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© American Roentgen Ray Society

Genitourinary ImagingChou et al.CT Urethrography and Virtual Urethroscopy

Original Report

Chen-Pin Chou1

Jer-Shyung Huang1

Ming-Ting Wu1,2

Huay-Ben Pan1,2

Fong-Dee Huang3

Chia-Cheng Yu3

Chien-Fang Yang1,2Chou C-P, Huang J-S, Wu M-T, Pan H-B, Huang F-D, Yu C-C, Yang C-F

Received January 18, 2004; accepted after revision September 15, 2004.

Supported by Kaohsiung Veterans General Hospital research program (VGHKS93-82).

1Department of Radiology, Kaohsiung Veterans General Hospital, 386 Da-Chung First Rd., Kaohsiung 813, Taiwan, ROC. Address correspondence to C.-P. Chou (r2207759@ms19.hinet.net).2National Yang-Ming University School of Medicine, Taipei, Taiwan, ROC.3Department of Urology, Kaohsiung Veterans General Hospital, Kaohsiung, Taiwan, ROC.

CT Voiding Urethrography and Virtual Urethroscopy: Preliminary Study with 16-MDCT

OBJECTIVE. The purpose of this study was to demonstrate CT voiding urethrography andCT virtual urethroscopy. Fourteen CT voiding urethrography examinations on 13 men (meanage, 30 years) were prospectively performed with 16-MDCT. The clinical diagnoses of thosepatients included urethral injury, urethral stricture, and hypospadia. The CT voiding urethro-gram was obtained with transverse CT of the voiding, contrast-filled urethra and display of 2Dmultiplanar and 3D virtual images.

CONCLUSION. The full urethral structure was clearly shown by CT voiding urethrog-raphy and virtual urethroscopy in all patients. The results of CT voiding urethrography and con-ventional methods correlated closely with the urethral diseases being imaged.

ost diagnostic imaging of theurethra continues to be per-formed using conventional radi-ography with luminal distention

by iodine contrast media. However, radio-graphic contrast material–enhanced studiesare invasive and do not provide informationabout periurethral tissue. Other, more mod-ern imaging techniques such as sonographyand MRI can contribute, in some specificcircumstances, to the diagnosis of urethraldiseases. Sonography has a small field ofview, and the technique is operator-depen-dent. MRI is not widely used to examine theurethra because the technique is somewhatcomplex and expensive. CT is used onlyrarely to study the urethra. Its usefulness islimited to the evaluation of inflammatoryfluid collections or the identification of gasformed during necrosis or trauma [1]. Re-cent advances in MDCT, rapid image acqui-sition, and software have made 2D and 3Dreformatted images available for the newerdiagnostic techniques. These techniqueshave been applied to many organs, includ-ing the colon, bronchus, stomach, and uri-nary bladder [2, 3].

The thin-section transverse images andhigh scanning speed of CT have led to the de-velopment of promising new techniques forurethral evaluation: CT voiding urethrogra-phy and virtual urethroscopy. With thesetechniques, the voiding, contrast-filled ure-thra is scanned with 16-MDCT in approxi-mately 6 sec. Real-time 3D rendering of CTimages is performed to visually simulate ure-throscopic examination. In this study, we in-vestigated the technique of 16-MDCT in thedetection of urethral diseases.

Subjects and MethodsPatients

From January 2003 to May 2004, 13 men (agerange, 18–50 years; mean, 30 years) in whom ure-thral diseases were suspected were referred from thegenitourinary or emergency department for urethralimaging studies. In total, 14 CT voiding urethrogra-phy examinations were performed. One man withhypospadia underwent CT voiding urethrographybefore and after surgery. The micturating conditionof patients was checked before examination. If thepatients reported an inability to void before and dur-ing CT examination, they were not considered forthis study. Our series included suspected urethral in-

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juries, urethral stricture, and hypospadia. All pa-tients were alert and oriented and had stable vitalsigns when they arrived at the radiology depart-ment. The patients signed informed consent formsfor CT voiding urethrography.

