CT patient positioning: more important than you think
April 20, 2022
Published on auntminnie.com
By Anna Romanyukha, PhD, Steve Nzitunga, Radiographer, Ana Dolcet, MPE, Application & Training team at Qaelum
Techniques to reduce patient radiation dose during computer tomography (CT) examinations have improved drastically since their first introduction into clinical practice.
Automatic tube current modulation (ATCM) is one of the most important improvements in the history of computed tomography. Since the concept was introduced by Haaga et al.  in 1981, and made commercially available by GE Medical Systems in 1994 , ATCM has become one of the principal techniques for the optimization of patient radiation dose and image quality. In order to use ATCM, the scanner must estimate the attenuation characteristics of the patient over the range of the scan provided by one or two localizer radiographs (LRs, or topograms) . ATCM then automatically adjusts the tube current according to the patient size and attenuation to maintain the noise level indicated by the user.
The bowtie filter was introduced to compensate for the variation of patient attenuation at the level of the detector during scan rotation . The filter is composed of a thinner and thicker segment. The thinner segment allows maximum beam intensity for anatomical regions with high attenuation while the thicker segment is used to reduce beam intensity for anatomical regions with lower attenuation, in the peripheral areas of the patient . A Teflon bowtie filter is shown in Figure 1.
Figure 1. A Teflon bowtie filter. Reproduced from M.A.Habibzadeh et al. .
In order to ensure that ATCM and bowtie filter techniques work as intended, the patient must be correctly centered with respect to the CT gantry. Proper centering will lead to accurate localizer radiographs and therefore optimal ATCM performance (Fig. 2a). If the patient is positioned too close to the X-ray source, the scanner will overestimate the patient size and thus the patient attenuation (Fig. 2b). This will cause the ATCM system to transmit a higher tube current, causing an unnecessary increase in dose. If the patient is positioned too far from the X-ray source, the opposite will happen: an underestimation of the patient size and attenuation, leading to potentially noisy images (Fig. 2c) .
Figure 2. The grey circle at the bottom represents patient size when a) the patient is centered at the gantry b) an anterior shift is present in patient positioning c) a posterior shift is present in patient positioning. Reproduced from Barreto et al. .
The correct function of the bowtie filter also relies on centering, and the assumption that the thickest region of the patient is positioned in the center of the beam (Fig. 3). If the patient is not centered with respect to the gantry, dose will be higher than necessary in some regions of the patient, and lower than necessary in other regions.
Figure 3. Anterior (top) and lateral (bottom) positions of the x-ray tube when the patient is a) in the center of the gantry b) shifted anteriorly c) shifted posteriorly with respect to the beam output. Reproduced from Barreto et al. .
This can cause non-uniform noise in the image following reconstruction. Moreover, possible shading artifacts may occur due to beam hardening from changes in the mean beam energy that can lead to artifactual changes in organ density (Fig. 4) .
Figure 4. Shading artifacts as a result of beam hardening can be seen on the CT scan in the posterior region of the patient, causing an artifactual change in liver density. Reproduced from Szczykutowicz et al. .
Vertical and lateral shifts: quantified
Vertical miscentering has been reported for 73%  to 95%  of patients in various hospitals [9,10]. Maximum reported vertical miscenterings were 6.6 cm and 3.4 cm below and above the isocenter, respectively , with mean shifts in the range of -2.3-2.6 cm [10,11].
Miscentering was found to be more likely for smaller patients: slim adults [4,6] and smaller pediatric  patients. Moreover, many technologists often use only the AP topogram, as opposed to both AP and lateral topograms . Vertical shifts of > 3 cm were found to occur in 7.7-22% of the cases, and such shifts are expected to be detected by technologists [5,9,10].
Lateral miscentering with respect to the x-ray tube was reported for 80% of patients in one study , but the shift was > 0.5 mm, and supports the overall small shifts in lateral patient position across multiple studies [4, 10, 11]. It is assumed that this is due to the fact that the borders of the patient table provide sufficient reference for the technologist.
How miscentering affects dose and image quality
Effect on dose
A magnification of 4 cm was found to increase dose by 67%, increasing CTDIvol and SSDE by 10 and 11.9 mGy, respectively .
Patient positioning of 2.5-3.5 cm below the isocenter increases dose by up to 15 % in chest exams . Miscentering by 6 cm with respect to the bowtie filter increases surface dose by 41 - 49% [10, 11], whereas a 4-cm anterior shift increases CTDIvol by 8.5 - 10% [5, 14]. CTDIvol in general increases linearly with horizontal anterior distance from the isocenter .
