Influence of tube and patient positioning in thoracoabdominal CT examinations on radiation exposure–towards a better patient positioning

Although iso-centric patient positioning is enormously important in computed tomography (CT), it is complicated in thoracoabdominal imaging by the varying dimensions of the body. Patient positioning can affect the appearance of the patient on the localiser. Positioned too close to the x-ray tube, a patient appears considerably more voluminous. The goal of this study is to assess the difference in radiation exposure of combined chest and abdomen CT scans between scans with prior 0°- and 180°-localisers in conjunction with patient positioning. In this IRB-approved retrospective study, patients who had two routine thoracoabdominal CT scans on the same CT scanner, one with a prior 0°- and one with a prior 180°-localiser, were included. To evaluate the radiation exposure of the thoracoabdominal CT examination regarding the tube position during the localiser, volumetric computed tomography dose index (CTDIvol), size-specific dose estimate (SSDE), patient diameter and positioning within the iso-centre for three positions (heart, abdomen, femur level) were compared with regard to the tube position during the prior localiser. CT examinations of 114 patients were included. Despite similar patient weight and diameter between the two examinations, SSDE and CTDIvol was significantly larger (up to 73%) with 180°-localisers. Patient offset from the iso-centre ranged between −9 mm at the centre slice (abdomen level) to −43 mm at the most caudal slice at the pelvis (femur level), causing a significant magnification (p < 0.001) on 180°-localisers with a subsequent increase of the apparent attenuation. The results of this study emphasise the use of 0°-localisers in thoracoabdominal CTs, since 180°-localisers caused patient magnification with subsequent increase in radiation exposure. The advantage of 180°-localisers, namely reducing the dose in thyroid and breast, is eliminated if the dose of the CT scan increases significantly in the abdomen and pelvis.


Introduction
Patient positioning is known to influence radiation exposure in computed tomography (CT) acquisitions. The positioning in the iso-centre of the scanner gantry is the basis for an optimal function of the automatic tube current modulation (ATCM). For a small scan coverage, such as the chest or the hip, the patient can be positioned in the iso-centre without a larger deviation. However, for longer scan ranges, positioning in the iso-centre is complicated due to the different patient diameters in the axial scan range. Usually, the patient centre varies along the body, especially in the lung, abdominal and pelvic regions. Positioning the patient using the chest centre as reference might result in an off-centreing of the rest of the body in the gantry.
Another influence on radiation exposure of CT scans may be caused by the tube positioning for localisers. Schmidt et al. describe a fourfold dose reduction for the breast for 180 • -localisers (0.24 mGy) compared to 0 • -localisers (1.01 mGy) [1]. Whereas for 0 • -localisers (tube over bed), the non-attenuated beam reaches the thyroid and breast, for 180 • -localisers (tube below bed), the attenuated x-ray beam reaches these radiosensitive organs. Thus, localisers with x-ray tube in 180 • -position can reduce the radiation in exposure in the thyroid and breast, when the patient is scanned in supine position [1][2][3]. Hence, localisers acquired in p.a.-positioning are advantageous with regard to the radiation exposure of breast and thyroid tissue. This can be implemented with the patient in supine position and the tube positioned below the patient (180 • -localiser).
In general, patients appear magnified on localisers when they are positioned too close to the x-ray tube [4]. This happens when a patient is positioned too low during a 180 • -localiser. Equivalently, a patient appears smaller when positioned further away from the x-ray tube. According to the literature, patients are often positioned below the iso-centre [5,6]. This might be caused by the concave curvature of the bed, being lower at the centre/position of the spine compared to the sides where the shoulders are placed. For 0 • -localisers, patients appear smaller on the localiser image; for 180 • -localisers, this causes a magnification of the patient on the localiser image.
Achieving an off-positioning below 10 mm is not easy in everyday clinical practice; especially when patient shape varies or patients are covered by blankets. The majority of studies assessing patient positioning and tube position during localisers are based on phantom acquisitions or short scan coverages, such as the chest or pelvis. For CT scans with shorter scan length, optimal or close-to-optimal patient position can be more easily achieved. However, a large percentage of CT scans is combined chest and abdominal scans for cancer staging and treatment follow-up. Here, the positioning is more complicated due to the different anatomical body centres of thorax, abdomen, and the pelvic region.
The goal of this study is twofold. The first goal is to assess the difference in radiation exposure of combined chest and abdomen CT scans when comparing scans with prior 0 • -and 180 • -localisers. The second goal is to develop a guide to improve patient positioning in patients with varying diameters along the patient axis.

