Aryana K et al. / Efficacy of high energy collimator 79 TABLE 4. Comparison of mean count rates for injection site as well as sentinel node(s) on low energy high resolution (LEHR) collima- tor and high energy (HE) collimators Injection site counts on lateral views Injection site counts on anterior views Sentinel node counts on lateral views High energy collimator 9127632±78784 9252343±86772 1799±67 Low energy high resolution col- limator 8312982±67565 8811054±76453 1576±55 P value <0.001 <0.001 <0.0001 patients the sentinel node was visible by HE col- limator but not LEHR collimator. Tumour location in both of these patients was in the upper lateral quadrant and both had history of excisional biop- sy (Figure 2). In two patients, additional sentinel node was visible adjacent to the first one only on the LEHR images (Figure 3). Table 4 compares the counts of injection site as well as sentinel node(s) for HE and LEHR collimators. Discussion Imaging of the axillary sentinel lymph nodes (lym- phoscintigraphy) was recommended by many as a necessary part of breast cancer axillary staging using sentinel node biopsy.13,14 This imaging can be somehow challenging especially when the injec- tion site is near the axilla which can obscure the sentinel nodes due to “shine through” and star artifacts.7,15-17 Sub-areolar injection of the tracer by increasing the distance between injection site and axilla can obviate this problem to some extent. However, a problem still persists especially for tu- mours located in the upper lateral quadrants par- ticularly in patients with the history of previous excisional biopsy of the primary breast lesion.17 A method to decrease the masking effect of in- jection site count on the sentinel node(s) was using other types of collimators with less septal penetra- tion.9 For the first time in the literature we used HE collimators for lymphoscintigraphy imaging in the current study. Our results showed better visu- alization of axillary sentinel lymph nodes in two patients with history of excisional biopsy and the primary tumour location in the upper lateral quad- rants by HE collimator. This is most likely due to minimal septal penetration by 140 KeV photons of 99mTc using the HE collimator (minimal septal pen- etration using HE versus 1.5% using LEHR colli- mators). Injection sites in these two patients were very near the axilla and resulting septal penetra- tion masked the axillary sentinel nodes by LEHR collimator. Our findings were also supported by Tsushima et al. who used medium energy collima- tors. They reported a decrease in star-shaped arte- facts with a better chance of sentinel node visuali- zation when injection site is near the axilla.7 The count rate of the injection site as well as of sentinel nodes was consistently higher using HE collimator as compared to LEHR one in our study. This was most likely due to higher sensitivity of HE collimator for 140 KeV photons compared to LEHR (285.4 counts per minute per μCi for HE and 261.5 counts per minute per μCi for LEHR). This effect could also contribute to better sentinel node visualization in the above mentioned patients us- ing HE collimators. Star-shaped artefact which is the result of sep- tal penetration is not the only determinant of sen- tinel node masking by adjacent high counts. The spatial resolution of the collimators is also of ut- most importance in this regard. Several authors reported better performance of high resolution collimators for lymphoscintigraphy.5,6,10,18 We also find the same findings in two of our patients. Due to low resolution of HE collimator, the activity of the adjacent sentinel nodes were merged with each other and they can not be separable as two distinct nodes as they were on the LEHR images. Fenestra et al.10 and Lemstra et al.5 both found the same find- ings comparing medium energy and low energy collimators and attributed this to a higher spatial resolution. This can be especially true in our study since spatial resolution for 99mTc photons (in mm at 10 cm) is 12.66 for HE and 6.64 for LEHR collima- tors respectively. Conclusions HE collimators can be used for sentinel lymph node mapping and lymphoscintigraphy of the breast cancer patients. This collimator can almost eliminate star-shaped artefacts due to septal pen- etration which can be advantageous in some cases. However, to separate two adjacent sentinel nodes from each other LEHR collimators perform better. Radiol Oncol 2012; 46(1): 75-80.