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Volume 8 Issue 7 - July 2010
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Hybrid SPECT/CT: Enhancing Nuclear Medicine

SPECT/CT combines the anatomic details seen in CT images with functional information from single photon emission computed tomography (SPECT) nuclear imaging by acquiring both sets of images on the same scanner

SPECT/CT scans provide better anatomic localization of lesions than standard nuclear medicine images

SPECT/CT is primarily used in oncologic applications, but can be applied to a broad range of nuclear medicine procedures

In selected cases, SPECT/CT can provide better diagnostic accuracy than either modality alone



Clinical Applications of SPECT/CT
The SPECT/CT Examination
Scheduling
Further Information
References

PET/CT

SPECT/CT

CT is integral part of study

CT is selective adjunct to SPECT

Diagnostic CT may be included

Only non-diagnostic CT included

CT can be used for attenuation correction

CT can be used for attenuation correction

Primarily used for imaging FDG metabolic activity (glucose analog)

Used for imaging a variety of physiologic uptake markers

Table 1. Comparison of PET/CT and SPECT/CT Applications at MassGeneral Imaging
Single photon emission computed tomography (SPECT) imaging depicts the uptake of radioactive physiological tracers in cross sectional images that are similar in orientation to standard CT imaging. SPECT imaging can be applied to a wide range of nuclear medicine imaging applications. However, the low resolution of SPECT images, combined with the lack of anatomic detail, can make it difficult to accurately determine the anatomic location of the focal site of tracer uptake or in some cases to distinguish between physiological and abnormal tracer uptake.

In many cases, localization of the source of radioactivity can be achieved using side-by-side comparisons of separately acquired SPECT and CT images, or with the use of co-registration software. However, co-registration is much more straightforward when SPECT and CT are combined in a single scanner because the patient remains in the same position during the acquisition of both sets of images. Automated software can accurately co-register the SPECT and CT data to create fused SPECT/CT images, in a manner analogous to PET/CT, resulting in images that show the focal point of tracer uptake and good anatomical detail. However, there are significant differences between PET/CT and SPECT/CT (Table 1). SPECT/CT is useful in selected cases where there is a need for precise localization, for example, in regions with complex anatomy, such as in the abdomen and in the head and neck. In addition, the CT data can be used to correct for attenuation inherent in the SPECT data, in some cases providing a more accurate rendering of the actual tracer distribution.

Figure 1. A) Axial image from diagnostic CT performed without intravenous contrast demonstrates soft tissue nodule adjacent to the adrenal gland in a patient with neurofibromatosis. B) Coronal image from an adrenal protocol CT again demonstrates the lesion, which was shown to have enhancement characteristics similar to that of the spleen, suggesting that it could represent an unusual splenule.
Figure 1. A) Axial image from diagnostic CT performed without intravenous contrast demonstrates soft tissue nodule adjacent to the adrenal gland in a patient with neurofibromatosis. B): Coronal image from an adrenal protocol CT again demonstrates the lesion, which was shown to have enhancement characteristics similar to that of the spleen, suggesting that it could represent an unusual splenule.


Clinical Applications of SPECT/CT
Established oncological applications of SPECT/CT include the evaluation of a variety of tumors, such as neuroendocrine tumors, adrenocortical tumors, and parathyroid tumors. Many neuroendocrine tumors express somatostatin analogs, which can be detected by SPECT using radiolabeled somatostatin receptors such as 111In-pentetreotide. Scintigraphic imaging using this agent has a high sensitivity (80-100%), but its specificity is reduced because of physiological uptake of radioactivity and the difficulty of precise anatomical localization. Because SPECT/CT provides anatomical information, it can increase specificity and improve localization of tracer uptake, which can have a direct impact on patient management.

Tumors that produce catecholamines, such as pheochromocytomas and neuroblastomas, can be successfully imaged with radiolabeled guanethidine analogs, such as 123II-metaiodobenzylguanidine (MIBG). However, the interpretation of scintigraphic images can be confounded by physiological uptake, which can be better identified with SPECT/CT. For example, in a limited series of patients with biochemically or clinically suspected pheochromocytoma, SPECT/CT has been shown to add incremental diagnostic value by identifying sources of physiological tracer uptake. In cases of neuroblastoma, CT or MRI is used initially to assess the primary tumor and its potential resectability. MIBG scintigraphy is commonly used for disease staging to guide management, assessment of treatment response, and detection of disease recurrence. SPECT/CT improves tumor localization, especially when tumors are found adjacent to organs with physiological MIBG uptake, such as the heart, liver, and kidneys.

Parathyroid adenomas are the most common cause of primary hyperparathyroidism. Accurate localization of the tumor is essential to minimize surgical procedures. This is usually accomplished with the aid of SPECT, using 99mTc-methoxyisobutylisonitrile (MIBI), and high-resolution sonography. However, in selected patients with thyroid nodules, ectopic adenomas, or distorted neck anatomy from previous surgery, SPECT/CT has an incremental value for tumor localization.