Patient PreparationCT voiding urethrography was performed as an

independent CT examination (n = 13) or as part ofserial CT examinations (n = 1). The urinary bladderwas opacified by renal excretion of iodine contrastmedium administered IV (n = 13) or through infu-sion into a suprapubic tube (n = 1). Sixty millilitersof IV iopamidol (Iopamiro 300, Bracco) was ad-ministered 30 min before scanning. We urged thepatients to drink water or increased their IV salinefluid supplement to distend the urinary bladder rap-idly. For pelvic trauma patients who might needemergent surgery, we increased the rate of IV fluidadministration to accelerate urine production anddistend the urinary bladder satisfactorily. If the pa-tient had suprapubic tubes or Foley urethral cathe-ters, we directly infused 400 mL of diluted water-soluble iopamidol via catheters. We performed CTwhen the patient expressed a strong desire to void.No oral contrast medium was used in our series.

Patients wore urinary bags over their penises tocollect urine and avoid wetting the CT machine. CTwas performed with the patient prone (n = 9) or su-pine (n = 5). Patients with pelvic fractures were su-pine while scanned because they could not lie

prone. In contrast to CT colonography or CT cys-tography, for which both supine and prone scan-ning is necessary, CT urethrography requiresscanning in only one position.

Image AcquisitionAfter an anteroposterior topogram had been ob-

tained for slice selection, the patients were asked topress a handheld wireless bell controller when theybegan voiding. As soon as technologists heard thebell, they began scanning. The CT topogram wasused as a pelvic radiogram. We did not perform pre-voiding CT routinely because we wanted to avoidexposing the patients to additional radiation.

All serial thin-section images of the lower urinarytract were obtained using a 16-MDCT scanner (So-matom Sensation 16, Siemens). Scanning parame-ters included a 0.5-sec gantry rotation speed, high-quality scanning mode (0.75-mm collimation × 16-detector array, 512 × 512 matrix, 120 mA, and 120kVp), a 1.0-mm reconstructed slice width at inter-vals of 0.7 mm (0.3-mm overlap), a total scanningtime of 6 sec, and a total scan length of 16–20 cm.Data acquisition was craniocaudad and resulted inabout 300 transverse images for each scan.

Image EvaluationThe transverse thin sections were transferred to

a workstation (Syngo, Siemens) with manufac-turer-provided software that allows generation of2D maximum intensity projection, 2D multiplanar

reconstruction, 3D shaded-surface display, and 3Dvolume-rendered technique (Fig. 1). Two experi-enced abdominal radiologists (who were trained ininteractive navigation and interpretation of 3D vir-tual-reality CT colonography or CT cystographyand were unaware of the results of other examina-tions) prospectively and independently interpretedthe CT voiding urethrography. The urethra wasevaluated with 2D transverse images and a soft-tis-sue window setting (window level, 40 H; windowwidth, 300 H).

Virtual endoscopy was performed using sur-face-rendered or volume-rendered techniques. Weadjusted the attenuation coefficient range for voxelcategorization to the contrast material in the urethrauntil the normal mucosal surface appeared smoothand no noise was seen in the lumen. The lower limitof the attenuation coefficient range for voxel cate-gorization on virtual urethroscopy was 200–250 H;the upper limit range was 700–850 H. Because theattenuation coefficient of the urethral lumen variedfrom patient to patient, variable ranges were triedfor each patient to optimize the setting. With CTendoscopic fly-through navigator software, radiol-ogists performed interactive intraluminal naviga-tion from the urinary bladder to the urethra.Interactive standard axial, sagittal, and coronal ref-erence images were obtained automatically duringnavigation (Fig. 2A).

The new vessel-view tool of the Syngo worksta-tion is a semiautomatic protocol-driven analysis

Fig. 1.—Imaging of urethra with 2D and 3D techniques in 27-year-old man after vehicle accident.A, Three-dimensional volume-rendered urogram shows comminuted fractures of left ilium bones extending to left acetabulum with displaced bone fragments, and diastasis of bilateralsacroiliac joints and symphysis pubis. Urethra (arrows), urinary bladder, and ureters (arrowheads) are shown well. No urethral interruption or contrast medium extravasation is noted.B, Two-dimensional curve reformatted sagittal image using maximum intensity projection shows normal segmental anatomy of male urethra. M = membranous urethra, P =prostatic urethra, B = bulbous urethra, Pe = penile urethra.

