The practice of urology has changed dramatically in recent years, perhaps more than any other medical specialty. There have been three primary motivators. The first is advances in technology that now permit most diagnostic and therapeutic procedures to be performed on the genitourinary tract in minimally invasive ways using imaging, endoscopy, or both: what is called "endourology."¹ The second is the aging of the population and the greater reluctance to accept such accompanying problems as stress urinary incontinence, benign prostatic hyperplasia, and erectile dysfunction.  The third is the widespread use of prostate specific antigen screening, which is identifying more prostate cancers when they are small, and of cross-sectional abdominal imaging for various purposes, leading to the discovery of a considerable number of small renal cancers.² Such subclinical cancers might safely be treated by measures short of the radical surgery that has long been the standard of care (eg, "nephron-sparing" operations for kidney cancer³).

For decades, most imaging of the urinary tract has been by radiography after contrast injection: the intravenous urogram (formerly called the intravenous pyelogram or IVP), which often was obtained and interpreted by urologists. Today, according to Ralph V. Clayman, MD, professor and chairman of urology, University of CaliforniaIrvine, "IVU has died except when you need exquisite outlining of the collecting system," he says. "Some medical centers have shut down their IVU facilities. Imaging in urology today is mostly ultrasonography and CT."

Ultrasonography

Urologists and their interventional radiology colleagues were early users of ultrasonography to guide percutaneous renal punctures for diagnostic and therapeutic purposes such as stone removal. As the technology improved and new methods such as color Doppler were introduced, the value of the modality increased. Today, ultrasonography is a rapid method of detecting dilation of the upper-tract collecting system, a sign of urinary obstruction, and a less expensive alternative to CT for this indication.4 Ultrasonography also can be used for initial characterization and staging of kidney lesions, helping, for example, to distinguish solid tumors from cysts.⁵ A recent report from the Lister Hospital in Hertfordshire, UK, indicated that ultrasonography and a plain abdominal film were superior to an IVU in evaluating men with urinary tract infection.⁶ Ultrasonography also is useful as an alternative to catheterization for measuring residual urine volume in patients with lower-tract obstruction,⁷ avoiding the risk of infection. The modality also has been used during open and laparoscopic surgery to identify the proximal extent of thrombi from kidney cancers in the renal vein and inferior vena cava⁸ and to aid in nephron-sparing surgery by clearly delineating the boundaries of a given renal tumor.

Computed Tomography

The appeal of CT is its ability to acquire anatomic (including vascular) and functional information in one

quick study. A noncontrast scan, analogous to the plain radiograph (kidneysuretersbladder/KUB) film of years past, is performed. This scan by itself often is sufficient to identify the cause of renal colic.⁹ If further information is needed, contrast medium is injected, followed by a 250-mL saline bolus and a diuretic such as furosemide, and a second set of scans is obtained, capturing arterial, corticomedullary, and nephrographic phases.¹⁰ For those, usually two more sets of scans are obtained, one early to see the vessels and the early renal stage and then a later set to show the collecting system. Three-dimensional reconstructions can be produced, such as for planning nephron-sparing surgery¹¹ or laparoscopic donor nephrectomy.¹² In the latter application, CT generally can replace selective arteriography,¹³ a situation one transplant team called "the marriage of minimally invasive imaging with minimally invasive surgery."¹² In patients with unusual anatomy, CT may be valuable in guiding the creation of a percutaneous nephrostomy tract for extraction of stones.¹⁴ Multidetector array CT is being examined for urography¹⁵ and for defining and staging renal cancers.¹⁶

Magnetic Resonance Imaging

Magnetic resonance imaging is a latecomer to urologic imaging, partly because it is of no value in one of the most common urologic diseases, namely stones. Its only widely accepted indications are for the study of adrenal tumors and vena caval thrombi from renal-cell cancers. However, its excellent depiction of the soft tissues, absence of radiation, and utilization of non-nephrotoxic contrast medium have led to exploration of other applications.

Pelvic (or, more accurately, vaginal) prolapse leading to urinary and defecatory dysfunction is common, with approximately 11% of US women requiring surgery before age 8017 at an estimated cost in 1997 of more than $1 billion.¹⁸ There are numerous forms of prolapse, and the signs and symptoms are not a reliable guide to the spectrum of anatomic defects in a particular patient. Dynamic fluoroscopy, in which the pelvis is imaged during urination and defecation after contrast medium has been instilled into the bladder, vagina, and rectum, can be used to clarify the anatomic defects, but the technique requires considerable time and expense.¹⁹ A faster method is dynamic MRI in which the supine patient is imaged in the coronal, axial, and sagittal planes at rest and during straining.²⁰ It is possible to measure the degree of pelvic descent precisely and to assess muscular and fascial failure so that subsequent surgery can be tailored to the patient. Perk and associates²¹ and Lorenzo and colleagues²² have described the value of MRI in determining the cause of urethral problems (incontinence, obstruction) in women.

