8 SPECT and SPECT/CT for the Skeletal System


8 SPECT and SPECT/CT for the Skeletal System

Hyewon Hyun, Chun K. Kim, and Katherine A. Zukotynski

8.1 Introduction

Skeletal scintigraphy is one of the most commonly requested general nuclear medicine studies due to its wide availability, ease of performance, low cost, and ability to assess the entire skeletal system. Often, abnormalities can be identified on skeletal scintigraphy earlier than on anatomical imaging. Since radiotracer uptake in the skeleton occurs as a result of osteoblastic activity, it can be seen in traumatic, infectious, and neoplastic conditions. Thus, although the sensitivity of skeletal scintigraphy is high, correlation with clinical history and additional imaging is crucial for accurate image interpretation (► Fig. 8.1). Single-photon emission computed tomography (SPECT; ► Fig. 8.2) and SPECT/computed tomography (SPECT/CT; ► Fig. 8.3) can improve image contrast as well as cross-sectional anatomical detail, resulting in higher sensitivity, specificity, diagnostic accuracy, and reader confidence compared with planar skeletal scintigraphy alone.

Fig. 8.1 A 74-year-old man with left foot pain due to simultaneous metastatic lung cancer and osteomyelitis shown with SPECT/CT. (a) Skeletal phase planar, (b) axial SPECT, and (c) Maximum intensity projection (MIP) images from a skeletal scintigraphy study of the feet show multifocal radiotracer uptake at sites of active osteoblastic turnover. Although skeletal scintigraphy with SPECT helps to detect and localize several sites of osseous disease, it cannot reliably distinguish osteomyelitis from metastatic disease. (d) Axial CT, (e) SPECT, and (f) fused SPECT/CT images from a 99mTc-labeled WBC study focusing on the left midfoot show that the osteoblastic turnover in the region of the left first metatarsal head (orange arrow in b) correlates with intense white blood cell accumulation at a site of active osteomyelitis (orange arrow in f), while the osteoblastic turnover in the region of the left fourth metatarsal head (yellow arrow in b) correlates with a destructive mass at a site of lung cancer metastasis (yellow arrow in f).
Fig. 8.2 A 65-year-old man with prostate cancer and rising prostate-specific antigen. SPECT helped to detect a left iliac bone metastasis adjacent to the sacroiliac joint (SIJ), which was equivocal on planar bone scintigraphy. (a) Anterior and posterior projection skeletal phase whole-body images from a skeletal scintigraphy study show subtle active osteoblastic turnover in the left iliac bone adjacent to the SIJ (arrow) that is indeterminate for osseous metastasis. (b) Coronal, (c) sagittal, and (d) axial SPECT images from the skeletal scintigraphy study improve the contrast of the metabolically active osseous metastasis (arrows) and localize it to the left iliac bone.
Fig. 8.3 A 50-year-old woman with breast cancer for staging. SPECT/CT helped identify an L3 vertebral body metastasis, which was equivocal on planar bone scintigraphy. (a) Posterior projection skeletal phase whole-body image from a skeletal scintigraphy study shows subtle osteoblastic turnover in L3 that is indeterminate for osseous metastasis. (b) Coronal, (c) sagittal, and (d) axial SPECT and fused SPECT/CT images from the skeletal scintigraphy study improve the contrast of the metabolically active osseous metastasis and localize the site of disease to the junction of the L3 vertebral body posteriorly and the left pedicle.

Although adding CT to SPECT increases the radiation dose to the patient, the benefits often far outweigh the risks associated with the additional radiation exposure. The CT data can be used for attenuation and scatter correction, thus improving the quality of SPECT. The addition of CT has allowed us to clarify SPECT findings and has also increased our ability to determine the anatomical location and extent of disease and enabled concurrent evaluation of anatomical and metabolic changes, thereby assisting with surgical planning and determination of appropriate management. Today, some of the most common indications for skeletal scintigraphy with SPECT or SPECT/CT include evaluation of suspected radiographically occult fractures and stress changes; postsurgical complications, such as pain and hardware instability; and suspected neoplastic disease. Skeletal scintigraphy with SPECT or SPECT/CT is also often used when magnetic resonance imaging (MRI) cannot be performed because of contraindications or a high likelihood of artifact.