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tool for CT angiography. This tool also works wellfor the urethra. The vessel navigator displays a lon-gitudinal cut along the centerline of the urethra, theso-called ribbon multiplanar reformation, withmultiplanar reference images along the curve of theurethra (Fig. 2B). The required longitudinal multi-planar reformatted section view and cine imagingare created using the angle slider to rotate the imageplane. We could create urethral boundaries usingsemiautomatic segmentation functions. We easilycould find the urethral path distance by clicking ei-ther the multiplanar reformation or the volume-ren-dering techniques to place the first seed point andterminal point. Also, maximum luminal diameterand luminal area could be measured perpendicu-larly to the urethral axis (Fig. 2C).

Further Urethral StudyEleven men were assessed using conventional ret-

rograde urethrography on the same day. Because CTvoiding urethrography obtains antegrade urethral im-ages similar to voiding cystourethrograms (VCUG)and we did not wish to expose the patients to addi-tional radiation, we did not routinely obtain VCUG.An experienced abdominal radiologist interpreted theconventional urethral examinations without knowingthe results of CT voiding urethrography.

CystourethroscopyThree men were examined with cystourethros-

copy by experienced urologists on the same or nextday. Cystourethroscopy was performed if the pa-tient sensed a foreign body in the urethra, had a Fo-

ley catheter placed because of urethral injury, orrequired optic urethrostomy for urethral stricture.

ResultsThe time required for the CT procedure

ranged from 4 to 20 min (mean, 9 min), depend-ing on how soon patients began to void. Thetime required for interpretation of the CT ure-thrographic data ranged from 6 to 20 min(mean, 10 min). CT voiding urethrography ex-aminations were completed successfully for allpatients. No significant difference in imagequality was noted between supine and prone po-sitioning. Images on CT voiding urethrographywere of excellent quality, with adequate contrastfilling of both the anterior and the posterior ure-

Fig. 2.—37-year-old man with straddle injury.A, Display panel of 3D volume-rendered virtual urethroscopy shows axial, sagittal,and coronal reference images. Verumontanum is viewed at 6-o’clock position(arrow). Fly path of virtual urethroscopy is identified on synchronized multidirectionalreference images.B, Vessel-view display panel shows entire urethra with 2D curve multiplanar refor-mation technique and measurement tools. Focus pointer (arrows) displays as line invessel navigator. When focus pointer is moved, reference imaging segments are syn-chronized to position of pointer.C, Maximum transverse diameter and area in axial cross-section of urethra aredetermined automatically by clicking required position of urethral path.

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thra in all patients. The final diagnosis wasbased on retrograde urethrography, cystoure-throscopy, or surgical findings (Table 1).

For urethral injury, CT voiding urethrogra-phy and conventional methods were of simi-lar accuracy. Nine patients with clinicallysuspected urethral injury underwent retro-grade urethrography. Pelvic fractures werenoted in three. A type 5 urethral injury ac-cording to the classification by Goldman et al.[4] was diagnosed in two patients. Patients 1and 10 had a straddle injury, and the retro-grade urethrograms showed contrast mediumextravasation in the bulbous urethra. CT void-ing urethrography also revealed extravasa-tion, an intraluminal blood clot, and mucosalabnormality (Fig. 3). Patient 12 experiencedbleeding from the urethra after sexual inter-course; contrast medium extravasation fromthe penile urethra was detected with CT void-ing urethrography but was missed on retro-grade urethrography (Fig. 4). A blood clot at

the site of injury was found in two of the threepatients with urethral injury.

