Development of Upright Computed Tomography With Area Detector for Whole-Body Scans: Phantom Study, Efficacy on Workflow, Effect of Gravity on Human Body, and Potential Clinical Impact

Abstract

Objectives: Multiple human systems are greatly affected by gravity, and many disease symptoms are altered by posture. However, the overall anatomical structure and pathophysiology of the human body while standing has not been thoroughly analyzed due to the limitations of various upright imaging modalities, such as low spatial resolution, low contrast resolution, limited scan range, or long examination time. Recently, we developed an upright computed tomography (CT), which enables whole-torso cross-sectional scanning with 3-dimensional acquisition within 15 seconds. The purpose of this study was to evaluate the performance, workflow efficacy, effects of gravity on a large circulation system and the pelvic floor, and potential clinical impact of upright CT.

Materials and methods: We compared noise characteristics, spatial resolution, and CT numbers in a phantom between supine and upright CT. Thirty-two asymptomatic volunteers (48.4 ± 11.5 years) prospectively underwent both CT examinations with the same scanning protocols on the same day. We conducted a questionnaire survey among these volunteers who underwent the upright CT examination to determine their opinions regarding the stability of using the pole throughout the acquisition (closed question), as well as safety and comfortability throughout each examination (both used 5-point scales). The total access time (sum of entry time and exit time) and gravity effects on a large circulation system and the pelvic floor were evaluated using the Wilcoxon signed-rank test and the Mann-Whitney U test. For a large circulation system, the areas of the vena cava and aorta were evaluated at 3 points (superior vena cava or ascending aorta, at the level of the diaphragm, and inferior vena cava or abdominal aorta). For the pelvic floor, distances were evaluated from the bladder neck to the pubococcygeal line and the anorectal junction to the pubococcygeal line. We also examined the usefulness of the upright CT in patients with functional diseases of spondylolisthesis, pelvic floor prolapse, and inguinal hernia.

Results: Noise characteristics, spatial resolution, and CT numbers on upright CT were comparable to those of supine CT. In the volunteer study, all volunteers answered yes regarding the stability of using the pole, and most reported feeling safe (average rating of 4.2) and comfortable (average rating of 3.8) throughout the upright CT examination. The total access time for the upright CT was significantly reduced by 56% in comparison with that of supine CT (upright: 41 ± 9 seconds vs supine: 91 ± 15 seconds, P < 0.001). In the upright position, the area of superior vena cava was 80% smaller than that of the supine position (upright: 39.9 ± 17.4 mm vs supine: 195.4 ± 52.2 mm, P < 0.001), the area at the level of the diaphragm was similar (upright: 428.3 ± 87.9 mm vs supine: 426.1 ± 82.0 mm, P = 0.866), and the area of inferior vena cava was 37% larger (upright: 346.6 ± 96.9 mm vs supine: 252.5 ± 93.1 mm, P < 0.001), whereas the areas of aortas did not significantly differ among the 3 levels. The bladder neck and anorectal junction significantly descended (9.4 ± 6.0 mm and 8.0 ± 5.6 mm, respectively, both P < 0.001) in the standing position, relative to their levels in the supine position. This tendency of the bladder neck to descend was more prominent in women than in men (12.2 ± 5.2 mm in women vs 6.7 ± 5.6 mm in men, P = 0.006). In 3 patients, upright CT revealed lumbar foraminal stenosis, bladder prolapse, and inguinal hernia; moreover, it clarified the grade or clinical significance of the disease in a manner that was not apparent on conventional CT.

Conclusions: Upright CT was comparable to supine CT in physical characteristics, and it significantly reduced the access time for examination. Upright CT was useful in clarifying the effect of gravity on the human body: gravity differentially affected the volume and shape of the vena cava, depending on body position. The pelvic floor descended significantly in the standing position, compared with its location in the supine position, and the descent of the bladder neck was more prominent in women than in men. Upright CT could potentially aid in objective diagnosis and determination of the grade or clinical significance of common functional diseases.

FIGURE 1

Upright CT machine and tools for safety. Our upright CT system enables up-and-down movements with a transverse 320 row-detector gantry. A, Gantry in the up position. B, Gantry in the down position. C, Knee-high acrylic wall encircling the body. D, Pole to lightly support the back of the patient.

FIGURE 2

CT images of Catphan 504 consisted of CTP528, CTP486, and CTP404. A, CTP528 for MTF (modulation transfer function) contains a bead point source. B, A region of interest (ROI) of 100 cm² (256 × 256 pixels) was placed on the center of the image of CTP486 for NPS (noise power spectrum). C, CTP404 for CT number measurements contains inserts made of air, polymethylpentene (PMP), low-density polyethylene (LDPE), polystyrene, acrylic, Delrin, and Teflon.