Urinary tract malformations are among the most common congenital abnormalities, and in this pediatric population, avoidance of radiation and of potentially nephrotoxic contrast medium is particularly desirable. Urologists at Philipps University in Marburg, Germany, described the use of magnetic resonance urography with gadolinium in 12 children with a mean age of 3 months and were able to obtain accurate images of the anomalies in all cases.²³ Those investigators also noted that, unlike IVU, magnetic resonance urography is not compromised by the presence of gas in the overlying bowel.

Despite the increasingly early discovery of prostate cancers since the introduction of prostate specific antigen, thousands of men with newly diagnosed disease have sufficiently large tumors to suggest the likelihood of pelvic lymph node metastases. Because patients with metastases are not candidates for radical surgery, much effort has been devoted to detecting disease spread by such means as laparoscopic lymph node sampling. A new application of MRI promises a better way. Radiologists and urologists at Massachusetts General Hospital, Boston, and the University Medical Center in Nijmegen, The Netherlands, obtained high-resolution MR images after injection of highly lymphotropic superparamagnetic iron nanoparticles in a series of 80 men with confirmed prostate cancer.²⁴ Nodal metastases were found at surgery in 33 patients, 71% of whom would have been considered free of disease by standard pelvic imaging. All 33 patients were correctly identified by the MR technique. On a node-by-node basis, MR was 90.5% sensitive vs 35.4% for standard imaging.

Monitoring Tissue Ablation

Looking toward the future, Clayman remarked that "imaging is going to be very important to the new fields of needle ablative therapy, such as cryotherapy and radiofrequency ablation, which are developing rapidly."

Publications attesting to the medical value of low temperatures date to the mid-19th century, and a cryosurgical apparatus was described in 1961. Soon thereafter, urologists began experimenting with cryosurgery of renal tumors in animals^25,26 and during open surgery in humans or as a substitute for surgery.^27,28 Both liquid nitrogen and liquid argon have been used to induce temperatures that cause thrombosis and intracellular ice crystal formation (lower than -20º C). Repeated cycles of freezing and thawing may be used to increase the effect.

Cryoablation requires monitoring in order to avoid the complications that plagued early efforts. The goals of intraprocedural imaging are to differentiate the tumor from the normal tissue, to obtain accurate measurements of the iceball, and to use these measurements to control energy delivery. Intraoperative ultrasonography accurately depicts tumor size, cryoprobe position, and depth of freezing.²⁹ However, because frozen tissue reflects nearly all of the acoustic energy, only the near edge of the iceball is visible, and temperature measurements often are used in addition.

A team at the Cleveland Clinic first reported laparoscopic renal tumor cryoablation with intraoperative ultrasound monitoring in 1998, and their series now encompasses more than 60 patients.³⁰ Percutaneous renal cryotherapy under interventional MRI guidance was applied in a series of 65 patients, nine of whom received two treatments.³¹ Gadolinium-enhanced MRI generally is used for follow-up or renal cryoablation, with nonenhancement being the criterion of success.³² Serial MR examinations demonstrate lesion shrinkage.^33,34

Also under intense investigation is radiofrequency ablation, in which alternating electrical current is used to heat the tumor to a lethal temperature. Two approaches are being explored. With "dry" ablation, needles are inserted into the tumor tissue. With "cool tip" ablation, the tip of the needle is cooled to allow a deeper penetration of the heat and less tissue charring. With both methods, a combination of thrombosis, desiccation, protein denaturation, and metabolic impairment causes tissue necrosis.

The first use of radiofrequency ablation as a treatment for kidney cancer was reported in 1999.³5 A probe was placed percutaneously under ultrasound guidance in an elderly patient who refused surgery, and the tumor was treated for 12 minutes. Contrast-enhanced CT scans at 1 and 3 months suggested necrosis of the treated region. A later paper from the same surgical team described an additional seven patients, who had no evidence of recurrent disease with a short follow-up of 3 to 21 months.³⁶ The largest series described to date³⁷ included 21 patients treated under ultrasound and CT guidance. Contrast-enhanced CT at 2 months indicated ablation of 19 of the lesions. In this and another series,³⁸ continued enhancement of the lesions in a few patients led to retreatment. The role of radiofrequency ablation is still being refined, as viable tumor has been recovered in some patients at later biopsy or surgery.^39,40

The Future

Some other imaging applications that promise to advance the goals of minimally invasive urology are under development. For example, MRI and MR spectroscopy focusing on choline and citrate may improve local staging and histologic typing of prostate cancer. 18-Fluorocholine and positron emission tomography may likewise improve prostate cancer evaluation.⁴³ Positron emission tomography is superior to CT in evaluating the extent of a testicular cancer and determining the nature of residual retroperitoneal masses after chemotherapy.44 The modality also shows promise as a method of follow-up of surgery or ablative therapy for renal cancers.44 Clearly, just as the first percutaneous nephrostomy for drainage of an obstructed kidney was the harbinger of a revolution in urologic practice, the replacement of the IVU is only the beginning of the changes in urologic imaging.

