ORIGINAL ARTICLE
Correlations of Anatomical Parameters in Dynamic Pelvic CT andConventional Defecography for Patients with Rectal Prolapse
Norihiro Okamoto

Department of Surgery, Shinshiro Municipal Hospital, Aichi, Japan(Received for publication on June 5, 2008)(Revised for publication on August 29, 2008)(Accepted for publication on September 18, 2008)

Abstract. Purpose: To evaluate the correlations of anatomical parameters between dynamic pelvic CT (D-PCT) and conventional defecography (CD) for patients with rectal prolapse.
Material and Methods: Anatomical parameters in multislice CT scanning of the pelvis performed at rest and during simulated defecation (D-PCT) were studied with those of CD toevaluate the correlations in both methods for 10 patients with rectal prolapse.
Result: The correlation coefficients of the pubococcygeal line and the pubosacral line werer=0.6 and r=0.8 respectively. The length from anal verge to pubococcygeal line and to thepubosacral line showed a good correlation of r=0.7. The length of puborectal muscle showed
a good correlation of r=0.8. Anorectal angle was significantly well correlated between twomethods (r=0.9, p<0.05). The lengths of anococcygeal length and anosacral length showed agood correlation.
Conclusion: The anatomical parameters measured by D-PCT were well correlated withthose by CD. D-PCT might be an alternative tool for anatomical evaluation of the anorectal
region in patients with rectal prolapse. (Keio J Med 57 (4) : 205－210, December 2008)
Key words: defecography, dynamic pelvic CT, rectal prolapse, defecation disorders, anorectal angle

Introduction

Materials and Methods

Conventional defecography (CD) has been the mostcommonly employed diagnostic imaging modality formorphological and physiological evaluation of the anorectal region in patients with functional anorectal diseases.
Lateral images of the pelvic region have been used to obtain various anatomical measurements. The pubic bone
and the coccyx were often used as indicators in CD.1- 4
However, the anatomy of the coccyx varies from person
to person with frequent incomplete visualization of thebone on X-ray images. In the present study, the author
employed multislice CT during simulated defecation (DPCT) to evaluate various anatomical measurements byusing the pubococcygeal line and the pubosacral line,
which are considered to be morphologically consistentand poorly visualized by plain X-ray. The correlations ofmorphological or physiological parameters between
D-PCT and CD were investigated.

Ten patients (6 men, median age 62.8) with rectal prolapse were enrolled in this study. Rectal prolapse wasclassified as type II in 9 patients and III in 1 patient according to the Tuttle classification.1 Seven patients wereconstipated and 1 patient had soiling (Table. 1).
CD: Laxative was administered on the previous day. Aglycerin enema was performed prior to the study on thedate of the examination, and defecation was completed
at least 1 hour. The patient was placed in the left lateral
decubitus position prior to the examination. A 100 ml ofbarium mixed with rice bran was administered throughthe anus, and the running of the intestinal tract was initially determined. The barium was confirmed to reachthe sigmoid colon for adequate filling of the rectum. A
portable commode was mounted on the fluorography table, and right lateral images were obtained with the patient at rest in the sitting position. Images were also ob-Corresponding author: Norihiro Okamoto, MD. 32-1, Kitahata, Shinshiro, Aichi, 441-1387, Japan, Tel: +81-536-22-2171, Fax: +81-536-22-2850, E-mail:norihiro_okamoto@yahoo.co.jp
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Table 1 Patient characteristics
Number of patients
Gender (male / female)Age (year), median (range)
Bowel habit
normalconstipation
diarrhea
Soiling
Classification (Tuttles)
ⅠⅡⅢ

10
6/4
62.8 (32-75)
2
70109
1

tained during squeezing and straining. Imaging wascompleted in approximately 20 minutes. Anatomicalmeasurements were performed by using X-ray films.
D-PCT: One day before imaging, a laxative was administered and intestinal lavage was performed. The patient was placed in the prone position on CT scanner.
Immediately before imaging, a small catheter (14Fr) wasplaced in the anus and 300 ml of air was infused to expand the rectum. CT images were acquired by using a
multislice CT system with a 16-row detector and 16 dataacquisition systems (Aquilion 16 DAS, Toshiba). Helicalscanning was performed with a detector collimation of 1mm, a beam pitch of 0.94 (helical pitch 15), and a table
movement speed of 30 mm/sec. The contrast medium,
100 ml of Iopamidol (370 mgI/ml), was injected at a rateof 2.5 ml/sec to enhance contrast with other structures,and scan start was triggered using a function for automatically determining the optimal scan timing (SureStart, Toshiba). Images were obtained during four phases, at rest of plain, arterial, venous, and equivalent phase,at squeeze and strain of equivalent phase. CT images
were obtained at rest, and contrast medium was injected
in about 40 seconds, at squeeze and strain phases wereafter this in 1 to 2 minutes. Imaging was completed inapproximately 5 minutes. The scan data was reconstructed into 1 mm slice thickness and 0.5 mm overlap. Multiplanar reconstruction (MPR) images of this data set werecreated on a workstation (ALATOVIEW, Toshiba).
Based on the MPR images, anatomical measurements
were obtained.
Measurements: Lateral images of both D-PCT and CD
were used to evaluate anatomical measurements. The pubococcygeal line (Fig 1, the superior margin of the pubicbone to the tip of coccyx) and the pubosacral line (Fig 1,the superior margin of the pubic bone to the inferior margin of the 5th sacral bone) were used for measurements.
The distance between the superior margin of the pubic
bone and the upper margin of the anal canal was mea-Fig. 1 Points of anatomical measurement
Lateral images of both conventional defecography and dynamic
pelvic CT
P : superior margin of the pubic bone
S : inferior margin of the 5th sacral bone
C : tip of coccyx
AV : anal verge

