PubMed · 8046483
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S C Port. 1994. The first-pass ventricular function study: why now?. https://pubmed.ncbi.nlm.nih.gov/8046483/
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The mechanical tolerances in building collimators for scintillation cameras are studied. A simulation method has been used to quantify the effects of defects in hole inclination and hole diameter on the uniformity of planar and tomographic images. The calculation takes into account the geometry of the hexagonal hole collimator, the camera intrinsic resolution, the object size, the pixel size, the effect of low-pass filtering, as well as the type, size and position of the defect. For instance, a 0.03 mm diameter defect on several holes located in the central region of a very high resolution collimator can result in a 12% uniformity artefact in tomographic imaging of an 18 cm diameter cylinder, using a 3.45 mm resolution camera, 4.5 mm pixel size, and Hamming filtering with a Nyquist frequency cut-off. A 0.17 degree inclination defect of a few holes can result in the same uniformity artefact. These results show that the building of a collimator has to be very precise.
There is increasing clinical interest in the use of quantitative imaging for radiopharmaceuticals labelled with 111In. Dual-energy-window (DEW) scatter correction is a frequently used component of planar geometric mean quantitative imaging, but it is known that the scatter multiplier k suffers from significant dependence on the characteristics of the scatter medium. Phantom studies with a variety of source geometries were carried out to determine the clinical impact of this dependence on the quantitative accuracy of tumour imaging carried out in conjunction with attenuation correction. Spheres of various sizes (5-20 ml volumes) containing approximately 3.7 MBq (100 microCi) 111In were imaged at a variety of depths (4.8-10.5 cm) within an elliptical water-filled phantom, as well as in air. Geometric mean emission images were acquired using a 20% photopeak window at 247 keV and a 10% scatter window at 205 keV. These emission images were corrected for attenuation using measured 99Tcm transmission data that were scaled to 111In photon energies. Scatter correction was performed in two ways: (1) using the standard DEW method and (2) using a modified DEW method that takes into account benign scatter in the detector crystal. Errors in the activity estimates ranged from -4% to +3% for method 1 in water, and -5% to +3% for method 2 in water. In air, method 1 ranged from -13% to -5%, and method 2 ranged from -10% to -1%. Method 1 was found to yield an accuracy equivalent to that of method 2, except in conditions of very low patient scatter, when the modified method behaved significantly better. We conclude that in a variety of realistic geometries, variations in scatter fraction as determined by the DEW scatter correction method combined with appropriate attenuation correction need not inhibit accurate absolute quantitation of spherical 'tumours' labelled with 111In when using planar imaging.
Split and total renal function are commonly assessed from time-activity curves obtained from gamma camera renography with 99Tcm-diethylenetriamine pentaacetate (99Tcm-DTPA) and 99Tcm-mercaptoacetyl-triglycine (99Tcm-MAG3). The impact of the statistical noise associated with these curves on the total precision in split and total renal function impairment was studied at different levels of administered activity by simulation. The simulation consisted of generated time-activity curves, with statistical noise corresponding to the administration of 10-400 MBq 99Tcm-DTPA or 7.5-150 MBq 99Tcm-MAG3. Our results indicate that the error induced by statistical noise in split renal function calculations is generally very low (< 3% at 100 MBq 99Tcm-DTPA or 75 MBq 99Tcm-MAG3) and only affects precision slightly in calculations at 45-50 MBq 99Tcm or more. Similar results were obtained for precision in GFR calculations, where the total error increased from 12.3% at 100 MBq 99Tcm-DTPA to 12.8% at 50 MBq, but decreased to 12.0% at 400 MBq. Consequently, statistical noise has a minor effect on the precision in split and total renal function calculations at commonly used activity levels when it is reduced to approximately 50 MBq.