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AJR 2005; 184:1882-1888
© American Roentgen Ray Society

Original Report

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

Chen-Pin Chou1, Jer-Shyung Huang1, Ming-Ting Wu1,2, Huay-Ben Pan1,2, Fong-Dee Huang3, Chia-Cheng Yu3 and Chien-Fang Yang1,2

1 Department of Radiology, Kaohsiung Veterans General Hospital, 386 Da-Chung First Rd., Kaohsiung 813, Taiwan, ROC.
2 National Yang-Ming University School of Medicine, Taipei, Taiwan, ROC.
3 Department of Urology, Kaohsiung Veterans General Hospital, Kaohsiung, Taiwan, ROC.

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

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

Address correspondence to C.-P. Chou (r2207759{at}ms19.hinet.net).


Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
OBJECTIVE. The purpose of this study was to demonstrate CT voiding urethrography and CT virtual urethroscopy. Fourteen CT voiding urethrography examinations on 13 men (mean age, 30 years) were prospectively performed with 16-MDCT. The clinical diagnoses of those patients included urethral injury, urethral stricture, and hypospadia. The CT voiding urethrogram was obtained with transverse CT of the voiding, contrast-filled urethra and display of 2D multiplanar and 3D virtual images.

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


Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
Most diagnostic imaging of the urethra continues to be performed using conventional radiography with luminal distention by iodine contrast media. However, radiographic contrast material-enhanced studies are invasive and do not provide information about periurethral tissue. Other, more modern imaging techniques such as sonography and MRI can contribute, in some specific circumstances, to the diagnosis of urethral diseases. Sonography has a small field of view, and the technique is operator-dependent. MRI is not widely used to examine the urethra because the technique is somewhat complex and expensive. CT is used only rarely to study the urethra. Its usefulness is limited to the evaluation of inflammatory fluid collections or the identification of gas formed during necrosis or trauma [1]. Recent advances in MDCT, rapid image acquisition, and software have made 2D and 3D reformatted images available for the newer diagnostic techniques. These techniques have been applied to many organs, including the colon, bronchus, stomach, and urinary bladder [2, 3].

The thin-section transverse images and high scanning speed of CT have led to the development of promising new techniques for urethral evaluation: CT voiding urethrography and virtual urethroscopy. With these techniques, the voiding, contrast-filled urethra is scanned with 16-MDCT in approximately 6 sec. Real-time 3D rendering of CT images is performed to visually simulate urethroscopic examination. In this study, we investigated the technique of 16-MDCT in the detection of urethral diseases.


Subjects and Methods
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 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Patients
From January 2003 to May 2004, 13 men (age range, 18-50 years; mean, 30 years) in whom urethral diseases were suspected were referred from the genitourinary or emergency department for urethral imaging studies. In total, 14 CT voiding urethrography examinations were performed. One man with hypospadia underwent CT voiding urethrography before and after surgery. The micturating condition of patients was checked before examination. If the patients reported an inability to void before and during CT examination, they were not considered for this study. Our series included suspected urethral injuries, urethral stricture, and hypospadia. All patients were alert and oriented and had stable vital signs when they arrived at the radiology department. The patients signed informed consent forms for CT voiding urethrography.

Patient Preparation
CT voiding urethrography was performed as an independent CT examination (n = 13) or as part of serial CT examinations (n = 1). The urinary bladder was opacified by renal excretion of iodine contrast medium administered IV (n = 13) or through infusion into a suprapubic tube (n = 1). Sixty milliliters of IV iopamidol (Iopamiro 300, Bracco) was administered 30 min before scanning. We urged the patients to drink water or increased their IV saline fluid supplement to distend the urinary bladder rapidly. For pelvic trauma patients who might need emergent surgery, we increased the rate of IV fluid administration to accelerate urine production and distend the urinary bladder satisfactorily. If the patient had suprapubic tubes or Foley urethral catheters, we directly infused 400 mL of diluted water-soluble iopamidol via catheters. We performed CT when the patient expressed a strong desire to void. No oral contrast medium was used in our series.

