Simplified approach to estimation of organ absorbed doses for patients undergoing abdomen and pelvis CT examination

The volumetric computed tomography (CT) dose index (CTDI_vol) is the measure of output displayed on CT consoles relating to dose within a standard phantom. This gives a false impression of doses levels within the tissues of smaller patients in Southeast Asia. A size-specific dose estimate (SSDE) can be calculated from the CTDI_volto provide an assessment of doses at specific positions within a scan using size-specific conversion factors. SSDE is derived using the water equivalent diameter (D_w) of the patient, but calculation ofD_wrequires sophisticated computer software. This study aimed to evaluate relationships betweenD_Wand effective diameter (D_Eff), which can be measured more readily, in order to estimate SSDE at various positions within a routine clinical abdomen and pelvis CT examination for Thai patients. An in-house ImageJ algorithm was developed to measureD_w, effective diameter (D_Eff), and SSDE on CT slices located at the heart, liver, kidneys, colon, and bladder, on 181 CT examinations of abdomen and pelvis. Relationships betweenD_EffandD_wwere determined, and values of organ absorbed dose usingD_Effwere estimated. This approach was validated using a second cohort of 54 patients scanned on a different CT scanner. The results revealed that ratios betweenD_EffandD_wat the heart level were 1.11-1.13 and those for the others were about 1.00. Additionally, the SSDE/CTDI_volratio was estimated for each organ in terms of exponential functions using the relationships betweenD_wandD_Efffor individual organs. In summary, this study proposed a simple method for estimation of organ absorbed doses for Southeast Asian patients undergoing abdomen and pelvis CT examinations where sophisticated computer software is not available.

Effects of tube potential selection together with computed tomography automatic tube current modulation on CT imaging performance

The effects of tube potential selection with a computed tomography (CT) automatic tube current modulation (ATCM) system on radiation dose and image quality have been investigated on a Canon CT scanner. The use of different values of tube voltage for imaging, and the appropriate settings of the ATCM system, were evaluated. The custom-made phantom consisted of three sections of different sizes with inserts of various materials. It was scanned using tube potentials of 80-140 kV and different image quality ATCM settings. CTDI_voland image quality in terms of noise, contrast, and contrast-to-noise ratio (CNR) for air, polyethylene (PE), acrylic, polyoxymethylene (POM) and polyvinylchloride (PVC) were analysed. A figure of merit (FOM) was estimated by combining CNR and CTDI_vol. CTDI_volvalues were similar for all values of tube voltage and individual image quality ATCM settings when tube current was not restricted by the maximum value. The contrasts were independent of ATCM image quality setting, but CNR increased at the higher image quality level as image noise decreased. Both contrast and CNR decreased with increasing tube voltage for PVC and PE, but increased for POM and acrylic. PVC was the only insert material for which there was a significant improvement in contrast at lower tube potentials. FOM indicated that standard (SD = 10) and low dose (SD = 12.5) ATCM settings might be appropriate. The optimum tube voltage settings for imaging the PVC was 80-100 kV, but not for the lower contrast POM and acrylic, for which the standard tube voltage setting of 120 kV was better. The tube potential should be carefully set to gain radiological protection optimisation and keep the radiation dose as low as possible. Results indicate that 100 kV is likely to be appropriate for imaging small and medium-sized Thai patients when iodine contrast is used.

Design and use of a phantom for testing and comparing the performance of computed tomography automatic tube current modulation systems

Automatic tube current modulation (ATCM) is now available on all computed tomography (CT) scanners, but there is no standard phantom for testing its operation. For this study, a phantom comprising five elliptical sections of varying diameters in the shape of a pagoda has been made to represent the range of sizes for patients in Thailand and the Far East. ATCM performance can be evaluated in terms of how tube current and image noise vary with changes in the sizes of the sections. Additional rods of different materials with similar properties to human tissues have been inserted to allow the measurement of contrast-to-noise ratios (CNRs) for assessment of image quality. The phantom has been used to test and compare the performance of CT ATCM systems for the major four CT scanner vendors. The results showed that the ATCM systems of Toshiba and GE CT scanners maintained image noise and CNR within narrower ranges by varying tube current aggressively along the scan length, and commencing modulation before the high attenuation sections are reached. In contrast, the ATCM systems of Philips and Siemens scanners adjusted the tube current within narrower ranges, allowed larger variations in image noise and CNR, and commenced modulation at the start of sections with higher attenuation. The phantom can be used to confirm functionality of the system for acceptance testing, as well as providing information on the tradeoff between radiation dose and image quality when setting up clinical protocols during commissioning of new CT scanners. The phantom can also be used on a routine basis to check that performance is maintained, and could be used in the future for selecting protocol settings to give required values of CNR or adjusting protocol settings on different CT scanners to provide similar levels of clinical performance.