Judith Gunn Bronson, MS, is a contributing writer for Decisions in Imaging Economics.

References:

1. Smith AD, Lange PH, Fraley EE. Applications of percutaneous nephrostomy: new challenges and opportunities in endo-urology. J Urol. 1979;121:382.
2. Smith SJ, Bosniak MA, Megibow AJ, Hulnick DH, Horii SC, Raghavendra BN. Renal cell carcinoma: earlier discovery and increased detection. Radiology. 1989;170:699â€"703.
3. Novick AC. Nephron-sparing surgery for renal cell carcinoma. Br J Urol. 1998;82:321â€"324.
4. Mostbeck GH, Zontsich T, Turetschek K. Ultrasound of the kidney: obstruction and medical diseases. Eur Radiol. 2001;11:1878â€"1889.
5. Helenon O, Correas JM, Balleyguier C, Ghouadni M, Cornud F. Ultrasound of renal tumors. Eur Radiol. 2001;11:1890â€"1901.
6. Andrews SJ, Brooks PT, Hanbury DC, et al. Ultrasonography and abdominal radiography versus intravenous urography in investigation of urinary tract infection in men: prospective incident cohort study [see comment]. BMJ. 2002;324:454â€"456.
7. Goode PS, Locher JL, Bryant RL, Roth DL, Bugrio KL. Measurement of postvoid residual urine with portable transabdominal bladder ultrasound scanner and urethral catheterization. Int Urogynecol J. 2000;11:296â€"300.
8. Hsu THS, Jeffrey RB Jr, Chon C, Presti JC Jr. Laparoscopic radical nephrectomy incorporating intraoperative ultrasonography for renal cell carcinoma with renal vein tumor thrombus. Urology. 2003;61:1246â€"1248.
9. Dalla Palma L, Pozzi-Mucelli R, Stacul F. Present-day imaging of patients with renal colic [see comment]. Eur Radiol. 2001;11:4â€"17.
10. Schreyer HH, Uggowitzer MM, Ruppert-Kohlmayr A. Helical CT of the urinary organs. Eur Radiol. 2002;12:575â€"591.
11. Remer EM, Herts BR, Veniero JC. Imaging for nephron-sparing surgery. Semin Urol Oncol. 2002;20:180â€"191.
12. Rydberg J, Kopecky KK, Tann M, et al. Evaluation of prospective living renal donors for laparoscopic nephrectomy with multisection CT: the marriage of minimally invasive imaging with minimally invasive surgery. Radiographics. 2001;21(spec no.):S223â€"S236.
13. El Fettouh HA, Herts BR, Nimeh T, et al. Prospective comparison of 3-dimensional volume rendered computerized tomography and conventional renal arteriography for surgical plann zing in patients undergoing laparoscopic donor nephrectomy. J Urol. 2003;170:57â€"60.
14. Matlaga BR, Shah OD, Zagoria RJ, Dyer RB, Streem SB, Assimos DG. Computerized tomography guided access for percutaneous nephrostolithotomy. J Urol. 2003;170:45â€"47.
15. Caoili EM, Cohan RH, Korobkin M, et al. Urinary tract abnormalities: initial experience with multi-detector row CT angiography [see comment]. Radiology. 2002;222:353â€"360.
16. Sheth S, Scatarige JC, Horton KM, Corl FM, Fishman EK. Current concepts in the diagnosis and management of renal cell carcinoma: role of multidetector CT and three-dimensional CT. Radiographics. 2001;21(spec no.):S237â€"S254.
17. Kobashi KC, Leach GE. Pelvic prolapse. J Urol. 2000;164:187â€"1890.
18. Subak LL, Waetjen LE, van den Eeden S, Thom DH, Vittinghoff E, Brown JS. Cost of pelvic organ prolapse surgery in the United States. Obstet Gynecol. 2001;98:646â€"651.
19. Brubaker L, Retzky S, Smith C, Saclarides T. Pelvic floor evaluation with dynamic fluoroscopy. Obstet Gynecol. 1993;82:863â€"868.
20. Singh K, Reid WMN, Berger LA. Assessment and grading of pelvic organ prolapse by use of dynamic magnetic resonance imaging. Am J Obstet Gynecol. 2001;185:71â€"77.
21. Perk H, Oral B, Yesildag A, Serel TA, Özsay M, Turgut T. Magnetic resonance imaging for stress incontinence: evaluation of patients before and after surgical correction. Eur J Radiol. 2002;44:44â€"47.
