TABLE 1

CNR Calculations for 2 Block Thicknesses: 10 cm (Thin Patient) and 20 cm (Average Patient)

⁵⁷Co source activity (MBq)	Node 1 (total counts)	Node 2 (total counts)	Background (mean counts)	No. of pixels in background ROI	CNR node 1	CNR node 2
10-cm-thick acrylic
37	169	505	0.52	27	41.4	131.0
126	158	514	0.89	27	27.3	100.0
184	176	537	1.15	27	26.0	90.8
220	185	582	2.00	27	17.8	71.9
269	213	550	2.48	27	17.8	59.0
20-cm-thick acrylic
37	139	469	0.35	25	44.0	155.6
126	153	442	0.42	25	44.0	133.2
184	156	471	0.62	25	35.7	115.7
220	178	480	0.81	25	35.1	102.2
269	133	483	0.85	25	24.2	100.2