IMPORTANT ADVANCES IN NUCLEAR MEDICINE Despite the ability of magnetic resonance imaging (MRI) to demonstrate physiology without radiation burden, nuclear medicine remains the modality of choice for capturing functional information. Since the introduction of 2-dimensional imaging to nuclear medicine, there has been a steady improvement in gamma camera technology due to the development of equipment and computer software. Planar imaging has been highly valuable for a wide range of pathology, although there have always been limitations such as the highly specific tracer uptake (no anatomical landmarks) associated with colloid imaging for sentinel nodes12 in technology to improve imaging.
. This type of difficulty has driven the advancement
SPECT was introduced in the mid to late 1980s to produce cross-sectional images of radionuclide uptake, allowing improved specificity, due in part to the ability of SPECT to differentiate between overlying structures. The popularity of SPECT led to the introduction of dual-headed gamma cameras and the technique became the standard for some investigations. Methods were developed to correct for radionuclide attenuation within the patient (eg Gd-153) to help quantify uptake and improve image quality.
SPECT/CT is the most recent notable advancement in nuclear medicine technology and is already popular for certain techniques. SPECT (emission data) is combined with CT (transmission data) on a single scanner gantry (Figure 2)13 fused data set of images showing functional and anatomical data.
to provide an accurately
The CT component of SPECT/CT has three distinct roles: attenuation correction (AC), lesion localisation and diagnosis. There are two different types of CT system available to support these roles: low-resolution (most suitable for AC) and diagnostic quality with the same functionality as a stand-alone CT scanner.
ROLES OF SPECT/CT IN LYMPHOSCINTIGRAPHY
Attenuation Correction (AC) Photons emanating from radioactive foci deep within the body are susceptible to absorption by the body before they can be detected by the gamma camera. Consequently, deep sited foci, such as a sentinel node, can go undetected in both planar and SPECT imaging negatively affecting the sensitivity and specificity of the diagnostic procedure.
CT (low-dose) can generate an attenuation map that, when applied to SPECT data, can correct for photon attenuation. This technique is now commonplace for certain patients and techniques in SPECT imaging; indeed, AC has been shown to have value for obese patients undergoing lymphoscintigraphy, with improved identification and a greater number of nodes being marked as sentinel.14
Lesion localisation and diagnosis Whether using low-dose CT or diagnostic quality CT, foci of radionuclide
GAMMA CAMERA 2
Figure 2. SPECT-CT Unit: CT Studies with SPECT. CT = computed tomography; SPECT = single photon emis- sion CT. Courtesy of Siemens press pictures
http://www.siemens.com/press/pool/de/pp_med/2005/ sc_upload_file_mednm200504048_03_300dpi_1456207.jpg Accessed January 201113. Annotated by the authors.
SCANNER
CT
GAMMA CAMERA 1
33 2011
IMAGING & ONCOLOGY
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