22. Lorenzo AJ, Zimmern P, Lemack GE, Nurenberg P. Endorectal coil magnetic resonance imaging for diagnosis of urethral and periurethral pathologic findings in women. Urology. 2003;61:1129â€"1133.
23. Wille S, von Knobloch R, Klose KJ, Heidenreich A, Hofmann R. Magnetic resonance urography in pediatric urology. Scand J Urol Nephrol. 2003;37:16â€"21.
24. Harisinghani MG, Barentsz J, Hahn PF, et al. Noninvasive detection of clinically occult lymph-node metastases in prostate cancer. N Engl J Med. 2003;348:2491â€"2499
25. Barone GW, Rodgers BM. Morphologic and functional effects of renal cryoinjury. Cryobiology. 1988;25:363â€"371.
26. Stephenson RA, King D, Rohr RL. Renal cryoablation in a canine model. Urology. 1996;47:772â€"776.
27. Uchida M, Imaide Y, Sugimoto K, Uehara H, Watanabe H. Percutaneous cryosurgery for renal tumours. Br J Urol. 1995;75:132â€"136; discussion 136â€"137.
28. Delworth MG, Pisters LL, Fornage BD, von Eschenbach AC. Cryotherapy for renal cell carcinoma and angiomyolipoma. J Urol. 1996;155:252â€"254; discussion 254â€"255.
29. Zegel HG, Holland GA, Jennings SB, Chong WK, Cohen JK. Intraoperative ultrasonographically guided cryoablation of renal masses: initial experience. J Ultrasound Med. 1998;17:571â€"576.
30. Levin HS, Meraney AM, Novick AC, et al. Needle biopsy histology of renal tumors 3â€"6 months after laparoscopic renal cryoablation [abstract]. J Urol. 2000;163(suppl):682.
31. Shingleton WB, Sewell PE. Percutaneous tumor cryoablation: 24 and 36 month follow-up [abstract]. J Endourol. 2002;16(suppl):A133.
32. . Gill IS, Novick AC. Renal cryosurgery. Urology. 1999;54:215â€"219.
33. Moon TD, Lee FT, Nakada SY, et al. Laparoscopic cryotherapy for small renal tumors [abstract]. J Endourol. 2002;16(suppl):A151.
34. Soble JJ, Gill IS, Novick AC, et al. Ultrasound and MRI characteristics of laparoscopic renal cryolesions [abstract]. J Urol. 1999;161(suppl):368.
35. McGovern FJ, Wood BJ, Goldberg SN, Mueller PR. Radio frequency ablation of renal cell carcinoma via image guided needle electrodes. J Urol. 1999;161:599.
36. Gervais DA, McGovern FJ, Wood BJ, Goldberg SN, McDougal WS, Mueller PR. Radiofrequency ablation of renal cell carcinoma: early clinical experience. Radiology. 2000;217:665â€"672
37. Pavlovich CP, Walther MM, Choyke PL, et al. Percutaneous radio frequency ablation of small renal tumors: initial results [see comment]. J Urol. 2002;167:10â€"15.
38. Ogan K, Jacomides L, Dolmatch BL, et al. Percutaneous radiofrequency ablation of renal tumors: technique, limitations, and morbidity. Urology. 2002;60:954â€"958.
39. Rendon RA, Kachura JR, Sweet JM, et al. The uncertainty of radio frequency treatment of renal cell carcinoma: findings at immediate and delayed nephrectomy [see comments]. J Urol. 2002;167:1587â€"1592.
40. Michaels MJ, Rhee HK, Mourtzinos AP, Summerhayes IC, Silverman ML, Libertino JA. Incomplete renal tumor destruction using radio frequency interstitial ablation. J Urol. 2002;168:2406; discussion 2409.
41. Jager JG, Ruijter ET, van de Kaa CA, et al. Dynamic turboFLASH subtraction technique of contrast-enhanced MR imaging of the prostate: correlation with histopathologic results. Radiology. 1997;203:645â€"652.
42. Swanson MG, Vigneron DB, Tran TK, Kurhanewicz J. Magnetic resonance imaging and spectroscopic imaging of prostate cancer. Cancer Invest. 2001;19:510â€"523.
43. DeGrado TR, Coleman RE, Wang S, et al. Synthesis and evaluation of 18F-labeled choline as an oncologic tracer for positron emission tomography: initial findings in prostate cancer. Cancer Res. 2001;61:110â€"117.
44. Shvarts O, Ilan KR, Seltzer M, Pantuck AJ, Belldegrun AS. Positron emission tomography in urologic oncology. Cancer Control. 2002;9:335â€"342.