Page 1 Physics Case of the Day - Thursday During routine QA testing of a computed tomography scanner, the physicist recorded the data below for the tube output at isocenter and the dose efficiency at several mAs settings. She then calculated the tube output per mAs and recorded it in the fourth column. What causes the shift in mR/mAs and in the dose efficiency between 500 and 600 mAs? Why does the scanner do this? mAs Measured exposure (mR) Dose efficiencymR/mAs % % % % % % % %11.96 Author: David M. Gauntt, Ph.D.

Page 2 Physics Case of the Day - Thursday During routine QA testing of a computed tomography scanner, the physicist measured exposure at isocenter at 120kV for several mAs settings, and then recorded the data below. She then calculated the tube output per mAs and recorded it in the fourth column. Each measurement was fora single 1 second rotation of the gantry. This model scanner uses the small focal spot at 500 mA and below, and the large focal spot at 600 mA and above. The collimator opens slightly wider when using the large focal spot than when using the small focal spot; this is reflected in the increased tube output and decreased dose efficiency. mAs Measured exposure (mR) Dose efficiencymR/mAs % % % % % % % %11.96

Page 3 Physics Case of the Day - Thursday If the focal spot were infinitely small, the collimator blades could be set so only the active rows of the detector array are irradiated with primary x-rays. In this case the system is making the most efficient possible use of the patient dose, so the dose efficiency is 100%.

Page 4 Physics Case of the Day - Thursday In a real tube, the focal spot has a non-zero size. As a result, part of the detector array is irradiated by the entire focal spot (the “umbra” of the beam) and part is irradiated by only part of the focal spot (the “penumbra”. If the collimator width is set as if the focal spot were infinitely small, part of the active area of the detector would be in the beam penumbra, and these detector elements would have more photon noise than those in the umbra.

Page 5 Physics Case of the Day - Thursday To ensure that the active area of the detector is covered, the collimator blades are moved apart slightly; this increases the size of the umbra. The dose efficiency is the fraction of the primary x-rays that are travelling toward the active area of the detector; in this case, these are the x-rays in the umbra. Thus, the dose efficiency drops slightly below 100%, and the measured tube output increases slightly.

Page 6 Physics Case of the Day - Thursday When the large focal spot is used, the collimator is widened further to ensure that the entire active area of the detector is covered by the umbra. The total tube output is increased, and the dose efficiency is decreased, by the same fractional amount as the increase in the actual collimation width. This effect is greater for narrower beam widths; for 1.25mm widths often used in high resolution chest, the dose efficiency can drop below 50% because the penumbra is wider than the active area of the detector.