is cumbersome and lesion characterization is not always straightforward. The localization and characterization of lesions can, however, be aided by dual-phase Tc-99m MDP bone scanning followed by SPECT/CT imaging of the wrist and hand (Figure 6).

Back pain is an extremely common problem that can originate from bone or soft tissues. Many different imaging modalities are used in the diagnosis and management of back pain. Tc-99m MDP bone SPECT is useful in diagnosing facet joint disease and osteoporotic vertebral collapse, though precise localization of disease can be difficult in some cases. SPECT/CT is much better at identifying the precise location of spinal disease. The combined imaging modality may also help with image-directed therapeutic management, such as steroid injections or vertebroplasty.

We routinely see patients with unexplained hip pain. Nonspecific uptake around the hip joint on planar wholebody Tc-99m MDP scintigraphy is typically dismissed as degenerative. The use of SPECT/CT in selected cases may allow more specific causes of unexplained hip pain, such as femoral acetabular impingement syndrome, to be diagnosed.

The sensitivity of radionuclide techniques in imaging infection and inflammation is high. Specificity is often limited by poor anatomical localization and nonspecific tracer uptake. These limitations may be overcome by the complementary use of CT in specific cases. For example, when SPECT/CT was evaluated as an adjunct to gallium-67 or In-111–labeled white blood cell scintigraphy (planar/ SPECT), scintigraphy and SPECT/CT results were concordant for the diagnosis and localization of infection in 50% of cases and discordant in 50% of patients.29 SPECT/CT improved anatomic localization of infection in 85% of discordant studies.

SPECT/CT is useful in selective cases of infection and inflammation where localization was difficult and other investigations were equivocal, non-specific with multiple coexisting pathology (Figure 7).

SPECT/CT is additionally reported to be useful in the localization of ectopic parathyroid adenomas (Figure 1).

Any new modality must demonstrate clinical efficacy and cost-effectiveness or advantages in the current healthcare system if it is to be a success. SPECT/CT has yet to do this (see table, below). As published evidence increases, appropriate roles for SPECT/CT in various clinical scenarios will no doubt emerge.

Most studies support the usefulness of SPECT/CT for improving diagnostic accuracy (accurate localization and characterization) and increasing confidence in interpreting scans. Although dedicated multislice SPECT/CT is relatively new, it is rapidly becoming a popular imaging modality. Healthcare professionals must work together to tackle existing technical challenges, establish the clinical applications where SPECT/CT will be most useful, and define appropriate scanning protocols to ensure that SPECT/CT becomes a robust and reliable imaging technique.

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