Figure 4: Cross-sectional SPECT image of the breast. Pectoralis minor muscle clearly visible on CT for cutoff between LI/II and LII/ III. Top to bottom: SPECT; CT (pectoralis minor highlighted in green); fused SPECT/CT images, axial plane. Reprinted with permission from Husarik and Steinert. Semin Nucl Med. 2007; 37:29-33.21
colloid injection should be small in volume and activity (10-20MBq in less than 1ml), consist of a suitable range of particle sizes, and should be taken up quickly and trapped by the sentinel node to minimise uptake by secondary and tertiary nodes.
The number of primary lesions and previous surgery may dictate that the injection be split and administered in two sites. Other considerations include: • An absorbent sheet to minimise the risk of skin contamination; • Drawing air into the syringe to ensure the whole dose is administered and aid radiopharmaceutical dispersion;
• Massage of the injection site to encourage further dispersion.
Static Planar Imaging Planar imaging (figure 3)19
requires careful patient positioning to ensure all relevant
nodes will be included. For breast lymphoscintigraphy it is important to include the shoulder, neck, axilla, costal margin and beyond the midline of the chest. Anterior and anterior-oblique images should be acquired immediately post injection. If the sentinel node fails to appear, then delayed imaging at 2-4 hours is recommended. Table 1 shows a typical set of acquisition parameters.
Figure 3 shows the specific nature of the colloid uptake and the associated lack of anatomical landmarks. Images of this type have limited value in isolation and many centres use a cobalt-57 flood source to provide a low-resolution transmission image to aid localisation. This was deemed suboptimal and led to investigation of the use of SPECT.
SPECT imaging SPECT is a natural progression from planar imaging and, as before, should include
the extent of the lymphatic chain of interest. Acquisition parameters can vary, but an example is shown in Table 1. The patient is usually supine and may require immobilising since the SPECT acquisition can take 20-30 minutes. Using a dual-headed gamma camera is preferable because it reduces acquisition time and reduces the risk of patient movement. Camera heads should be positioned as close to the patient as possible to maintain image resolution. Figures 3 and 4 demonstrate how the lack of anatomical landmarks provided by planar imaging and SPECT, make it difficult to be sure of the precise position of the sentinel node.
Body area Chest Abdomen and pelvis
Typical CT radiation doses (mSv) Low-resolution CT
1 1.5
SPECT/CT imaging Following a SPECT acquisition, a CT can be performed, without moving the patient, to
Diagnostic quality CT
5.8 7.1
Table 2. Typical radiation dose (mSv) associated with CT. Data from Griffiths et al.22
enable anatomical detail to be overlaid on the foci of uptake. Accurate image registration allows precise localisation to occur. The patient must not be moved between the two acquisitions as image misregistration can occur with movement. As with all ionising radiation, the dose received by the patient must be a consideration. Table 2 shows that there can be a significant increase in radiation dose when using CT in addition to SPECT, and there should be good justification and clinical need prior to making a decision on its use22
.
35 2011
IMAGING & ONCOLOGY
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