Abstract
BackgroundApplied tube voltage (kilovolts, kV) and tube current (milliampere seconds, mAs) affect CT radiation dose and image quality and should be optimised for the individual patient. CARE kV determines the kV and mAs providing the lowest dose to the patient, whilst maintaining user-defined reference image quality. Given that kV changes affect CT values which are used to obtain attenuation maps, the aim was to evaluate the effect of kV changes on PET quantification and CT radiation dose using phantoms.MethodFour phantoms (‘Lungman’, ‘Lungman plus fat’, ‘Esser’ and ‘NEMA image quality’ (NEMA IQ)) containing F-18 sources underwent 1 PET and 5 CT scans, with CARE kV on (automatic kV selection and mAs modulation) and in semi mode with specified tube voltages of 140, 120, 100 and 80 kV (mAs modulation only). A CARE kV image quality reference of 120 kV/50 mAs was used. Impact on PET quantification was determined by comparing measured activity concentrations for PET reconstructions from different CT scans with the reconstruction using the 120 kV reference, and dose (DLP, CTDIvol) differences calculated by comparing doses from all kV settings with the 120 kV reference.ResultsCARE kV-determined optimal tube voltage and CARE kV ‘on’ dose (DLP) savings compared with the 120 kV reference were: Lungman, 100 kV, 2.0%; Lungman plus fat, 120 kV, 0%; Esser, 100 kV, 9.3%; NEMA IQ, 100 kV, 3.4%. Using tube voltages in CARE kV ‘semi’ mode which were not advised by CARE kV ‘on’ resulted in dose increases ≤ 65% compared with the 120 kV reference (greatest difference Lungman plus fat, 80 kV). Clinically insignificant differences in PET activity quantification of up to 0.7% (Lungman, 100 kV, mean measured activity concentration) were observed when using the optimal tube voltage advised by CARE kV. Differences in PET quantification of up to 4.0% (Lungman, 140 kV, maximum measured activity concentration) were found over the full selection of tube voltages in semi mode, with the greatest differences seen at the most suboptimal kV for each phantom. However, most differences were within 1%.ConclusionsCARE kV on can provide CT radiation dose savings without concern over changes in PET quantification.
Highlights
Applied tube voltage and tube current affect Computed tomography (CT) radiation dose and image quality and should be optimised for the individual patient
Using tube voltages in CARE kV ‘semi’ mode which were not advised by CARE kV ‘on’ resulted in dose increases ≤ 65% compared with the 120 kV reference
In positron emission tomography (PET)-CT imaging, CT is performed in addition to the PET scan, for one or more of the following purposes: ‘attenuation correction’ (AC) of PET data, ‘localisation’ of abnormal tracer uptake shown in the PET images, ‘characterisation’ of the underlying disease associated with abnormal tracer uptake or with ‘diagnostic’ intent [2, 3]
Summary
Applied tube voltage (kilovolts, kV) and tube current (milliampere seconds, mAs) affect CT radiation dose and image quality and should be optimised for the individual patient. Computed tomography (CT) scanning is known to be the greatest contributor to radiation burden associated with medical imaging examinations [1]. The level of CT image quality needed and the radiation dose delivered to the patient can be very low for AC, localisation and characterisation scans but increases markedly when performing fully diagnostic scans [3]. Despite L/C level CT scans allowing relatively low CT exposure settings compared with fully diagnostic scans [2, 3], the fact that PET-CT scans cover the wholebody in many clinical applications means the CT component still contributes a significant proportion of the overall radiation dose in whole-body PET-CT [4, 5]. Given the high CT radiation burden in medical imaging, there has been much focus on CT dose reduction techniques
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