Setting up computed tomography automatic tube current modulation systems

Automatic tube current modulation (ATCM) on CT scanners can yield significant reductions in patient doses. Modulation is based on x-ray beam attenuation in body tissues obtained from scan projection radiographs (SPRs) and aims to maintain the same level of image quality throughout a scan. Noise level is important in judging image quality, but tissues in larger patients exhibit higher contrast resulting from the presence of fat. CT scanner manufacturers use different metrics to assess image quality. Some employ a simple measure of image noise, while others adopt a measure related to a reference image that accepts higher noise levels in more attenuating parts with higher contrast. At the present time there is no standard method for testing ATCM. This paper reviews the operation of different ATCM systems, considers options for testing, and sets out a framework that could be used for optimizing clinical protocols. If dose and image quality can be established for a reference phantom, the modulation performed by ATCM systems can be characterised using anatomical phantoms or geometrical elliptical phantoms which may be conical or include sections of varying dimension. For scanners using a reference image or mAs, selection of the image quality reference determines other factors. However, for scanners using a noise reference, a higher noise level should be selected for larger patients to avoid high doses, and the operator should ensure that appropriate limits are set for mA modulation. Other factors that need to be considered include the SPRs used to plan the ATCM and image thickness. Users should be aware of the mode of operation of the ATCM system on their CT scanner, and be familiar with the effects of changing different protocol parameters. The behaviour of ATCM systems should be established through testing of each CT scanner with suitable phantoms during commissioning.

Influence of CT automatic tube current modulation on uncertainty in effective dose

Computed tomography (CT) scanners are equipped with automatic tube current modulation (ATCM) systems that adjust the current to compensate for variations in patient attenuation. CT dosimetry variables are not defined for ATCM situations and, thus, only the averaged values are displayed and analysed. The patient effective dose (E), which is derived from a weighted sum of organ equivalent doses, will be modified by the ATCM. Values for E for chest-abdomen-pelvis CT scans have been calculated using the ImPACT spreadsheet for patients on five CT scanners. Values for E resulting from the z-axis modulation under ATCM have been compared with results assessed using the same effective mAs values with constant tube currents. Mean values for E under ATCM were within ±10 % of those for fixed tube currents for all scanners. Cumulative dose distributions under ATCM have been simulated for two patient scans using single-slice dose profiles measured in elliptical and cylindrical phantoms on one scanner. Contributions to the effective dose from organs in the upper thorax under ATCM are 30-35 % lower for superficial tissues (e.g. breast) and 15-20 % lower for deeper organs (e.g. lungs). The effect on doses to organs in the abdomen depends on body shape, and they can be 10-22 % higher for larger patients. Results indicate that scan dosimetry parameters, dose-length product and effective mAs averaged over the whole scan can provide an assessment in terms of E that is sufficiently accurate to quantify relative risk for routine patient exposures under ATCM.

Relationships between patient size, dose and image noise under automatic tube current modulation systems

Automatic tube current modulation (ATCM) systems are now used for the majority of CT scans. The principles of ATCM operation are different in CT scanners from different manufacturers. Toshiba and GE scanners base the current modulation on a target noise setting, while Philips and Siemens scanners use reference image and reference mAs concepts respectively. Knowledge of the relationships between patient size, dose and image noise are important for CT patient dose optimisation. In this study, the CT patient doses were surveyed for 14 CT scanners from four different CT scanner manufacturers. The patient cross sectional area, the tube current modulation and the image noise from the CT images were analysed using in-house software. The Toshiba and GE scanner results showed that noise levels are relatively constant but tube currents are dependent on patient size. As a result of this there is a wide range in tube current values across different patient sizes, and doses for large patients are significantly higher in these scanners. In contrast, in the Philips and Siemens scanners, tube currents are less dependent on patient size, the range in tube current is narrower, and the doses for larger patients are not as high. Image noise is more dependent on the patient size.

Approaches to aspects of optimisation of protection in diagnostic radiology in six continents