8.1.1 Technique

Bone is made up of a crystalline lattice consisting of calcium, phosphate, and hydroxyl ions that form hydroxyapatite. Bone-targeting radiopharmaceuticals are analogues of calcium, hydroxyl groups, or phosphates and are taken up in osteoblasts at the time of bone remodeling or healing. 1 Among them, technetium-99 m (99mTc)-labeled methylene diphosphonate (MDP) is the most widely used radiopharmaceutical for skeletal scintigraphy. Images are typically acquired as either a single- or a three-phase study. No patient preparation is required prior to the study.

  • In a three-phase bone scan, angiographic phase (blood flow) images are acquired by dynamic imaging, usually for 60 seconds, immediately after the injection of 740 to 925 MBq (20-25 mCi) of 99mTc-MDP to assess perfusion to the area of interest. Tissue phase (blood pool) static images are acquired once the dynamic imaging has been completed and are used to assess hyperemia or the amount of activity that extravasates into the soft tissue in the area of interest. Skeletal phase imaging is performed 2 to 4 hours afterward to assess skeletal uptake. Three-phase bone scans are often done to evaluate traumatic injury where knowledge regarding perfusion is needed, such as in cases of suspected complex regional pain syndrome or osteomyelitis. They are also useful for the evaluation of neoplastic osseous disease, such as for a suspected osteoid osteoma, among other pathologies.

  • In a single-phase bone scan, only skeletal phase imaging is performed. Single-phase bone scans are commonly done for follow-up of patients with known osseous metastases.

  • SPECT or SPECT/CT is obtained at the time of skeletal phase imaging as needed and is usually targeted to a site of abnormality on planar images or to improve the sensitivity and specificity in a region of clinical concern. SPECT is typically acquired using a 128 x 128 matrix with 25 seconds per step, iterative reconstruction, and resolution recovery. Images are displayed in coronal, sagittal, and axial projections alongside a rotating maximum intensity projection (MIP) image. For the CT portion of the study, the typical kVp range is 80 to 110 kVp and slice thickness is 2 to 2.5 mm with overlapping cuts; however, the CT may be tailored to the clinical question, varying from very low dose for attenuation correction, to low dose for anatomical correlation, to normal dose for more precise anatomical imaging when needed. Intravenous contrast administration is not performed, and dose modulation is applied to minimize radiation exposure. Also, radiation exposure can be minimized by avoiding the use of CT when unlikely to provide additional value to SPECT. Bone and soft tissue windows as well as reformats in coronal and sagittal projections are available for studies with three-dimensional rendering as required. Fused and nonfused SPECT and CT images are reviewed. Since CT images are rapidly acquired (in a matter of seconds), motion is rarely an issue. SPECT typically requires 12 to 15 minutes to complete and can lead to potential misregistration artifact between the CT and SPECT images. Thus it is important to ensure that patients are comfortably positioned whenever possible.

8.1.2 Pearls

  • Intense activity in the bladder can cause artifact on reconstructed SPECT images, including streak artifact and “cold” areas close to the bladder due to pixel overload (► Fig. 8.4). Therefore, if the patient cannot void and there is clinical concern for sacral, pelvic bone, or proximal femoral abnormality, bladder catheterization may be helpful.

Fig. 8.4 Streak artifact (small arrows on the sagittal and axial images) and “cold” areas close to the bladder (large arrows on the coronal and axial images) are caused by intense bladder activity.

8.2 Trauma/Arthropathy/Unexplained Pain

Skeletal scintigraphy has long been used for the evaluation of traumatic injury, and there are a number of indications as to when SPECT or SPECT/ CT should be considered. In particular, SPECT or SPECT/CT can be particularly useful (1) to diagnose a clinically suspected fracture in the setting of negative anatomical imaging, (2) to diagnose a cause of mechanical back pain or pars stress, and (3) for the evaluation of pain after surgical intervention when MRI is subject to significant artifact. Whole-body skeletal scintigraphy with additional SPECT or SPECT/CT where needed is also good to provide a rapid whole-body assessment with limited radiation exposure for the detection of multifocal traumatic injury that may be difficult to obtain using anatomical imaging modalities, such as radiographs, CT, or MRI.