Patients wore urinary bags over their penises to collect urine and avoid wetting the CT machine. CT was performed with the patient prone (n = 9) or supine (n = 5). Patients with pelvic fractures were supine while scanned because they could not lie prone. In contrast to CT colonography or CT cystography, for which both supine and prone scanning is necessary, CT urethrography requires scanning in only one position.

Image Acquisition
After an anteroposterior topogram had been obtained for slice selection, the patients were asked to press a handheld wireless bell controller when they began voiding. As soon as technologists heard the bell, they began scanning. The CT topogram was used as a pelvic radiogram. We did not perform prevoiding CT routinely because we wanted to avoid exposing the patients to additional radiation.

All serial thin-section images of the lower urinary tract were obtained using a 16-MDCT scanner (Somatom Sensation 16, Siemens). Scanning parameters included a 0.5-sec gantry rotation speed, high-quality scanning mode (0.75-mm collimation x 16-detector array, 512 x 512 matrix, 120 mA, and 120 kVp), a 1.0-mm reconstructed slice width at intervals of 0.7 mm (0.3-mm overlap), a total scanning time of 6 sec, and a total scan length of 16-20 cm. Data acquisition was craniocaudad and resulted in about 300 transverse images for each scan.



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  		Fig. 1A. —Imaging of urethra with 2D and 3D techniques in 27-year-old man after vehicle accident. Three-dimensional volume-rendered urogram shows comminuted fractures of left ilium bones extending to left acetabulum with displaced bone fragments, and diastasis of bilateral sacroiliac joints and symphysis pubis. Urethra (arrows), urinary bladder, and ureters (arrowheads) are shown well. No urethral interruption or contrast medium extravasation is noted.

 



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  		Fig. 1B. —Imaging of urethra with 2D and 3D techniques in 27-year-old man after vehicle accident. 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.

 
Image Evaluation
The transverse thin sections were transferred to a workstation (Syngo, Siemens) with manufacturer-provided software that allows generation of 2D maximum intensity projection, 2D multiplanar reconstruction, 3D shaded-surface display, and 3D volume-rendered technique (Figs. 1A, and 1B). Two experienced abdominal radiologists (who were trained in interactive navigation and interpretation of 3D virtual-reality CT colonography or CT cystography and were unaware of the results of other examinations) prospectively and independently interpreted the CT voiding urethrography. The urethra was evaluated with 2D transverse images and a soft-tissue window setting (window level, 40 H; window width, 300 H).

Virtual endoscopy was performed using surface-rendered or volume-rendered techniques. We adjusted the attenuation coefficient range for voxel categorization to the contrast material in the urethra until the normal mucosal surface appeared smooth and no noise was seen in the lumen. The lower limit of the attenuation coefficient range for voxel categorization on virtual urethroscopy was 200-250 H; the upper limit range was 700-850 H. Because the attenuation coefficient of the urethral lumen varied from patient to patient, variable ranges were tried for each patient to optimize the setting. With CT endoscopic fly-through navigator software, radiologists performed interactive intraluminal navigation from the urinary bladder to the urethra. Interactive standard axial, sagittal, and coronal reference images were obtained automatically during navigation (Fig. 2A).



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  		Fig. 2A. —37-year-old man with straddle injury. 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 multidirectional reference images.

 
The new vessel-view tool of the Syngo workstation is a semiautomatic protocol-driven analysis tool for CT angiography. This tool also works well for the urethra. The vessel navigator displays a longitudinal cut along the centerline of the urethra, the so-called ribbon multiplanar reformation, with multiplanar reference images along the curve of the urethra (Fig. 2B). The required longitudinal multiplanar reformatted section view and cine imaging are created using the angle slider to rotate the image plane. We could create urethral boundaries using semiautomatic segmentation functions. We easily could find the urethral path distance by clicking either the multiplanar reformation or the volume-rendering techniques to place the first seed point and terminal point. Also, maximum luminal diameter and luminal area could be measured perpendicularly to the urethral axis (Fig. 2C).



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  		Fig. 2B. —37-year-old man with straddle injury. Vessel-view display panel shows entire urethra with 2D curve multiplanar reformation technique and measurement tools. Focus pointer (arrows) displays as line in vessel navigator. When focus pointer is moved, reference imaging segments are synchronized to position of pointer.

