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Alternative methods of obtaining the computed tomography dose index.

The most direct way of getting the value of the multiple scan average dose (MSAD) in computed tomography is to employ a pencil chamber for integration of a single scan dose profile. Because the active length of the pencil chamber is fixed, the measurement can represent the value of the MSAD from a different number of contiguous scans depending on the slice thickness. This characteristic makes it difficult to compare the value of MSAD using the pencil chamber to the information required by Federal regulations on the computed tomography dose index (CTDI). The CTDI, which is the MAD at the center of a set of 14 contiguous scans, is the dose descriptor used in the Federal Performance Standard. Two alternative methods were developed to make the CTDI measurements at the center of a CT dosimetry phantom. These alternative methods were compared to the results of thermoluminescent dosimeter (TLD) measurements from more than 20 different CT scanners. One alternative method involved the use of radio-opaque sleeves with the pencil chamber to limit the length of the single scan dose profile incident on the pencil chamber. In addition, the TLD data were also used to obtain a set of conversion factors for converting the results of a measurement with the pencil chamber without a radio-opaque sleeve to a value of the CTDI. The alternative methods of obtaining the CTDI agree on the average to better than 10% for all values of slice thickness on the different CT scanners.