8.2.1 Fractures

Skeletal scintigraphy with SPECT or SPECT/CT is helpful in patients with pain, a clinical history of trauma or osseous stress, and negative diagnostic imaging. Stress injury occurs following repetitive activity causing marrow edema and microtrabecular fractures that ultimately coalesce to form a cortical break. Skeletal scintigraphy is exquisitely sensitive for stress injury and fractures. Indeed, active osteoblastic turnover can be seen prior to anatomical changes and often correlates with the site of pain. 2 , 3 SPECT/CT has improved diagnostic accuracy compared with planar imaging alone or with SPECT and can be particularly helpful to localize abnormalities in patients with radiographically occult stress fractures (► Fig. 8.5) or osteochondral defects (► Fig. 8.6). Linke et al studied 71 patients with extremity pain and found that planar imaging and SPECT/CT led to a different diagnosis compared with planar imaging and SPECT in 23 patients (p <0.01). 4 Scheyerer et al found improved diagnosis of occult pelvic fractures on SPECT/CT compared with radiographs and CT, 5 while Allainmat et al found improved diagnosis of occult wrist fractures on SPECT/CT compared with CT. 6 MRI is sensitive for bone marrow edema; however, the extent of edema is protocol dependent, and image interpretation can be difficult and/ or misleading. Skeletal scintigraphy with SPECT or SPECT/CT can also identify ongoing active osteoblastic turnover, which can be useful to suggest when healing is complete as well as the presence of nonunion. Active osteoblastic turnover 12 to 18 months after a fracture is suggestive of nonunion. 2

Fig. 8.5 A 29-year-old athletic woman with bilateral foot pain and negative radiographs. SPECT/CT confirmed stress fractures of bilateral navicular bones suggested on planar skeletal scintigraphy. (a) Skeletal phase images of both feet in the anterior projection and (b) lateral projection of the left foot show active osteoblastic turnover in the region of both the right and left navicular bone (arrows). (c) Axial CT, fused SPECT/CT, and SPECT images of both feet as well as (d) sagittal CT, fused SPECT/CT, and SPECT images of the left foot show metabolically active osteoblastic turnover in both the right and left navicular bone correlating with subtle sclerosis on CT at sites of bilateral navicular stress fractures (arrows). Stress is often associated with athletes who engage in forceful push-offs for sprinting or jumping. Symptoms can be nonspecific. Although radiographs are often the first imaging study performed, the sensitivity of skeletal scintigraphy is significantly higher. Navicular bone stress fractures are at high risk of delayed union and nonunion, in part, related to the relatively avascular central area, making early diagnosis and management important for good long-term results and early resumption of physical activity. 29
Fig. 8.6 A 20-year-old athletic woman with bilateral knee pain and negative radiographs. SPECT/CT clarified the location and extent of several osteochondral defects involving the left patella and both femora. (a) Skeletal phase images in anterior and lateral projections of both knees show several sites of active osteoblastic turnover in the knees (arrows). (b) Sagittal and axial CT, SPECT, and fused SPECT/CT images of the left knee localize osteochondral defects in the left patella and femur (arrows). (c) Axial CT, SPECT, and fused SPECT/CT images of the right knee localize additional osteochondral defects (arrows). Articular cartilage abnormalities of the knees are common in active patients, and early, accurate characterization of size and location is helpful to ensure appropriate management. 30

8.2.2 Back Pain

SPECT or SPECT/CT is recommended for the evaluation of back pain. SPECT has been shown to have improved specificity compared with planar skeletal scintigraphy, particularly for the evaluation of lesions in the lumbar spine. 7 , 8 CT increases the specificity still further. 9 Moderately to markedly active facet arthropathy is generally evident on planar images, demonstrating a characteristic appearance. 10 However, mild facet arthropathy may not be apparent on planar images, while it is clearly evident on SPECT or SPECT/CT (► Fig. 8.7). Skeletal scintigraphy with SPECT or SPECT/CT may also be helpful to identify active osteoblastic turnover at sites of pain associated with transitional vertebra or stress to the region of the pars interarticularis. 2

Fig. 8.7 A 57-year-old woman with low back pain. (a) Planar images show mild irregular tracer activity in the lower lumbar spine but no obvious focal abnormalities. (b) SPECT/CT shows intense tracer uptake in the bilateral facet joints, right greater than left, at the L4/L5 level.

8.2.3 Accessory Ossicles

Skeletal scintigraphy with SPECT/CT may be helpful to precisely localize the site of active osteoblastic turnover to an accessory ossicle. Active osteoblastic turnover associated with an accessory ossicle may suggest a cause of pain (stress, fracture, inflammation, etc.) versus absent activity, which is more commonly associated with a normal anatomical variant, an important distinction if surgical resection is considered. 2

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May 24, 2020 | Posted by in GENERAL RADIOLOGY | Comments Off on 8 SPECT and SPECT/CT for the Skeletal System
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