sured, and this measurement was defined to represent thelength of the puborectal muscle. In addition, the distances from the inferior margin of the 5th sacral vertebra to
the upper margin of the anal canal and from the tip of thecoccyx to the upper margin of the anal canal were measured. Measurements were obtained during three phases:
at rest, at squeeze and at strain (Fig. 1).
The correlation values were calculated as Pearson correlation coefficients (r), and statistical analysis was performed using the Fisher (z) conversion (p) value. A pvalue less than 0.05 was considered as significant.
Results
The mean measurement and standard deviation (SD)values for the pubococcygeal line (P-C) at rest, during
squeezing, and during straining obtained by CD were168.4±10.8 mm (160-196 mm), 166.3±11.2 mm(158-190 mm), and 177.8±11.9 mm (167-201 mm), respectively. The corresponding measurement values obtained by D-PCT were 114.9±7.4 mm (108-149 mm),112.6±8.2 mm (106-138 mm), and 118.8±7.8 mm(107-141 mm), respectively. A positive correlation wasobserved in all phases: at rest (r=0.6, p=0.10), during
squeezing (r=0.6, p=0.11), and during straining (r=0.6,p=0.10). The mean measurement and SD values for thepubosacral line (P-S) at rest, during squeezing, and during straining obtained by CD were 176.5±10.1 mm(165-191 mm), 176.5±11.6 mm (163-190 mm), and177.8±12.1 mm (164-189 mm), respectively. Those obtained by D-PCT were 129.6±9.1 mm (122-147 mm),129.5±8.1 mm (123-146 mm), and 130.0±8.4 mm(125-146 mm), respectively. A strong positive correlation
was observed in all phases: at rest (r=0.8, p=0.02), during squeezing (r=0.7, p=0.03), and during straining (r =Keio J Med 2008; 57 (4): 205－210

207

Fig. 2 Pubococcygeal and pubosacral length

Fig. 3 Anorectal angle

0.7, p=0.06) (Fig. 2).
The mean measurement and SD values for the anorectal angle (AV-a-S, AV-a-C) at rest, during squeezing, andduring straining obtained by CD were 122.0±8.6º(113-146º), 107.5±9.2º (97-128º), and 139.0±9.1º(123-154º), respectively. Those obtained by D-PCT were
125.8±4.2º (109-145º), 112.2±6.1º (93-124º), and 137.2±5.2º (114-158º), respectively. A statistically significant
strong positive correlation was observed in all phases: atrest (r=0.9, p=0.0008), during squeezing (r=0.9,p=0.0015), and during straining (r=0.9, p<0.001) (Fig.
3).
The mean measurement and SD values for length fromanal verge relative to the pubococcygeal line (AV-b) atrest, during squeezing, and during straining obtained by

CD were 58.6±8.2 mm (30-85 mm), 51.4±7.4 mm(38-70 mm), and 91.2±8.1 mm (32-137 mm), respectively. Those obtained by D-PCT were 29.5±5.1 mm (18-50mm), 26.9±4.8 mm (15-34 mm), and 53.6±5.7 mm(20-65 mm), respectively. A positive correlation was observed in all phases: at rest (r=0.7, p=0.09), during
squeezing (r= 0.4, p=0.31), and during straining (r=0.7,p=0.04). The mean measurement and SD values forlength from anal verge relative to the pubosacral line
(AV-c) at rest, during squeezing, and during straining obtained by CD were 71.5±9.8 mm (52-80 mm), 63.0±9.6mm (53-67 mm), and 108.0±10.1 mm (80-135 mm), respectively. Those obtained by D-PCT were 46.7±6.1 mm(35-70 mm), 38.7±7.2 mm (27-52 mm), and 73.0±7.6mm (40-83 mm), respectively. A positive correlation was

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Fig. 4 Length from anal verge to pubococcygeal and pubosacral line