There has been an expansion in the use of x-ray imaging during the last 20 years. Effective arrangements for justification of exposures as well as for optimisation of protection are crucial. The amount of effort put into the latter, the way in which it is organised and the groups carrying this out vary across the globe. A simple survey of organisational arrangements relating to performance testing of x-ray equipment, management of patient dose and other aspects of implementing optimisation has been undertaken. A total of 137 completed survey forms were received from medical physicists in 48 countries. Results for individual countries from which more responses were received, or for groups of neighbouring ones, are compared to portray variations. Some performance testing of x-ray equipment was mandated in most countries (more than 90%), with the tests being performed primarily by hospital or private medical physicists, although other groups are involved. Testing of equipment prior to clinical use was generally high for most regions, but the frequency was lower in Latin America. There was considerable variation in the frequency and regularity of subsequent testing. The prevalence of patient dose surveys was high in Europe, but lower in other continents. Organisational arrangements for testing performance of x-ray equipment, patient dose surveys and implementing optimisation of protection in medical exposures across the globe can be divided into five main groups. Hospital medical physicists take the lead in western Europe and Australia with the involvement of radiographers. Private medical physicists test equipment in Brazil, the USA and New Zealand, and have some responsibility for optimisation in Brazil. University personnel have significant involvement, together with medical physicists in eastern Europe, but the extent of the coverage is uncertain. Government personnel and service engineers carry out equipment testing in many countries of Africa and Asia, while radiographers have a significant role in Thailand and other countries where the number of medical physicists is limited. In order for dose surveys to have an impact, action must be taken upon the findings, but there must be an effective link between surveyors and radiology facility staff to ensure that this is done.

Comparison of different phantom designs for CT scanner automatic tube current modulation system tests

Modern CT scanners modulate tube current during scans according to patient size, shape and attenuation. However, the ATCM (automatic tube current modulation) systems for different CT manufacturers work on different principles. Although the systems are used for the majority of patients and examinations, there is no standard phantom for routine quality control of CT scanner ATCM operation. The ideal phantom for testing these systems should be capable of evaluating how tube current and image quality as well as dose vary according to changes in patient size and shape. For this study, a conical phantom designed by ImPACT has been compared with two phantoms made from elliptical sections with varying dimensions. The concept of the designs is to reflect the ATCM performance for the varying shapes and dimensions along the length of the human body. The first phantom comprises five elliptical sections with a wide range of different dimensions and the second has three sections that are more similar in size. The phantoms have been used to test ATCM systems for Philips, Siemens, GE and Toshiba scanners. Although the results of the tube current modulation patterns were similar for all CT scanners, the abrupt changes in attenuation for the first sectional phantom provoked an abnormal ATCM response for the GE and Toshiba scanners. The second sectional phantom was developed from the results of the first, and was more effective for ATCM system testing and could be used for dose and image quality assessment in standard positions. However, the ImPACT conical phantom provided the best overall assessment of performance in terms of tube current modulations and noise pattern.

A study of CT dose distribution in an elliptical phantom and the influence of automatic tube current modulation in the x-y plane

Computed tomography (CT) performance assessments relating to patient dose to the body are made conventionally in 320 mm diameter cylindrical acrylic phantoms. The cross section of the human trunk is closer to an ellipse and automatic tube current modulation (ATCM) systems adjust the exposure level with orientation in the x-y plane, changing the dose distribution within the body. This study has investigated differences in the distributions of dose within a standard cylindrical body phantom and an elliptical dosimetry phantom for Toshiba, General Electric and Philips CT scanners, and recorded changes with the application of the ATCM. Single slice dose profiles have been recorded within the phantoms using Gafchromic film. CT dose indices along 100 mm lengths have been calculated and data sets combined to simulate helical scans, from which values for cumulative doses have been derived. The doses in the centre of the elliptical phantom are 70-100% larger than for the cylindrical one and in the anterior are around 20-40% larger, while the doses in the lateral positions are similar for the two phantom shapes. The differences between the anterior and lateral doses were larger for the Toshiba scanner and this is thought to be linked to the narrower profile of the beam produced by the bow-tie filter. When the ATCM mode for the Toshiba scanner is implemented, the doses in the anterior and posterior positions are reduced preferentially, bringing them closer to the doses in the lateral positions.

Application of Gafchromic film in the study of dosimetry methods in CT phantoms

Gafchromic film has been used for measurement of computed tomography (CT) dose distributions within phantoms. The film was calibrated in the beam from a superficial therapy unit and the accuracy confirmed by comparison with measurements with a 20 mm long ionisation chamber. The results have been used to investigate approaches to CT dosimetry. Dose profiles were recorded within standard CT head and body phantoms and scatter tail data fitted to exponential functions and extrapolated to predict dose levels in longer phantoms. The data have been used to simulate both CT dose index (CTDI) measurements with ionisation chambers of differing length and measurements of cumulative doses with a 20 mm chamber for scans of varying length. The results show that the length of a pencil ionisation chamber is the most significant factor affecting measurements of weighted CTDI (CTDI(w)) and a 100 mm chamber would record 50-61% of the dose measured with a 450 mm one. The cumulative dose measured at the centre of a 150 mm long body phantom records over 70% of the equilibrium dose from a helical scan of a longer phantom. For routine CT dosimetry tests, the determination of correction factors could allow measurements with a 100 mm chamber to be used to derive the CTDI that would be recorded with a longer chamber, and cumulative doses measured with a 20 mm chamber in shorter phantoms to be used to calculate equilibrium doses for helical scans.