 


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  		Fig. 2C. —37-year-old man with straddle injury. Maximum transverse diameter and area in axial cross-section of urethra are determined automatically by clicking required position of urethral path.

 
Further Urethral Study
Eleven men were assessed using conventional retrograde urethrography on the same day. Because CT voiding urethrography obtains antegrade urethral images similar to voiding cystourethrograms (VCUG) and we did not wish to expose the patients to additional radiation, we did not routinely obtain VCUG. An experienced abdominal radiologist interpreted the conventional urethral examinations without knowing the results of CT voiding urethrography.

Cystourethroscopy
Three men were examined with cystourethroscopy by experienced urologists on the same or next day. Cystourethroscopy was performed if the patient sensed a foreign body in the urethra, had a Foley catheter placed because of urethral injury, or required optic urethrostomy for urethral stricture.


Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
The time required for the CT procedure ranged from 4 to 20 min (mean, 9 min), depending on how soon patients began to void. The time required for interpretation of the CT urethrographic data ranged from 6 to 20 min (mean, 10 min). CT voiding urethrography examinations were completed successfully for all patients. No significant difference in image quality was noted between supine and prone positioning. Images on CT voiding urethrography were of excellent quality, with adequate contrast filling of both the anterior and the posterior urethra in all patients. The final diagnosis was based on retrograde urethrography, cystourethroscopy, or surgical findings (Table 1).


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  		TABLE 1 Patient Data and Examination Findings

 

For urethral injury, CT voiding urethrography and conventional methods were of similar accuracy. Nine patients with clinically suspected urethral injury underwent retrograde urethrography. Pelvic fractures were noted in three. A type 5 urethral injury according to the classification by Goldman et al. [4] was diagnosed in two patients. Patients 1 and 10 had a straddle injury, and the retrograde urethrograms showed contrast medium extravasation in the bulbous urethra. CT voiding urethrography also revealed extravasation, an intraluminal blood clot, and mucosal abnormality (Figs. 3A, 3B, 3C, and 3D). Patient 12 experienced bleeding from the urethra after sexual intercourse; contrast medium extravasation from the penile urethra was detected with CT voiding urethrography but was missed on retrograde urethrography (Figs. 4A, and 4B). A blood clot at the site of injury was found in two of the three patients with urethral injury.



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  		Fig. 3A. —25-year-old man who presented with hematuria after blunt perineum contusion. Retrograde urethrogram shows contrast medium extravasation (arrow) in bulbous urethra.

 


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  		Fig. 3B. —25-year-old man who presented with hematuria after blunt perineum contusion. Volume-rendered CT voiding urethrogram obtained with contrast infusion from suprapubic tube shows contrast extravasation and irregular mucosal surface (arrow) in bulbous urethra. Urethrocavernous and urethrovascular reflux (arrowhead) also were noted.

 


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  		Fig. 3C. —25-year-old man who presented with hematuria after blunt perineum contusion. Conventional cystourethroscopy image reveals bleeding and perforation at 5- to 7-o'clock position of bulbous urethra.

 


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  		Fig. 3D. —25-year-old man who presented with hematuria after blunt perineum contusion. Virtual urethroscopy image based on surface rendering shows mucosal disruption (arrows) in bulbous urethra.

 


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  		Fig. 4A. —50-year-old man with urethral bleeding after sexual activity. Retrograde urethrogram shows no finding.

 


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  		Fig. 4B. —50-year-old man with urethral bleeding after sexual activity. Contrast medium extravasation (arrow) in penile urethra is identified on CT voiding urethrogram, vessel view.

 
For three cases of urethral stricture, CT voiding urethrography was superior to conventional examinations for imaging of pathologic anatomy and measurement of lesions and created greater diagnostic confidence. The retrograde urethrograms did not allow proper evaluation of the posterior urethral stricture. Patient 9 had a history of surgical realignment for type 3 urethral injury with disruption of the urogenital diaphragm. After 3 months, CT voiding urethrography showed a short-segment urethral stenosis at a bulboprostatic anastomosis. Patient 12 had had a urethral injury and a complicated urethral stricture 1 year earlier. CT voiding urethrography revealed stricture in the membranous urethra with proximal urethral dilatation (Fig. 5).



