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 [1–3]. 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.

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

2.1. 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).

Figure 1. Patient cohort flowchart.

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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).

2.2. 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 kVp. Scan protocol parameters are presented in table 1.

Table 1. Protocol parameters for scanner A and scanner B.

ParameterScanner AScanner B Localiser Tube potential [kVp]120120Tube current [mA]3534Reconstruction kernelTr20T20fSingle collimation width [mm]0.60.6 Venous phase acquisition Reference tube potential [kVp]120120Reference tube current-time product [mAs]120120Pitch0.60.6Rotation time [s]0.50.5Collimation [mm]0.60.6Reconstruction parameters axial stacks  Slice thickness [mm]22Increment [mm]1.61.6Reconstruction kernelBr38, Br59I30f, I70f

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).

2.3. Data processing

For each acquisition, anonymised patient ID, CTDIvol [mGy], DLP [mGycm], scan length [cm], effective tube current-time product [mAs], tube potential [kVp], patient height [cm] and weight [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):

distances h (anterior skin surface to upper limit of FOV)distances i (posterior skin surface to lower limit of FOV) andCTDIvol.

Figure 2. Measurement positions. (a) Evaluated positions with measurements to calculate patient diameter and patient offset inside the scanner gantry. Position 1: centre of heart (heart level); Position 2: centre slice (abdomen level); Position 3: last slice (femur level). (b) Pelvis and hip diameter measurements on localiser and maximum intensity projections.

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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 (k1) and MIPs (k2) and the distance left to right of the spina iliaca anterior superior on localiser (g1) and multiplanar reconstruction (MPR) (g2) 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 figure 3.

Figure 3. Table height according to the DICOM header for a thin (left) and larger patient (right). The arrows highlight the difference in table height regarding the field of view of the CT-scanner.

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Patient AP diameter (PD) was calculated as follows (see figure 2(a)):

PD [mm] = 500 mm–h–i

Patient offset was calculated as follows, with negative values indicating an offset towards the posterior side of the patient (positioning too low with respect to the iso-centre) (see figure 2(a)): offset [mm] = (i–h)/2

2.4. Statistical analysis

Correlation between volumetric computed tomography dose index (CTDIvol), 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.

In total, 228 CT scans from 114 patients were evaluated. All images were of sufficient image quality. The median time between two examinations amounted to 217 (100–307) days (approximately 7 months). Patient weight was comparable between the 0°- and 180°-group (median weight 75.5 (64.8–90.0) kg (0°) and 74.0 (64.0–90.0) kg (180°), p = 0.675), see table 2. In 21/114 patients, the automatically chosen tube potential increased by 20 kVp in the 180°-group compared to the 0°-group.

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).

Parameter0°-group n = 114180°-group n = 114p-valueTotal scan range Weight [kg]75.5 (64.8–90.0)74.0 (64.0–90.0)n.s.Scan length [cm]68.7 (65.6–71.3)68.5 (66.1–72.2)n.s.CTDIvol [mGy]5.4 (4.2–7.2)8.2 (6.1–11.5)<0.001SSDE [mGy]6.5 (5.5–7.6)9.3 (8.1–12.2)<0.001DLP [mGycm]358.5 (276.0–483.6)543.9 (401.7–788.5)<0.001Table height [mm]162.5 (152.0–186.0)161.0 (148.5–175.0)0.019Heart level CTDIvol [mGy]2.9 (2.2–4.1)3.5 (2.5–4.9)<0.001SSDE [mGy]3.5 (2.8–4.7)4.2 (3.2–5.9)<0.001Patient diameter [mm]257.0 (238.8–281.0)256.5 (237.0–282.0)n.s.Offset from centre [mm]−18.5 (−31.1—− 7.4)−15.5 (−23.5—− 4.9)0.021Abdomen level CTDIvol [mGy]5.4 (3.7–7.8)6.7 (4.2–9.5)<0.001SSDE [mGy]6.4 (5.0–8.0)7.7 (6.0–9.8)<0.001Patient diameter [mm]266.0 (231.8–301.0)262.0 (230.0–299.3)n.s.Offset from centre [mm]−10.3 (−25.3—− 0.3)−8.8 (−16.3–4.3)0.040Femur level CTDIvol [mGy]6.0 (4.3–7.7)10.2 (7.5–15.9)<0.001SSDE [mGy]8.8 (7.2–11.1)15.2 (12.5–22.7)<0.001Patient diameter [mm]204.0 (181.0–231.3)202.0 (179.8–225.0)0.025Offset from centre [mm]−42.3 (−60.6—− 27.0)−41.0 (−50.4—− 29.4)n.s.Diametersg1 Pelvis—localiser [mm]281.0 (269.9–292.5)a283.0 (272.5–297.0)cn.s.g2 Pelvis—MIP [mm]285.8 (274.4–297.2)a285.6 (273.6–297.5)cn.s.k1 Hip—localiser [mm]372.1 (342.8–405.0)b447.0 (416.0–477.0)d<0.001k2 Hip—MIP [mm]396.1 (369.3–419.6)b390.9 (366.0–421.0)dn.s.

