the study. Histological diagnosis of breast cancer was based on the results of core needle biopsy or excisional biopsy. For patients in whom the diagnosis was estab- lished by core needle biopsy periareolar intrader- mal injections of 18.5 MBq (0.5 mCi)/0.2 mL 99mTc- antimony sulphide colloid were used. For patients with history of previous excisional biopsy of the primary lesion two intradermal injections of 18.5 MBq (0.5 mCi)/0.2 mL 99mTc-antimony sulphide colloid at each end of the excisional line were giv- en. Gentle massage was applied to the injection site subsequently for all injections for 1 minute. Anterior and lateral views were acquired 30 minutes after the injection (2 minutes/image, 128×128 matrix, 15% energy window cantered over 140 keV) using a dual head gamma camera (e.cam Siemens), equipped with a parallel hole low energy high resolution (LEHR) collimator on one head and HE energy collimator on another head. The order of imaging was: 1) lateral view with HE, 2) lateral view with low energy, 3) anterior view with HE and 4) anterior view with low energy col- limators. The outline of the patients was acquired simultaneously using the scattered photons as de- scribed by Momennezhad et al.11 The specifications of both HE and LEHR collimators are provided in Table 1. Some of the gamma camera specifications are shown in Table 2. Collimator performance was calculated using especial calculator provided by Nuclear Fields Company.12 All images were reviewed by two nuclear medicine specialists regarding: detectability of axillary sentinel nodes, number of visualized sen- tinel nodes, and presence of star artefact. Semi- quantitative evaluation was also performed using ROIs on the injection site and on detected axillary sentinel nodes. Quantitative data (count rates) were expressed as mean ± SD. For comparison of these quantitative data between two sets of images paired sample t tests was used. P values less than 0.05 were consid- ered statistically significant. The study was carried out according to the Declaration of Helsinki. Results Table 3 shows the summary of patients’ characteris- tics. All images taken by LEHR collimators showed star artefact of the injection site. No image taken by HE collimator showed this effect (Figure 1). In two Aryana K et al. / Efficacy of high energy collimator 77 FIGURE 2. Low energy high resolution collimator (LEHR) (upper row) as well as high energy (HE) collimator (lower row) images of a patient. Note that the sentinel node is only visible on the HE collimator images (arrow) due to star artefact on the low energy collimator imaging. The scatterograms are also shown on the left side of each original image. A B FIGURE 3. High energy (HE) collimator (A) as well as low energy high resolution col- limator (B) images of the same patient. Note that two separate sentinel nodes are obvious on the low energy high resolution images. These two sentinel nodes are not shown as discrete nodes on the HE collimator images. Radiol Oncol 2012; 46(1): 75-80.