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

 
Patient 13 had undergone plastic surgery for hypospadia 30 years earlier and complained of postvoid dripping. Retrograde urethrography was unsuccessful because of a small meatus opening and pain intolerance. CT voiding urethrography showed penile hypospadia and a diverticulum within the penile skin coverage (Fig. 6A). The patient underwent corrective surgery with flap reconstruction. Five months later, a penile urethral stricture requiring urethrotomy developed (Fig. 6B). CT voiding urethrography provided useful information before and after surgery.



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

 


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  		Fig. 6B. —38-year-old man with history of hypospadia after plastic surgery 30 years earlier. He arranged another surgical correction because of dripping after voiding. Urethral stricture (arrow) developed 5 months after surgical correction.

 

Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
CT voiding urethrography is a technique similar to conventional VCUG. The CT voiding urethrography protocol in this study involved 0.75-mm collimation and revealed the entire urethra in 6 sec. The more thinly collimated transverse images and the subsequent better quality of the reformatted images should further increase the ability of CT to depict the urinary tract accurately [5].

At a normal urinary flow rate of 15 mL/sec for men, the time needed to void 400 mL is longer than 20 sec. High-speed 16-MDCT can scan the entire urethra and urinary bladder in 6 sec. In this study, transverse CT images showed the full extent of the urethra. Complete evaluation of the entire urinary tract, kidney to urethra, is easy with the newly developed 16-MDCT.

Patient compliance is an important determinant of the success of CT voiding urethrography. CT voiding urethrography could play a role in lower urinary tract evaluation for clinically stable patients. The technique should not be used on patients with acute major trauma or acute pelvic fracture unless they already have shown an ability to void. Patients should be interviewed before the examination to evaluate their acceptance of it. Radiologists should know that the patients have no difficulty with voiding. Good communication between patients and CT technologists during the examination also is necessary. Patients need to understand and follow the instructions of technologists. Radiologists should participate in the whole procedure and interpret the real-time images on monitors.

Because CT easily depicts the high attenuation produced by contrast material, diluted contrast material is better appreciated on CT images than on conventional urethrograms. Missing of lesions obscured by bone structures, contrast media, or instruments can be avoided with use of axial and multiplanar images.

We preferred to position patients prone with pillows below their abdomen to increase intraabdominal pressure and enhance the force of micturation. A supine scanning position was preferred for patients with multiple pelvic fractures. The scan processes or imaging quality in our study were the same whether patients were prone or supine. Variations in patient positioning and penile traction during imaging can greatly alter the radiographic appearance of the urethra and strictures. Multiple views, including bilateral oblique, may be required on conventional radiographs [6]. When multiple pelvic fractures and associated patient discomfort are present, the oblique position for conventional radiography may not always be possible. CT voiding urethrography is more convenient because patients are required to adopt only one position and the scanning time is only 6 sec. CT voiding urethrography is more accurate with computer-aided tools for urethral measurement. The vessel view is longitudinal along the curve of the urethra and accurately measures stricture length, distance from the urethral meatus, and luminal area. Exact comparison of the luminal size and stricture length on clinical follow-up is possible.

Compared with retrograde urethrography and conventional cystourethroscopy, urethral imaging with CT voiding urethrography and virtual endoscopy can reduce organ injury and patient suffering. Conventional radiography requires positioning of the patient's urethra and avoidance of overlapping with bone structures. The patient's position is not critical with high-quality 3D images, and patients with complex pelvic fractures do not need to change positions. In our experience, CT voiding urethrography improved patient compliance. Some patients who could not tolerate conventional urethral examinations could accept CT voiding urethrography.

Display of CT data in the form of virtual urethroscopy images affords a number of advantages over transverse CT images alone. Virtual urethroscopy allows data from more than 300 slices of CT images to be compressed into one interactive data set. The data set can be manipulated easily for multidirectional viewing and can be recorded as cine files [7]. For urologists who are not familiar with transverse images, CT voiding urethrography and virtual endoscopy provide a global orientation for focal findings and aid navigation for endoscopists.