FIGURE 3

Measured points on the vena cava. Each coronal reformatted unenhanced CT image demonstrates the 3 points where the area size and flattening ([major axis – minor axis]/major axis) measurements of the vena cava were performed. These points include (1) directly below the junction of the bilateral cephalic veins, (2) at the height of the diaphragm, and (3) directly above the junction of bilateral common iliac veins. A, Supine position (thoracic). B, Upright position (thoracic). C, Supine position (abdominal). D, Upright position (abdominal).

FIGURE 4

Measurement of the pelvic floor. A, From bladder neck to PC line. B, From anorectal junction to PC line. Dotted line indicates PC line (pubococcygeal line); black line, bladder neck; white line, anorectal junction.

FIGURE 5

Noise power spectrum and MTF curves of upright CT and supine CT. Both noise characteristics (noise power spectrum) and spatial resolution (modulation transfer function) of upright CT were comparable to those of conventional CT. A, NPS (noise power spectrum). B, MTF (modulation transfer function) in xy-plane. C, MTF in z-axis.

FIGURE 6

Results of the questionnaire survey. The average rating for safety throughout examination (A) was 4.2. The average rating for comfortability throughout examination (B) was 3.8. Scale: 5 = excellent, 4 = good, 3 = fair, 2 = unsatisfactory, 1 = bad.

FIGURE 7

Total access time and total operation time. “Total access time” refers to the sum of the entry and exit times. Total access time (A) was significantly shorter in upright CT than in supine CT, while total operation time (B) was similar between the 2. Box plot shows the 5th and 95th (vertical lines), 25th and 75th (Boxes), and 50th (horizontal line) percentile values for total access time or operation time (excluding outliers and extreme values). *P < 0.001.

FIGURE 8

Area of vessels. When comparing the upright position with the supine position, the area of the vena cava (A) was smaller at the SVC; the area was not significantly different at the level of the diaphragm; and the area was larger at the iliac bifurcation of IVC. The area of the aorta (B) was similar at all 3 levels. ST indicates sinotubular. Box plot shows the 5th and 95th (vertical lines), 25th and 75th (Boxes), and 50th (horizontal line) percentile values for the area of vena cava and aorta. *P < 0.001.

FIGURE 9

Difference of area and flattening of the vena cava at 3 levels. At the superior vena cava, directly below the junction of the bilateral cephalic vein, the area was 143 mm² in supine CT (A) and 51 mm² in upright CT (D) (64% smaller). The flattening was 0.12 in supine CT (A) and 0.38 in upright CT (D) (3.2-fold increase). At the vena cava, at the level of the diaphragm, the area was 286 mm² in supine CT (B) and 259 mm² in upright CT (E). The flattening was 0.51 in supine CT (B) and 0.58 in upright CT (E). At the inferior vena cava, directly above the iliac bifurcation, the area was 199 mm² in supine CT (C) and 241 mm² in upright CT (F) (21% larger). The flattening was 0.3 in supine CT (C) and 0.1 in upright CT (F) (66% decrease).

FIGURE 10

Position of pelvic floor. Both bladder neck (A) and anorectal junction (B) descended significantly in the standing position compared with the supine position. Box plot shows the 5th and 95th (vertical lines), 25th and 75th (Boxes), and 50th (horizontal line) percentile values for the position of bladder neck and anorectal junction. *P < 0.001.

FIGURE 11

Difference in position of the pelvic floor. The distance from bladder neck to PC line (A) was 25 mm in the supine position (A), but 10 mm in the upright position (B) (15 mm descent). The distance from anorectal junction to PC line (B) was 12 mm in the supine position (A), but 20 mm in the upright position (B) (8 mm descent). Both bladder neck and anorectal junction descended significantly in the standing position compared with the supine position. A, From bladder neck to PC line. B, From anorectal junction to PC line. Dotted line indicates PC line (pubococcygeal line); black line, bladder neck; white line, anorectal junction.

FIGURE 12

A case of spondylolisthesis. Supine CT showed spondylolisthesis between the L4 and L5 vertebrae in the reconstructed midsagittal image (A), and foraminal stenosis in the parasagittal image at the level of foramen (B) and the axial image (C). Upright CT showed a marked increase of stenosis in the spinal canal (D) and foramen (E and F).

FIGURE 13

A case of pelvic floor prolapse. Bladder prolapse was not detected on conventional supine CT (A), but was detected on upright CT (B, arrow).

FIGURE 14

A case of inguinal hernia. Conventional supine CT shows mild left inguinal hernia (A, dot line, size of hernia orifice is 16.0 mm), and the hernia content is fat tissue only (A, arrow). Upright CT shows more apparent inguinal hernia (B, dot line, size of hernia orifice is 32.1 mm), and the hernia contents include small intestine and fat tissue (B, arrow).