Computed Tomography Dose Index Measurements Research Articles

Discrepancy between absorbed dose estimated from phantom measurements and from estimates using data on individual head sizes in head CT examinations

Patient doses during computed tomography (CT) scanning are relative high, as compared with doses received by patients during conventional X-ray examinations. Owing to this, every possible optimisation of CT examinations is of importance. Dose estimation in CT examinations of patients are normally based on measurements of CT dose index and dose length products in standardised CT phantoms, representing a standard person of 70 kg. Real patients may differ significantly from this standard, and hence the dose values calculated for a phantom are not relevant for the individual patient. In this investigation, the effective dose for 31 patients undergoing a CT examination of their head has been evaluated using measurements on a standard CT head phantom and the data corrected for the real size (perimeter) and exposed volume of the patient's head. It is shown that the actual doses to patients were higher in 50 % cases of all performed head CT examinations, compared with doses estimated from phantom measurements. A dose calculation model for head CT patients based on the individual patient's head size (perimeter) and exposed head volume is proposed and the possibility to optimise head CT examinations of patients is discussed.

CT Dosimetry: Comparison of Measurement Techniques and Devices

In x-ray computed tomography (CT), the most common parameter used to estimate and minimize patient dose is the CT dose index (CTDI). The CTDI is a volume-averaged measure that is used in situations where the table is incremented in conjunction with the tube rotation. Variants of the CTDI correct for averaging across the field of view and for adjacent beam overlaps or gaps. CTDI is usually measured with a pencil-shaped ionization chamber, although methods have been developed that use alternative detectors, including an optically stimulated luminescence probe and a solid-state real-time dosimeter. Because the CTDI represents an averaged dose to a homogeneous cylindrical phantom, the measurements are only an approximation of the patient dose. Furthermore, dose from interventional or perfusion CT, in which the table remains stationary between multiple scans, is best evaluated with point dose measurements made with small detectors. CTDI and point dose values are nearly the same for measurement of surface dose from spiral CT. However, for measurement of surface dose from perfusion CT, the dose is overestimated by a factor of two or more with CTDI values in comparison with point dose values. Both CTDI and point dose measurement are valuable for evaluating CT scanner output and estimating patient dose.

A Monte Carlo-based method to estimate radiation dose from spiral CT: from phantom testing to patient-specific models

The purpose of this work is to develop and test a method to estimate the relative and absolute absorbed radiation dose from axial and spiral CT scans using a Monte Carlo approach. Initial testing was done in phantoms and preliminary results were obtained from a standard mathematical anthropomorphic model (MIRD V) and voxelized patient data. To accomplish this we have modified a general purpose Monte Carlo transport code (MCNP4B) to simulate the CT x-ray source and movement, and then to calculate absorbed radiation dose in desired objects. The movement of the source in either axial or spiral modes was modelled explicitly while the CT system components were modelled using published information about x-ray spectra as well as information provided by the manufacturer. Simulations were performed for single axial scans using the head and body computed tomography dose index (CTDI) polymethylmethacrylate phantoms at both central and peripheral positions for all available beam energies and slice thicknesses. For comparison, corresponding physical measurements of CTDI in phantom were made with an ion chamber. To obtain absolute dose values, simulations and measurements were performed in air at the scanner isocentre for each beam energy. To extend the verification, the CT scanner model was applied to the MIRD V model and compared with published results using similar technical factors. After verification of the model, the generalized source was simulated and applied to voxelized models of patient anatomy. The simulated and measured absolute dose data in phantom agreed to within 2% for the head phantom and within 4% for the body phantom at 120 and 140 kVp; this extends to 8% for the head and 9% for the body phantom across all available beam energies and positions. For the head phantom, the simulated and measured absolute dose data agree to within 2% across all slice thicknesses at 120 kVp. Our results in the MIRD phantom agree within 11% of all the different organ dose values published by the UK’s ImPACT group for a scan using an equivalent scanner, kVp, collimation, pitch and mAs. The CT source model was shown to calculate both a relative and absolute radiation dose distribution throughout the entire volume in a patient-specific matrix geometry. Results of initial testing are promising and application to patient models was shown to be feasible.

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.

Radiation dosimetry for electron beam CT.

To measure the radiation dose profile, multiple-scan average dose (MSAD), and computed tomography dose index (CTDI) for electron beam CT and to determine the accuracy of ionization-chamber and manufacturer estimates of patient dose. High-resolution dose profiles along the longitudinal axis were acquired at several positions within the scan plane with use of radiographic film. The full-width-at-half-maximum values, peak radiation dose, CTDI, and MSAD were calculated from the digitized film profiles. CTDI was also measured with an ionization chamber. The full-width-half-maximum value of the radiation profiles were significantly wider than the nominal scan width for the 6-mm single-section and 8-mm multisection modes. In the single-section mode, the CTDI underestimated the MSAD by 15%-30%. The multisection radiation profile was nonuniform and asymmetric. Patient doses in electron beam CT are approximately 125% of the ionization-chamber CTDI measurements in the single-section mode. For the multisection mode, the average patient dose over the scan volume is approximately 70%-85% of the ionization-chamber CTDI measurements.

Pediatric patient exposures from CT examinations: GE CT/T 9800 scanner.

This report presents Computed Tomography Dose Index (CTDI) values for typical CT examinations of children for a GE CT/T 9800 scanner and compares them with measured entrance skin absorbed doses of pediatric patients under clinical situations. Pediatric entrance skin absorbed doses were 1.1-2.4 rad (cGy) for chest and abdomen examinations, 2.0-3.4 rad (cGy) for pediatric head examinations, and 3.2-4.2 rad (cGy) for infant (less than or equal to 6 months) head examinations. CTDI measurements in a cylindrical Lucite head phantom predicted typical pediatric absorbed doses to within about 5% for chest and abdomen examinations and to within about 15% for head examinations, when corrections for amperage differences are taken into account.