Statistical evaluation between g1 and g2 (a for 0°-group, c for 180°-group), k1 and k2 (b for 0°-group, d for 180°-group). ap < 0.001, bp < 0.001, cp = n.s., dp < 0.001.

Median CTDIvol for the total scan amounted to 5.4 (4.2–7.3) mGy in case of a prior 0°-localiser, whereas in case of a prior 180°-localiser, the CTDIvol amounted to 8.2 (6.1–11.5) mGy. This difference (+52% in 180°-group) was statistically significant (p < 0.001). The SSDE increased significantly by 43% (p < 0.001). The DLP was significantly higher (+52%) in case of a prior 180°-localiser (358.5 (276.0–483.6) mGycm (0°) vs. 543.9 (401.7–788.5) mGycm (180°), p < 0.001). The scan length between both groups shows no significant difference. The table height between both groups is significantly different, but the difference is only 1.5 mm. This is small compared to the differences in the offset from iso-centre for all measured positions (heart, abdomen, femur level) with a minimum absolute value of 8.8 mm. Hence, table height between both groups is considered similar. At the heart level, patients were either positioned in the iso-centre (±15 mm deviation from iso-centre, 102/228 (45% of the patients)) or below the iso-centre (<−15 mm, 123/228, 54%). At the abdomen level, most patients were positioned in the iso-centre (124/228, 54%) or below (80/228, 35%). At the femur level, nearly all patients were positioned too low (213/228, 93%).

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 CTDIvol 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).

Figure 4. Localiser and axial slices of a female patient. Patient weight was 86 kg during the 0°-scan and 83 kg during the 180°-scan. Examinations were three months apart. Localiser scanned with a 0°- and 180°-tube position (top). Patient positioned in the centre of the gantry at heart level and at femur level. At both CT examinations, the patient is positioned in the centre around the heart level, but positioned below iso-centre at the femur level, causing the hip region to appear larger on the 180°-localiser and increasing the CTDIvol from 7.7 mGy to 14.6 mGy at this position.

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Figure 5. Localiser and corresponding axial slices of a female patient. Patient weight was 115 kg during the 0°-scan and 108 kg during the 180°-scan. Examinations were two months apart. Localiser scanned with an 0°- and 180°-tube position (left). Top left: 0°-localiser, bottom left: 180°-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.

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CTDIvol and SSDE values were positively correlated to hip diameters measured on MIPs (r = 0.675–0.781, p < 0.01) and localisers (r = 0.708–0.854, p < 0.01), see table 3. Correlation between hip diameter on the localiser and CTDIvol was higher for the 180°-group (r = 0.854, p < 0.01) than for the 0°-group (0.708, p < 0.01). Correlations between hip diameter measured on MIPs and CTDIvol and SSDE are lower than correlations between hip diameter measured on localisers. For both groups, CTDIvol-values were positively correlated to patient diameter (r = 0.767–0.862, p < 0.01). SSDE correlates highest with patient diameter at femur level (r = 0.721–0.758, p < 0.01).

Table 3. Correlation analysis. (a) 0°-group, (b) 180°-group. A correlation is considered strong if r > 0.75 (marked with blue cell color).

(a)0°-groupWeightCTDIvolCTDIvol pos 1CTDIvol pos 2CTDIvol pos 3SSDESSDE heart SSDE abd. SSDE femur PD heart PD abd. PD femur Hip localiserHip MIPWeight1             CTDIvol.884**1            CTDIvol heart.679**.860**1           CTDIvol abd..880**.909**.743**1          CTDIvol femur.674**.796**.558**.606**1         SSDE.753**.949**.850**.762**.830**1        SSDE heart.570**.781**.979**.627**.521**.813**1       SSDE abd..856**.912**.763**.988**.617**.797**.662**1      SSDE femur.528**.681**.483**.460**.954**.773**.480**.492**1     PD heart.868**.788**.637**.854**.493**.607**.497**.807**.334**1    PD abd..893**.810**.618**.899**.535**.602**.503**.853**.372**.916**1   PD femur.801**.811**.577**.756**.770**.721**.496**.727**.591**.683**.768**1  Hip localiser.579**.708**.520**.483**.861**.775**.522**.503**.868**.376**.448**.697**1 Hip MIP.619**.675**.516**.516**.762**.700**.499**.519**.764**.470**.529**.599**.918**1(b)180°-groupWeightCTDIvolCTDIvol pos 1CTDIvol pos 2CTDIvol pos 3SSDESSDE heart SSDE abd. SSDE femur PD heart PD abd. PD femur Hip localiserHip MIPWeight1             CTDIvol.891**1            CTDIvol heart.716**.857**1           CTDIvol abd..889**.955**.829**1          CTDIvol femur.765**.900**.685**.784**1         SSDE.753**.943**.806**.838**.928**1        SSDE heart.567**.750**.967**.706**.607**.746**1       SSDE abd..812**.928**.808**.970**.784**.875**.714**1      SSDE femur.565**.735**.505**