Figure 2. SPECT/CT imaging was performed with technetium-99m sulfur colloid liver and spleen imaging agent and low-dose, non-diagnostic localizing CT. A) The sulfur colloid SPECT image demonstrates ectopic tracer uptake medial to the spleen (arrow), which corresponds precisely to the soft tissue nodule on B) low dose CT and C) on SPECT/CT fusion image.
Figure 2. SPECT/CT imaging was performed with technetium-99m sulfur colloid liver and spleen imaging agent and low-dose, non-diagnostic localizing CT. A) The sulfur colloid SPECT image demonstrates ectopic tracer uptake medial to the spleen (arrow), which corresponds precisely to the soft tissue nodule on B) low dose CT and C) on SPECT/CT fusion image.


Another oncologic application of SPECT/CT is in selected patients undergoing lymphoscintigraphy. Lymph node staging is integral for treatment planning for a number of cancers. A combination of lymphoscintigraphy using a 99mTc-sulfur colloid injected intradermally prior to surgery and mapping with dye during surgery is a common approach to finding sentinel lymph nodes (SLN). Localization with CT may facilitate surgical planning and can increase detection rates. Anatomic co-registration using SPECT/CT can be beneficial for SLN detection in the pelvis, the mediastinum, and particularly in the head and neck region. In a pilot study of patients with melanoma, SPECT/CT enabled the detection of SLNs in a number of patients with negative planar scintigraphy examinations. Other studies have found that SPECT/CT improved SLN identification and/or localization of SLNs associated with head and neck tumors, and breast cancer. SPECT/CT is also superior to planar scintigraphy for detecting SLNs in patients with a body mass index over 25. In other patients undergoing lymphoscintigraphy, such as for truncal or extremity melanoma, conventional imaging is typically sufficient, as SPECT/CT does not appear to add substantial diagnostic information.

On occasion, SPECT/CT can be valuable for differential diagnosis. One example is shown here (Figures 1-3) in which uptake of 99mTc-sulfur colloid demonstrated that a soft tissue nodule adjacent to the adrenal gland represented a splenule.

Figure 3. Coronal images from the SPECT/CT correlate well with the diagnostic coronal CT (Figure 1B). The presence of sulfur colloid uptake within the nodule confirms that it represents a splenule.
Figure 3. Coronal images from the SPECT/CT correlate well with the diagnostic coronal CT (Figure 1B). The presence of sulfur colloid uptake within the nodule confirms that it represents a splenule.


The SPECT/CT Examination
Patients receive an injection of a radiotracer either the same day as the SPECT/CT examination (for example, 99mTc-MIBI, 99mTc sulfur colloid) or the day before the SPECT/CT examination (for example, 111In-pentetreotide). At the start of the examination, the patient is positioned comfortably on the imaging table and is asked to stay motionless for the duration of the imaging procedure. The SPECT/CT scan starts with a low-dose CT scan of the region of interest, which provides sufficient anatomic detail for localization purposes and for attenuation correction, if needed. The SPECT examination is performed immediately after the CT examination is completed and takes 10-30 minutes, depending on the body part being imaged.

Note that the CT scan in SPECT/CT is not of diagnostic quality and is performed as an adjunct to the nuclear images. This is an important difference between the techniques used for SPECT/CT and PET/CT. With PET/CT, a full diagnostic CT scan can be performed on the same scanner in addition to the PET images.

The radiation exposure for a SPECT/CT scan is about 47 mrem, or 0.47 mSv for an abdomen nondiagnostic localization and attenuation correction scan, in addition to the exposure from the nuclear medicine tracer. The half-life of the radiotracers commonly used in SPECT/CT (99mTc, 123I, 111In) ranges from 6 hours to 2.8 days. They are effectively fully decayed within ten half-lives after their administration.


Scheduling
SPECT/CT is currently available on a single scanner located on the main campus. At this time, it is not being used for nuclear cardiology examinations. Nuclear imaging examinations may be scheduled online through Radiology Order Entry (http://mghroe ) or by calling 617-724-9729(XRAY). A nuclear medicine specialist will choose whether to perform planar scintigraphy, SPECT, or SPECT/CT.

Further Information
For further questions on SPECT/CT, please contact Edwin L. Palmer, MD , Director of Clinical Nuclear Medicine (617-726-8350).

We would like to thank Edwin Palmer, MD, and Bob Liu, PhD, Medical Physicist for their advice on this issue.


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References

Buck AK, Nekolla S, Ziegler S, Beer A, et al. (2008). SPECT/CT. J Nucl Med 49: 1305-1319.

Bybel B, Brunken RC, DiFilippo FP, Neumann DR, et al. (2008). SPECT/CT imaging: clinical utility of an emerging technology. Radiographics 28: 1097-1113.

Chowdhury FU and Scarsbrook AF (2008). The role of hybrid SPECT-CT in oncology: current and emerging clinical applications. Clin Radiol 63: 241-251.

Patel CN, Chowdhury FU and Scarsbrook AF (2008). Clinical utility of hybrid SPECT-CT in endocrine neoplasia. AJR Am J Roentgenol 190: 815-824.