Standard practice dictates that trauma and stricture of the male urethra be evaluated with retrograde technique because of the belief that only it produces sufficient distention. Retrograde urethrography is not a physiologic examination. Contrast material often is injected under pressure to overcome the resistance of a stricture. Rapid and forceful injection of the contrast medium in retrograde urethrography may lead to rupture of the mucosal barrier and extravasation of the contrast material into the systemic circulation, with occasional resultant systemic complications such as sepsis and anaphylaxis. Reflex contraction of the pelvic muscle because of forceful injection of the contrast material may lead to a false-positive diagnosis of stricture [8]. Although retrograde urethrography also can be performed during CT, such as in CT voiding urethrography, some technical problems remain. These include inadequate contrast medium filling and radiation exposure to the operators. In this study, we showed in several instances that the new CT technology can show clear urethral imaging sufficient for diagnoses.

According to previously published articles, the disadvantages of CT virtual cystoscopy versus conventional cystoscopy include exposure to radiation, difficulty in detecting flat or small mucosal lesions, lack of information on the color and texture of the mucosa, and lack of biopsy [9].

The technical limitations are the same for CT voiding urethrography as for VCUG. Effective antegrade imaging may be impossible in patients with complete urethral disruption and severe posttraumatic urethral stricture [10]. Some patients are psychologically inhibited from micturating because of the required investigational procedures and surroundings. Simple VCUG cannot provide pressure as great as that provided by retrograde urethrography or double-balloon-catheter urethrography, and some urethral abnormalities can be missed with CT voiding urethrography. The combination of retrograde urethrography and CT voiding urethrography may help radiologists avoid potential pitfalls. The volume of contrast medium extravasation in CT voiding urethrography is usually less than that in retrograde urethrography, and radiologists might misinterpret extravasation as a negative finding or, if from the bulbous urethra, as a reflux into a normal Cowper's duct. Analysis of thin-section axial CT images and associated findings such as intraluminal blood clots and mucosal irregularity may improve the accuracy of diagnosis. Currently, experience with CT voiding urethrography is limited. Thus, conventional urethral examinations should be performed to confirm the diagnosis in doubtful cases. Because of the excessive time needed to create virtual images, radiologists need to select patients carefully.

A theoretic concern with MDCT voiding urethrography, in comparison with VCUG, is the possibility that patients will receive more radiation. The dose from MDCT can be estimated from the dose-length product, a measurement of radiation exposure that takes into account the volume of irradiation [11]. The effective dose of radiation from CT voiding urethrography for an average man is approximately 5 mSv. When VCUG is used for a child of 5-10 years old, the effective radiation dose is about 1.6 mSv [12]. As experience with CT voiding urethrography increases, it may become possible to reduce the radiation dose by adjusting CT parameters, as is done in low-dose CT colonography.

The benefits of CT voiding urethrography and virtual endoscopy over conventional imaging include accurate measurement of lesions, without magnification or distortion; production of both transverse and 3D images of urinary tract abnormalities; depiction of extraluminal anatomic landmarks; good patient compliance; and the ability to survey the whole urinary tract, from the kidney to the urethra.

To our knowledge, CT voiding urethrography has not been reported previously, and urethral pathology has not been described using virtual urethroscopy. Conventional urethral imaging is challenged by the new CT techniques. However, a large study of various urethral diseases is needed to determine the clinical value of CT voiding urethrography.


References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

  1. Pavlica P, Menchi I, Barozzi L. New imaging of the anterior male urethra. Abdom Imaging2003; 28:180 -186[Medline]
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  8. Mullin EM, Peterson LJ, Paulson DF. Retrograde urethrogram: diagnostic aid and hazard. J Urol1973; 110:462 -466[Medline]
  9. Lammle M, Beer A, Settles M, et al. Reliability of MR imaging-based virtual cystoscopy in the diagnosis of cancer of the urinary bladder. AJR 2002;178:1483 -1488[Abstract/Free Full Text]
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  12. Pediatric voiding cystourethrogram. RadiologyInfo Web site. Available at: http://www.radiologyinfo.org/content/v-cystourethrogrm-pd.htm. Accessed February 3, 2005

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