Vertical shifts were found to impact organ dose, specifically the lung, colon, uterus, ovary and skin by −35.0% to 22.0% in chest abdomen pelvis CT exams at a distance of up to 4 cm from the isocenter. Additional organs, such as liver, stomach, and breast had organ dose differences in the range of −13.0% to 15.0%. Organ dose also increases with vertical distance from the isocenter .
Furthermore, to help reduce dose, the PA localizer, as opposed to an AP localizer, is recommended for chest exams . When lateral topogram images were used, the impact on total patient dose i.e. CTDIvol, SSDE and DLP is lower than in the case of PA topogram images, however the risk to certain radiation sensitive tissues e.g. breasts and thyroid increases due to miscentering with respect to the bowtie filter .
Effect on image quality
Miscentering of 2.1 cm below the isocenter was found to increase image noise by 6% . Vertical miscentering of 6 cm was reported to increase image noise by 28-30 % [4,10].
Szczykutowicz et al. investigated the effect of patient positioning on CT number that is used to diagnose clinical conditions. They found that a mispositioning of 4 and 6 cm above the isocenter resulted in statistically significant differences in the Hounsfield unit (HU) standard deviation. A 4-cm magnification led to standard deviations of -15±5 and -8±2 HU for mid thorax posterior and abdomen posterior, respectively. A 6-cm magnification led to standard deviations in the range of 13-20 HU for high thorax and mid thorax posterior .
When the patient is positioned below the isocenter in supine position, noise especially increases in the posterior region due to presence of the spine and thus higher attenuation.
Vital role of technologists
Technologists play a vital role in the reduction of patient dose and image noise, as they are responsible for determining the patient positioning prior to each scan with the use of the laser lights mounted on the scanner. Multiple studies have recognized that incorrect patient positioning may outweigh the benefits of automatic exposure control [4,15,16]. The following recommendations for improving patient positioning have been offered for technologists by Mayo-Smith et al. , Kaasalainen et al. , and Habizadeh et al. :
- The tube should be positioned at the top of the gantry, rather than at the bottom, to avoid offsets below the isocenter .
- Both AP and LAT topograms should be used with the lowest acceptable tube current and tube voltage settings [6,12].
- Topograms should be repeated if the patient positioning is modified, to ensure proper ATCM operation .
- The AP topogram is more robust than the LAT, and should be performed last, since only the last topogram may be used for tube current modulation in some scanners .
- Special attention should be paid when positioning pediatric patients, due to their smaller size and organs, and pediatric weight-based protocols should be used to determine optimal settings .
Ultimately, of course, it is successful teamwork between technologists, radiologists and medical physicists that allows good image quality and dose reduction in the medical imaging department.
It is difficult or even impossible to retrospectively assess exactly how the patient was positioned by the technologist. In our experience, the best assessment is performed by a dose management system.
How DOSE by Qaelum can help
DOSE by Qaelum performs a detailed analysis of the actual patient positioning during each scan, allowing the end-user to detect and address potential technologist training needs. Patient positioning can be evaluated for different operators, protocols, study descriptions, patient body size, tube current, scanner, and much more.
Figure 5. Patient positioning during the scan is represented by the orange ellipse, while the position assumed by the scanner is represented by the blue circle. Exact horizontal, vertical, and radial offsets are displayed in the blue box on the top left.
In Figure 5 the blue circle indicates the position of the reference phantom assumed by the scanner, while the orange ellipse shows the actual position of the patient in the exam. A significant deviation can be easily spotted. The exact offset values are displayed for horizontal, vertical, and radial offsets in the blue box.
The software also offers charts that display positional offsets for every study (Fig. 6). Here any of the points i.e. series can be clicked to open the study and series details of interest for a full analysis of outliers.
Figure 6. Offsets in patient positioning for studies in two CT scanners. Patient positioning in all cases can be evaluated by clicking on the individual series.
Extended analyses can be performed and customized by the user, such as evaluating which operators need most training in positioning the patient (Fig. 7).
Figure 7. All study and series data can be evaluated and related to offsets in patient positioning.
DOSE by Qaelum does not only evaluate patient positioning, but all the components of a good CT examination. In our Advanced CT Analysis calculations of the size-specific dose estimate (SSDE), blindscan (i.e. area scanned in the spiral acquisition but excluded from the topogram), ATCM, image noise and more are performed. All data are exportable and easy to evaluate.
DOSE can help detect technologist training needs to ensure proper use of ATCM and bowtie filter. This can in turn help to reduce the patient dose and improve image quality in your department.
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