For three cases of urethral stricture, CT void-ing urethrography was superior to conventionalexaminations for imaging of pathologic anat-omy and measurement of lesions and createdgreater diagnostic confidence. The retrogradeurethrograms did not allow proper evaluation ofthe posterior urethral stricture. Patient 9 had ahistory of surgical realignment for type 3 ure-thral injury with disruption of the urogenitaldiaphragm. After 3 months, CT voiding ure-thrography showed a short-segment urethralstenosis at a bulboprostatic anastomosis. Patient12 had had a urethral injury and a complicatedurethral stricture 1 year earlier. CT voiding ure-thrography revealed stricture in the membranousurethra with proximal urethral dilatation (Fig. 5).

Patient 13 had undergone plastic surgery forhypospadia 30 years earlier and complained ofpostvoid dripping. Retrograde urethrography wasunsuccessful because of a small meatus opening

and pain intolerance. CT voiding urethrographyshowed penile hypospadia and a diverticulumwithin the penile skin coverage (Fig. 6A). The pa-tient underwent corrective surgery with flap re-construction. Five months later, a penile urethralstricture requiring urethrotomy developed (Fig.6B). CT voiding urethrography provided usefulinformation before and after surgery.

DiscussionCT voiding urethrography is a technique

similar to conventional VCUG. The CT void-ing urethrography protocol in this study in-volved 0.75-mm collimation and revealed theentire urethra in 6 sec. The more thinly colli-mated transverse images and the subsequentbetter quality of the reformatted imagesshould further increase the ability of CT to de-pict the urinary tract accurately [5].

At a normal urinary flow rate of 15 mL/secfor men, the time needed to void 400 mL islonger than 20 sec. High-speed 16-MDCT

TABLE 1 Patient Data and Examination Findings

Patient No.

Age (yr)

Position and Method of Contrast Administration Clinical History Findings of CT Voiding Urethrography Additional Study

1 25 Prone, IV Blunt perineum injury Contrast medium extravasation, irregular mucosa surface at injury region, and blood clot in bulbous urethra

Retrograde urethrography: contrast medium extravasation in bulbous urethra

2 27 Supine, IV Vehicle accident, urinary bladder rupture after primary suture 2 wk earlier

Negative Retrograde urethrography: negative

3 37 Prone, IV Blunt perineum injury Negative Retrograde urethrography: negative

4 32 Prone, IV Blunt perineum injury Negative Retrograde urethrography: negative

5 35 Prone, IV Foreign body sensation when voiding Negative Cystourethroscopy: negative

6 30 Supine, IV Vehicle accident Bladder wall contusion and blood clot at right vesicoureteric junction; no urethral injury

Retrograde urethrography and cystography: mass effect at right vesicoureteric junction; no urethral injury

7 24 Prone, IV Blunt perineum injury Negative Retrograde urethrography: negative

8 22 Prone, IV Blunt perineum injury Negative Retrograde urethrography: negative

9 23 Supine, IV Complete transection of urethra after surgical realignment

Short segment narrowing at bulboprostatic anastomosis region

Retrograde urethrography: posterior urethra cannot be evaluated well

10 26 Supine, suprapubic Foleycatheter

Falling injury Contrast medium extravasation in bulbous urethra

Retrograde urethrography: contrast medium extravasation in bulbous urethra

Cystourethroscopy: mucosa perforation in bulbous urethra

11 50 Supine, IV Bleeding from the urethra after sexual intercourse

Contrast medium extravasation in penile urethra

Retrograde urethrography: negative

12 18 Prone, IV Traumatic urethral stricture Membranous urethra stricture Retrograde urethrography: posterior urethra cannot be evaluated

13 38 Prone, IV Surgically repaired hypospadia 30 yr previously

Hypospadia at middle shaft of penis and a diverticulum within penile skin coverage

Retrograde urethrography: failure to insert a Foley catheter because of narrow meatus opening and pain

Prone, IV Difficult voiding 5 mo after corrective surgery

Stricture in penile urethra Cystourethroscopy: optic urethrotomy for penile urethral stricture

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can scan the entire urethra and urinary blad-der in 6 sec. In this study, transverse CT im-ages showed the full extent of the urethra.Complete evaluation of the entire urinarytract, kidney to urethra, is easy with the newlydeveloped 16-MDCT.

