Radiation doses, image noise and image uniformity

5.2.8. Radiation doses, image noise and image uniformity

5.2.8.1. Aim The aim of these tests is to ensure that: (a) Appropriate radiation doses are being used for patient CT scans;

(b) Image noise is typical of what would be expected for the specific radiation doses; (c) Computed tomography numbers (pixel values) are uniform over the image.

5.2.8.2. Frequency This test must be performed by a qualified medical physicist at the time of

acceptance testing, as part of the end-of-warranty testing, and whenever the CT system is serviced in a way that might have an impact on radiation dose, image noise and image uniformity, including, but not limited to, replacement of an X ray tube, system calibration, generator maintenance and software changes or upgrades.

5.2.8.3. Materials The materials required are: — Head (16 cm diameter) and body (32 cm diameter) acrylic dosimetry

phantoms; — A pencil ion chamber and electrometer; — DRLs for CT examinations from appropriate professional organizations.

Note that the noise measurements given by manufacturers are for specific phantoms with specific radiographic factors. It is essential that these phantoms and factors be used if the noise levels given by the manufacturer are to be used for evaluation purposes.

5.2.8.4. Procedure Place the 16 cm diameter dosimetry phantom on the table centred at the

isocentre of the scanner, with the long axis of the phantom aligned with the z axis of the scanner. Acquire a scout scan and a single 1 mm slice image of the phantom for alignment purposes.

Note that it is assumed that all the doses measured will be the CTDI and the DLP. See Refs [9, 47] for information on CT dosimetry. For DRL and comparison purposes, CTDI w should be used, which is defined as follows:

CTDI = CTDI 1 w 3 c + 2 3 CTDI p

where CTDI w is the weighted CTDI, CTDI c is the CTDI measured in the centre of the phantom and CTDI p is the peripheral CTDI defined as the average of the four peripheral CTDI measurements.

Place the ion chamber in the centre of the phantom. Use a scout scan image to select the volume or slice to be imaged. Reset the dosimeter readout to zero. Make an exposure in axial mode employing a technique used for clinical head CT scans, and record the technique factors and measured doses. Place the chamber in each of the four peripheral holes of the phantom, take additional exposures and record the doses. The peripheral dose is defined as the average of these four peripheral CTDI measurements.

With the ion chamber in the centre of the 16 cm phantom, select one technique, for example, 120 kVp and 100 mAs, and take dose measurements with various slice thicknesses over the clinical range. Ensure that one set of measurements includes the factors that the manufacturer uses for the reference noise measurements for acceptance testing. Include a complete CTDI w measurement at one clinical head CT setting.

Place the 32 cm phantom in the scanner and repeat the above procedure. Make CTDI w dose measurements using typical adult chest, abdomen and pelvis techniques with this phantom.

If the scanner is used for paediatric patients, CTDI w doses should be measured for paediatric techniques, assuming a 20 kg patient, using the 16 cm phantom and clinical techniques for paediatric head, chest and abdomen examinations.

Save the images produced during the dosimetry scans, as these will be used for noise and uniformity measurements.

5.2.8.5. Analysis For the analysis of radiation doses, image noise and image uniformity,

proceed as follows:

(a) Develop a spreadsheet showing all of the data for easy analysis. Compare the measured CTDI w doses obtained under the various clinical scan conditions with those displayed on the scanner console. Compare these clinical doses with doses from other CT scanners at the same institution using the same techniques. In addition, compare the doses with national DRLs. Plot the doses for a given technique as a function of slice thickness.

(b) Select areas in both head and body phantoms that appear to be free of artefacts, and with relatively uniform CT numbers in the centre and periphery of the image. These areas should be between 4 and 10 cm 2 . Measure the standard deviation and the average of the CT numbers in these areas. The noise is defined as the standard deviation of the CT

numbers divided by the average of the CT numbers in each area. 2 Plot the noise as a function of dose on a semi-logarithmic graph. (c) Determine the difference in the average CT number between the centre and periphery. This difference is the CT number uniformity value.

5.2.8.6. Suggested tolerances The suggested tolerances for radiation doses, image noise and image

uniformity are as follows: (a) Computed tomography doses for various scanners at one institution

should result in the same dose and noise levels for the same technical factors. Computed tomography doses should not exceed DRLs. Doses should be within ±20% of the manufacturer’s specification for both CTDI w and DLP.

(b) The doses for chest CT images should be significantly lower than those for abdomen CT images for the same size patient (phantom) for both adult and paediatric imaging. Doses for paediatric patients, using the 16 cm phantom, should be significantly lower than those for adult patients.

(c) With respect to graphs showing dose as a function of slice thickness, doses should be linear with slice thickness. However, these values will typically show an increase in dose at thinner slice thicknesses due to the increased width of the pre-patient collimators relative to the nominal slice thickness (or overbeaming). Compare the results with the manufacturer’s specifica- tions for radiation slice thickness.

2 Noise can also be defined using the contrast scale and attenuation coefficient of water [7].

(d) The image noise (the standard deviation divided by the average of the CT numbers) should be equal to or less than the values specified by the manufacturer. The average CT numbers should remain within ±5% of the values determined at acceptance testing.

(e) There should be a linear relationship between dose and noise when plotted on a semi-logarithmic graph. Noise levels should be comparable with those of other CT scanners at the facility for the same dose levels.

(f) The uniformity (difference between the average CT number in the centre compared with that at the periphery) should be between ±3 HU (CT numbers) in the head (16 cm) phantom and ±5 HU in the body (32 cm) phantom, where HU stands for Hounsfield unit.

5.2.8.7. Corrective actions The corrective actions for radiation doses and image noise are as follows: (a) Computed tomography doses that are above DRLs can be corrected by

changing the techniques used, i.e. using the appropriate kVp value and reducing the mAs value. Patient doses for the same examinations with similar CT scanners should be similar.

(b) Increased image noise can result from several factors, including selecting inappropriately low kVp or mAs values, or both, or from malfunctioning of the scanner electronics. The sources of increased noise should be determined and corrected.

(c) Any non-linearities in the noise versus dose figures may be due to increased electronic noise. The source of this noise should be determined and corrected.

Non-uniformity in the CT numbers from the centre to the edge of the phantoms can result from several causes including, but not limited to, inappro- priate selection of the beam shaping filter and an incorrect reconstruction algorithm.