Materials and methods
The institutional review board approved this retrospective study. Written informed consent was waived due to the retrospective character of this study.

Patient cohort
The study interval ranged between June 2021 and January 2023. Patients with clinically indicated combined CTs of the chest and abdomen (venous phase acquisition) and documented patient weight and height were included. The exclusion parameters were as follows: (a) patients with deviating localiser protocol (tube potential or tube current), (b) scan length < 55 cm and (c) patients without two CT-scans with each a 0 • -and a 180 • -localiser (see figure 1).
In the last step, '0 • -180 • -pairs' were formed for each remaining patient. Those two CT examinations were selected, which included both a 0 • -and a 180 • -localiser and which were closest in time to one another. One patient was excluded due to varying arm positioning during the examinations (one/both arm/s next to thorax and abdomen instead of elevated above the head).

Image acquisition
All examinations were carried out on a Somatom Definition Edge (scanner A) and Somatom Definition Flash (scanner B) CT scanner, both Siemens Healthineers, Forchheim, Germany. Venous scans were acquired 70 s after contrast medium injection. CARE kV and CARE Dose4D were enabled. Reference tube-current time product was 120 mAs at 120 kV p . Scan protocol parameters are presented in table 1.
Patients were imaged in supine position. The only parameter that varied between the examinations was the tube position during the localiser (tube placed above the patient for 0 • -localisers, tube placed below the patient for 180 • -localisers). The different tube positions were part of clinical CT protocol optimization.
Images were reconstructed with iterative reconstruction, soft (Br38 (A), I30f (B)) and sharp (Br 59 (A), I70f (B)) kernel in axial, coronal and sagittal stacks. For the evaluation, axial reconstructions with 2 mm slice thickness and 1.6 mm increment were used. 3D-maximum intensity projections (MIPs) were reconstructed in the picture archive and communication system (IDS7, Sectra, Linkoping, Sweden).  [kg] were automatically stored in the dose management software (DoseTrack, Sectra, Linkoping, Sweden). The size-specific dose estimate (SSDE) was calculated by using the patient antero-posterior diameter [7]. For each acquisition, the  following image parameters were manually derived at three axial slices throughout the image stack (through the centre of heart, centre slice (abdomen level) and most-caudal slice (femur level), see figure 2):

Data processing
• distances h (anterior skin surface to upper limit of FOV) • distances i (posterior skin surface to lower limit of FOV) and • CTDI vol .
Measurements of distances based on the actual CT scan (axial slices) are used as the gold standard. Furthermore, the patient lateral diameter (skin surface to skin surface) along height of the trochanter major on localiser (k 1 ) and MIPs (k 2 ) and the distance left to right of the spina iliaca anterior superior on localiser (g 1 ) and multiplanar reconstruction (MPR) (g 2 ) were measured.
Additionally, the table height (in mm, DICOM tag 0018,1130) was documented. The table height is inverse proportional to the value in the DICOM tag. Hence, the higher the provided table height from the DICOM tag (in mm), the lower the actual table position. Generally, a thin patient would be placed higher in the scanner gantry, see

Statistical analysis
Correlation between volumetric computed tomography dose index (CTDI vol ), hip, pelvis and PD were calculated. Median values with interquartile range in parentheses are provided for the evaluated parameters. Statistical evaluation was performed using SPSS statistics version 29 (IBM, New York, USA). A Wilcoxon test was employed for pairwise comparison with a significance level of α = 0.05.
On the three evaluation positions, patient diameters measured on the axial slices were larger in the 0 • -group than in the 180 • -group, although not significant for heart and abdomen level but significant for the femur level. In contrast to this, the median CTDI vol and SSDE of the evaluated slices were statistically significantly higher in the 180 • -group (up to +73%). The hip diameter measured on the localisers was significantly larger (+20%) in the 180 • -group, compared to the 0 • -group (p < 0.001).
On 0 • -localisers, pelvis and hip diameters were significantly (p < 0.001) smaller than the corresponding diameters on the MIPs (see figures 4 and 5). On 180 • -localisers, hip diameters were significantly larger than the corresponding diameter on the MIPs (p < 0.001).