Patient compliance is an important deter-minant of the success of CT voiding urethrog-raphy. CT voiding urethrography could play arole in lower urinary tract evaluation for clin-ically stable patients. The technique shouldnot be used on patients with acute majortrauma or acute pelvic fracture unless they al-ready have shown an ability to void. Patientsshould be interviewed before the examinationto evaluate their acceptance of it. Radiologistsshould know that the patients have no diffi-culty with voiding. Good communication be-tween patients and CT technologists duringthe examination also is necessary. Patientsneed to understand and follow the instruc-tions of technologists. Radiologists shouldparticipate in the whole procedure and inter-pret the real-time images on monitors.

Because CT easily depicts the high attenua-tion produced by contrast material, diluted con-

trast material is better appreciated on CT imagesthan on conventional urethrograms. Missing oflesions obscured by bone structures, contrastmedia, or instruments can be avoided with useof axial and multiplanar images.

We preferred to position patients pronewith pillows below their abdomen to in-crease intraabdominal pressure and enhancethe force of micturation. A supine scanningposition was preferred for patients with mul-tiple pelvic fractures. The scan processes orimaging quality in our study were the samewhether patients were prone or supine. Vari-ations in patient positioning and penile trac-tion during imaging can greatly alter theradiographic appearance of the urethra andstrictures. Multiple views, including bilat-eral oblique, may be required on conven-tional radiographs [6]. When multiple pelvicfractures and associated patient discomfortare present, the oblique position for conven-tional radiography may not always be possi-ble. CT voiding urethrography is moreconvenient because patients are required toadopt only one position and the scanningtime is only 6 sec. CT voiding urethrography

is more accurate with computer-aided toolsfor urethral measurement. The vessel view islongitudinal along the curve of the urethraand accurately measures stricture length,distance from the urethral meatus, and lumi-nal area. Exact comparison of the luminalsize and stricture length on clinical follow-up is possible.

Compared with retrograde urethrographyand conventional cystourethroscopy, urethralimaging with CT voiding urethrography andvirtual endoscopy can reduce organ injuryand patient suffering. Conventional radiogra-phy requires positioning of the patient’s ure-thra and avoidance of overlapping with bonestructures. The patient’s position is not criti-cal with high-quality 3D images, and patientswith complex pelvic fractures do not need tochange positions. In our experience, CT void-ing urethrography improved patient compli-ance. Some patients who could not tolerateconventional urethral examinations could ac-cept CT voiding urethrography.

Display of CT data in the form of virtualurethroscopy images affords a number of ad-vantages over transverse CT images alone.

Fig. 3.—25-year-old man who presented with hematuria after blunt perineum contusion.A, Retrograde urethrogram shows contrast medium extravasation (arrow) in bulbous urethra.B, Volume-rendered CT voiding urethrogram obtained with contrast infusion from suprapubic tube shows contrastextravasation and irregular mucosal surface (arrow) in bulbous urethra. Urethrocavernous and urethrovascularreflux (arrowhead) also were noted.C, Conventional cystourethroscopy image reveals bleeding and perforation at 5- to 7-o’clock position of bulbous urethra.D, Virtual urethroscopy image based on surface rendering shows mucosal disruption (arrows) in bulbous urethra.

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Virtual urethroscopy allows data from morethan 300 slices of CT images to be com-pressed into one interactive data set. The dataset can be manipulated easily for multidirec-tional viewing and can be recorded as cinefiles [7]. For urologists who are not familiarwith transverse images, CT voiding urethrog-raphy and virtual endoscopy provide a globalorientation for focal findings and aid naviga-tion for endoscopists.

Standard practice dictates that trauma andstricture of the male urethra be evaluated withretrograde technique because of the belief that

only it produces sufficient distention. Retro-grade urethrography is not a physiologic ex-amination. Contrast material often is injectedunder pressure to overcome the resistance of astricture. Rapid and forceful injection of thecontrast medium in retrograde urethrographymay lead to rupture of the mucosal barrier andextravasation of the contrast material into thesystemic circulation, with occasional resultantsystemic complications such as sepsis andanaphylaxis. Reflex contraction of the pelvicmuscle because of forceful injection of thecontrast material may lead to a false-positive

diagnosis of stricture [8]. Although retrogradeurethrography also can be performed duringCT, such as in CT voiding urethrography,some technical problems remain. These in-clude inadequate contrast medium filling andradiation exposure to the operators. In thisstudy, we showed in several instances that thenew CT technology can show clear urethralimaging sufficient for diagnoses.