Fig. 5 Puborectal muscle length

observed in all phases: at rest (r=0.7, p=0.05), during
squeezing (r=0.4, p=0.30), and during straining (r=0.7,p=0.10) (Fig. 4).
The mean measurement and SD values for the puborectal muscle length (P-C, P-S) at rest, during squeezing, and during straining obtained by CD were 133.1±10.1 mm (102-158 mm), 112.5±9.8 mm (100-130 mm),and 153.0±10.2 mm (116-173 mm), respectively. Thoseobtained by D-PCT were 87.1±6.2 mm (80-118 mm),81.3±7.4 mm (67-92 mm), and 109.7±9.1 mm (84-126mm), respectively. A positive correlation was observedin all the phases: at rest (r=0.8, p=0.04), during squeezing (r=0.5, p=0.16), and during straining (r=0.7, p=0.08)(Fig. 5).
The mean measurement and SD values for anococcy-geal length relative to the pubococcygeal line (a-C) at
rest, during squeezing, and during straining obtained by
CD were 84.3±11.2 mm (67-90 mm), 85.2±10.1 mm(62-117 mm), and 105.2±9.2 mm (86-114 mm), respectively. Those obtained by D-PCT were 46.6±8.7 mm(38-67 mm), 49.8±9.1 mm (38-61 mm), and 58.1±10.1mm (46-72 mm), respectively. A correlation was observed in all phases: at rest (r=0.5, p=0.07), during
squeezing (r= -0.5, p=0.21), and during straining (r=0.4,p=0.31). The mean measurement and SD values for anosacral length relative to the pubosacral line (a-S) at rest,during squeezing, and during straining obtained by CD
were 100.4±11.1 mm (97-145 mm), 96.8±10.2 mm(88-140 mm), and 118.2±11.7 mm (111-127 mm), respectively. Those obtained by D-PCT were 70.6±10.1

Keio J Med 2008; 57 (4): 205－210

209Fig. 6 Anococcygeal and anosacral lengthmm (65-88 mm), 49.7±11.2 mm (38-61 mm), and 84.9±9.7 mm (69-97 mm), respectively. A negative correlation
was observed in all phases: at rest (r= -0.5, p=0.09), during squeezing (r= -0.4, p=0.20), and during straining (r=-0.8, p=0.03) (Fig. 6).
Discussion
The current technique of CD, which is widely employed, has its origins in the method reported as cinefluorography by Wallden, Burhenn, and others.1 Later, inthe 1980s, the usefulness of CD for the evaluation ofanorectal morphological abnormalities associated withdefecation was reported in Europe and the UnitedStates.4, 6-1 0 In recent years, there have been significantadvances in MSCT, which permits high-speed helicalscanning at a speed of 30 mm/s or more, resulting insubstantial reductions in the times required for CT evaluation and examination. The authors therefore employed
CT scanning to obtain images during defecation an imaging method referred to as “Dynamic Pelvic CT”, andconducted clinical evaluations. Imaging systems such as
those employed in the present study allow scanningtimes to be substantially reduced and also permit pelvis
to be evaluated simultaneously. In lateral images obtained by CD, the anatomy and number of bones of thecoccyx differ from person to person, and the coccyx isoften abnormal due to fracture. Furthermore, due to thelimitations of X-ray examinations, visualization of thecoccyx is often difficult. Therefore, the author obtained
measurements relative to the inferior margin of the 5thsacral vertebra, which has a consistent anatomy and iseasy to detect in X-ray examinations. The results showeda correlation between the findings obtained by CD and
by D-PCT with regard to the pubococcygeal line, the pu-bosacral line, the length from anal verge, the anorectal
angle, and the puborectal muscle length. However, not
all of the actual measurement values showed statisticallysignificant correlations. Generally, the distances obtainedby D-PCT are considered to be the true values, which
tend to be increased in X-ray examinations. Therefore,
the author performed statistical analysis using both corrected and actual measurement values, but the results
were the same. The reason that the correlations were notstatistically significant was thought to be attributable topatient positioning. CD was performed with the patientin the sitting position, which is the usual position for defecation, while D-PCT was performed with the patient in
the prone position. The difference in patient positioning
was thought to result in differences related to gravitational effects and the forces applied by the patient during
straining and during squeezing. In addition, the intestines
were filled with a mixture of rice bran and barium (similar to feces) in CD, but with air in D-PCT, which may
have resulted in differences during defecation. The measurement error caused by slight body movement during
CD may also have been a factor. As a similar diagnostic
method, it would also be possible to perform examination using sagital and coronal images obtained by
MRI.11-1 3 However, this is not practical in the clinicalsetting due to a number of problems such as long imaging times, difficulties in patient positioning due to thelimited space within the system, and image distortion.
The author obtained measurements using both CD andD-PCT and evaluated the correlations between these two
diagnostic imaging methods. The results of the present
study indicate that D-PCT, although it is not equivalent
to CD in terms of clinical acceptance, is capable of providing comparable morphological measurements in patients with rectal prolapse. D-PCT permits not only the210

Okamoto N: Correlation of Anorectal Parameters in Dynamic Pelvic CT

rectum but also pelvic structure to be examined at thesame time and is therefore suitable for the evaluation ofpathologic changes in all of the intra pelvic organs.14, 15
The anatomical measurements obtained by D-PCT
showed correlations with those obtained by CD and are
therefore considered to be suitable for morphologicalevaluation of the anorectal region in patients with rectalprolapse.
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