Discussion
Off-centre patient positioning in CT examinations influences the automatic exposure control negatively. The tube position during localiser acquisition additionally influences radiation exposure if the patient is positioned off-centre. The results of this study emphasise the use of 0 • localisers in thoracoabdominal CT, as 180 • localisers cause patient magnification and thus increased radiation exposure. This is the first study that evaluates patient positioning, CTDI vol , SSDE and diameter at three different positions along the z-axis of the patient by means of an intraindividual comparison. Harri et al already compared the impact of the localiser tube position and table height on the dose [4]. According to them, perfect positioning is impossible since patients are not anatomically symmetric [4]. However, in their cohort, an intraindividual comparison was available in only eight patients. Furthermore, they evaluated CT scans of the abdomen and pelvis, whereas in the current study, the scan region covered thorax, abdomen and the hip.
Several publications have evaluated the influence of off-positioning of patients in CT scans of phantoms or in chest CT in patients [8,9]. An off-positioning of 20 mm might already cause deviations of 7%-14% from the radiation exposure compared to correct positioning in the iso-centre [9,10]. Off-centreing above 40 mm caused a change by 20%-33% [9,10]. Larger patients have been found to undergo larger deviations from ideal positioning than smaller patients in chest CT, according to Aly et al and Saltybaeva et al [5,11]. Table 2. Results of scan parameters, patient parameters and the statistical evaluation between 0 • -and 180 • -groups. Furthermore, results of the statistical evaluation between diameters g1 (pelvis on localiser) and g2 (pelvis on MPR) and between k1 (hip on localiser) and k2 (hip on MPR) are provided (in superscripts).  The results of the study confirm that patients are often positioned too low (median deviation −9 to −42 mm distance from iso-centre). At the same time, it demonstrates the different diameters of patients along the evaluated positions which make optimal positioning in the iso-centre difficult, if not impossible. Depending on the scan length, this problem becomes more apparent with a longer scan range. The low positioning very often resulted in magnifications of the patients on the 180 • -localisers, which significantly increased the applied dose compared to the images obtained with the previous 0 • -localiser. Especially in the area of the pelvis, which tends to be positioned lower than the thorax or abdomen, the low positioning led to large diameter deviations on the localisers. We assume that the diameters on the axial slices are not affected by the tube position during the localiser. The advantage of 180 • -localisers, namely reducing the dose in thyroid and breast, is eliminated by an increase in dose when scanning the thorax, abdomen and pelvis.
In this study, we found that hip diameters differed among localisers depending on the tube position selected, which altered CTDI vol in the hip region (femur level), but also at heart and abdomen level. The effect is well described in literature. Al-Hayek et al and Paolicchi et al even noticed that the magnification effect was more pronounced for 180 • -localisers, compared to 0 • -localisers [12,13]. Kaasalainen et al evaluated the effect of phantom positioning in chest CTs. A 180 • -localiser and low positioning caused a higher dose due to a larger appearing phantom [8]. Furthermore, the vertical off-centreing affected the tube potential selection, increasing the tube potential after performed 180 • -localisers. The magnification on 180 • -localisers with accompanying increase of dose was also described by Harri et al [4]. However, in their study, the tube potential did not change by changing the tube position. In our study, the automatically chosen tube potential increased in 18% of the patients from either 80 kV p to 100 kV p or from 100 kV p to 120 kV p with 180 • -localisers, presumably caused by the larger appearance on the localiser.  On the 0 • -scan, the patient is positioned in the iso-centre at heart and abdomen level but below iso-centre at femur level. On the 180 • -scan, the patient position is in the iso-centre at the heart level, above iso-centre at the abdomen level and again in the iso-centre at the femur level. The patient appears considerably larger on the 180 • -localiser.
The distance of the patient centre to the gantry iso-centre is often measured on a single slice rather than on several slices or the whole stack [5,14]. For symmetric phantoms, this distance is usually identical throughout the image stack; however, for patients or anthropomorphic phantoms, the distance to the iso-centre varies along the z-axis. In a number of studies, the off-centre distance is measured at several positions throughout the patient [6,11,15]. Kaasalainen et al published that the vertical off-centreing ranged between ±17 mm in chest CT examinations in adult patients. In their cohort (adults and pediatric patients), median off-centreing ranged between 25 and 35 mm below the iso-centre [6]. The median distances to the iso-centre in the current study ranged between −9 mm and −42 mm for the three evaluated positions. We barely saw patients being positioned above the iso-centre (29/684, 4%). This is in line with previously published articles [12]. Interestingly, in a study of Ryu et al., off-centreing has no clinical impact on radiation dose in the routine clinical practice [16]. They describe that off-centreing was not an issue in their hospital.
At the abdomen level, the deviation from the iso-centre was smallest with −9 mm. The area between heart and abdomen level is closest to the area on the patient that is used for estimation of the ideal table height (upper abdomen). Despite this fairly small deviation at abdomen level, the differences in CTDI vol between the 0 • -and 180 • -group were significant at this position.
Frequently, radiographers centre patients in the iso-centre, based on the organs of interest. This works well for small scan coverages, such as the brain or the hip. However, for a long scan coverage, as is evaluated in this study, there is not a single but several organs of interest, that should be placed in the iso-centre of the CT scanner.
The influence of patient positioning on the CTDI vol is smaller for the chest region compared to the hip region (difference of approximately 0.6-0.7 mGy CTDI vol and SSDE at heart level). In case of a 180 • -localiser, it seems better to perform patient centreing using the lower abdomen/hip region instead of the chest as reference to reduce the magnification of the hips on the localiser. However, then a large percentage of patients (chest and abdomen) are positioned higher than the iso-centre. Automatic exposure control can increase the applied radiation exposure in off-centreed patients, due to the shape of the bowtie filter, if a real-time angular tube current modulation is available.
Some vendors already offer camera systems that automatically position the patient inside the scanner gantry, based on the patient shape and contours using anatomical landmarks [11,17,18]. The optimal table height for the desired scan region is automatically calculated. Evaluated in chest CTs, the automatic positioning leads to a reduction in preparation time and radiation exposure due to a reduced off-centre distance from 4.0 cm with manual positioning to 1.6 cm with automatic positioning [14]. Saltybaeva et al found a reduction from 19 mm to 7 mm for chest CT and from 18 mm to 4 mm in abdomen CT [11] with a  [17]. In the future, the effect of combined lateral and frontal localisers needs to be evaluated more thoroughly. Peng et al describe that a combined 0 • -and lateral localiser can significantly reduce the radiation dose in chest CT [19,20]. Additional localisers come with an additional radiation exposure. However, the additional lateral localiser might also reduce the radiation exposure of the final CT-examination in combination with off-centreed 180 • -localisers [13]. Several studies described that lateral localisers can reduce the radiation exposure compared to 0 • -localisers for thoracic and abdominal CT, since the patient's width is larger on frontal localisers compared to lateral ones [13,21,22]. Image quality of the CTs planned with lateral localisers was only marginally lower compared to CTs planned with 0 • -localisers [21]. Additionally, vertical off-centreing had a smaller effect on the CT radiation exposure when lateral localisers were used [22]. However, setting the field of view can be more challenging in lateral localisers, i.e. for kidneys or the adrenal glands.