According to previously published articles,the disadvantages of CT virtual cystoscopyversus conventional cystoscopy include ex-posure to radiation, difficulty in detecting flat

Fig. 4.—50-year-old man with urethral bleeding after sexual activity.A, Retrograde urethrogram shows no finding.B, Contrast medium extravasation (arrow) in penile urethra is identified on CT voiding urethrogram, vessel view.

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Fig. 5.—18-year-old man with urethral stricture; he sustained urethral injury 1 yearearlier in motor vehicle collision. Multiplanar coronal reformatted image (curvedalong urethra) shows posterior urethral stricture (arrow) and prostatic urethral dila-tation (arrowhead).

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or small mucosal lesions, lack of informationon the color and texture of the mucosa, andlack of biopsy [9].

The technical limitations are the same for CTvoiding urethrography as for VCUG. Effectiveantegrade imaging may be impossible in patientswith complete urethral disruption and severeposttraumatic urethral stricture [10]. Some pa-tients are psychologically inhibited from mictur-ating because of the required investigationalprocedures and surroundings. Simple VCUGcannot provide pressure as great as that providedby retrograde urethrography or double-balloon-catheter urethrography, and some urethral ab-normalities can be missed with CT voiding ure-thrography. The combination of retrogradeurethrography and CT voiding urethrographymay help radiologists avoid potential pitfalls.The volume of contrast medium extravasation inCT voiding urethrography is usually less thanthat in retrograde urethrography, and radiolo-gists might misinterpret extravasation as a nega-tive finding or, if from the bulbous urethra, as areflux into a normal Cowper’s duct. Analysis ofthin-section axial CT images and associatedfindings such as intraluminal blood clots andmucosal irregularity may improve the accuracyof diagnosis. Currently, experience with CTvoiding urethrography is limited. Thus, conven-tional urethral examinations should be per-formed to confirm the diagnosis in doubtfulcases. Because of the excessive time needed tocreate virtual images, radiologists need to selectpatients carefully.

A theoretic concern with MDCT voidingurethrography, in comparison with VCUG, isthe possibility that patients will receive moreradiation. The dose from MDCT can be esti-mated from the dose–length product, a mea-surement of radiation exposure that takes intoaccount the volume of irradiation [11]. The ef-fective dose of radiation from CT voiding ure-thrography for an average man isapproximately 5 mSv. When VCUG is usedfor a child of 5–10 years old, the effective ra-diation dose is about 1.6 mSv [12]. As experi-ence with CT voiding urethrography increases,it may become possible to reduce the radiationdose by adjusting CT parameters, as is done inlow-dose CT colonography.

The benefits of CT voiding urethrographyand virtual endoscopy over conventional im-aging include accurate measurement of le-sions, without magnification or distortion;production of both transverse and 3D imagesof urinary tract abnormalities; depiction ofextraluminal anatomic landmarks; good pa-tient compliance; and the ability to surveythe whole urinary tract, from the kidney tothe urethra.

To our knowledge, CT voiding urethrogra-phy has not been reported previously, andurethral pathology has not been described us-ing virtual urethroscopy. Conventional ure-thral imaging is challenged by the new CTtechniques. However, a large study of variousurethral diseases is needed to determine theclinical value of CT voiding urethrography.

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Fig. 6.—38-year-old man with history of hypospadia after plastic surgery 30 years earlier. He arranged another surgical correction because of dripping after voiding.A, CT voiding urethrogram, vessel view, shows ectopic urethral orifice (black asterisk) in middle of penile shaft and diverticulum (arrow) within penile skin coverage (arrow-heads). White asterisk is at expected location of meatus.B, Urethral stricture (arrow) developed 5 months after surgical correction.

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A data supplement for this article can be viewed in the online version of the article at: www.ajronline.org.