Limitations
Some limitations need to be acknowledged. The scan coverage was chosen according to standard operation procedures, ranging from the apex of the lung to the symphysis. Since we chose the centre slice and last slice as positions 2 and 3, respectively, the exact anatomy of these slices might differ between patient images. We do not expect large differences between patients and within patients, since scan coverage was not significantly different in the intra-patient-comparison.
The median time between two examinations was approximately 7 months for both CT-scanners. Patients who receive thoracoabdominal CT examinations are often screened as part of their cancer treatment with follow-ups between every three and twelve months. Despite the time interval between the CT examinations, patient weight and diameter did not change considerably in the evaluated cohort and therefore do not explain the differences in CTDI vol between 0 • -and 180 • -groups.

Conclusion
The results of this study emphasise the use of 0 • -localisers in thoracoabdominal CTs, since 180 • -localisers caused a patient magnification with accompanying increase in radiation exposure. Since patient positioning is challenging, especially for large scan regions, we recommend to employ 0 • -localisers. The advantage of 180 • -localisers, namely reducing the dose in thyroid and breast, is eliminated if the dose of the CT scan increases significantly. The choice between the 0 • -localiser and the lateral-localiser or two localisers cannot be made with the current data and needs more investigation.

Data availability statement
The data cannot be made publicly available upon publication because they contain sensitive personal information. The data that support the findings of this study are available upon reasonable request from the authors.