Anton J Bueschen md, 3707 Peachtree Road NE, Unit 4, Atlanta, Georgia 30319, USA. Email: email@example.com
The evolution of urological imaging has had a major impact on the diagnosis and treatment of urological diseases since the discovery of the X-ray by Roentgen in 1895. Early developments included plain films of the abdomen, retrograde urographic techniques, development of contrast media, excretory urography, renal mass puncture, renal angiography, cystography and nuclear medicine procedures. These procedures led to the maturation of the specialties of diagnostic radiology and urology, and the development of the subspecialties of pediatric urology and urological radiology during the first seven decades of the 20th century. Subsequently, many imaging advances have occurred leading to changes in diagnosis and management of urological patients. Ultrasound and cross-sectional imaging technologies (computed tomography and magnetic resonance imaging) are increasingly applied in urological evaluation, treatment and surveillance. Current developments include dual energy computed tomography, positron emission tomography computed tomography, renal donor and renal transplant imaging, prostate magnetic resonance imaging, and microbubble contrast enhanced ultrasound. Imaging advances will continue. It is the responsibility of all physicians to assess the advantages of new developments while weighing those advantages against the additional radiation exposure and the costs associated with new procedures.
The evolution of urological imaging has had a major impact on the diagnosis and treatment of urological diseases since the discovery of the X-ray. It is quite remarkable how urological imaging and urology have developed together. This review discusses the development of urological imaging techniques, shows how new techniques have influenced urology, and finally discusses the most recent developments of imaging and techniques that are on the horizon for possible applications in the near future.
Roentgen discovered the X-ray in 1895 and plain films of the abdomen were first carried out around 1898. There were numerous early reports of plain film findings of renal and ureteral calculi. Retrograde urographic techniques were developed between 1905 and 1915, beginning with a report by Voelcker and von Lichtenberg showing that the renal pelvis and ureters could be demonstrated by introducing a radiopaque silver colloid suspension, Collargol.1,2 Braasch, of the Mayo Clinic, carried out many experimental and clinical trials of this new technique, resulting in a book titled Pyelography (1915), which described most of the structural lesions recognized now.3 Urological organizations were formed during the same period: American Urological Association (1902), Japanese Urological Association (1912) and several European associations. Before this period, the treatment of gonorrheal urethritis paid the bills for many urologists, and an effort was being made to exclude other aspects of venereal disease from the specialty of urology. However, the birth of modern urology began shortly after Roentgen's discovery.
Urological imaging continued to evolve slowly during the first half of the 20th century. As retrograde pyelography techniques improved, it was appreciated that a contrast medium was needed that would be safely excreted by the kidneys. It was not until 1929 that uroselectan, the precursor of later iodinated contrast agents, was first reported by Moses Swick; who did the initial work is controversial. Excretory urography (1929–1931) was first carried out with contrast that produced poor visualization and frequent toxicity. Tomography, which was first used for lungs (1928–1938), led to excretory nephrotomography (1950s).4 As excretory urography techniques were refined for four decades after 1930, it became one of the most useful and accurate examinations in clinical urology.
Iodinated contrast was developed during the 1950s and has been a mainstay for imaging of the urinary system. However, iodinated contrast media can cause reactions that might be dose-dependent or idiosyncratic. The reactions have been present historically as often as 12% of cases, but occur in only 3% with low osmolar non-ionic contrast agents. The three types of reactions that can occur are allergic reactions (most common), cardiovascular reactions and renal toxicity. Most allergic reactions are mild with nausea and vomiting, urticaria or facial edema. Treatment for the mild reactions is usually just an antihistamine. Bronchospasm can be treated with a β-adrenergic agonist or epinephrine if respiratory distress occurs. In the case of severe reactions with shock, epinephrine and hospitalization are required. Cardiovascular contrast toxicity causes minor electrocardiogram changes; decreased myocardial contractility or electrical activity of the sinoatrial and atrioventricular nodes is possible. Renal contrast toxicity (Fig. 1) occurs in less than 1% of patients if the patient had normal renal function before injection of contrast, but there is a greater risk if pre-existing renal damage was present. Medical literature is replete with references describing the evolution of contrast agents, the types of agents, types of reactions, and increased risk of reactions with renal failure, diabetes mellitus, cardiovascular disease and multiple myeloma.5
Seminal vesiculogram and epididymography
Seminal vesiculography, by injection of a 5% solution of colloidal silver into the vas deferens, to evaluate chronic vesiculitis as a cause of ill-defined pain in the perineal and sacrococcygeal area was first described by Belfield in 1913.6 Despite numerous reports during the next 30 years, it became an unpopular study. Epididymography was described by Boreau in 1951 to evaluate patients with azoospermia and active spermatogenesis on testicular biopsy.7 Vasography is indicated today to determine the site of obstruction in azoospermic patients that have active spermatogenesis documented by testis biopsy.
Renal mass puncture was introduced in 1939 by Dean for aspiration of fluid from masses thought to be benign.8 This procedure became popular transiently from the early 1970s until the utilization of computed tomography (CT) to confirm the benign character of a mass without the morbidity and mortality of an operation. Retrograde abdominal angiogram was first described by a right femoral arteriotomy approach in 1941 (Farinas) and by catheter over a guidewire through a percutaneous needle in 1953 by Seldinger.9,10 This technique with an aortogram and a selective renal arteriogram became the standard evaluation of renal masses, showing neovascularity of renal cancer and the renal arterial supply of the kidney to guide the surgeon to occlude the renal arteries before occlusion of the renal vein or veins and subsequent manipulation of the kidney for removal. Renal arteriography was also carried out to evaluate the renal vasculature before segmental nephrectomy and anatrophic nephrolithotomy for staghorn calculi. Renal arteriography used to evaluate renovascular hypertension and catheterization of renal veins is still occasionally carried out today to obtain renal venous samples to evaluate renovascular hypertension. Potential renal donors were evaluated with renal arteriography for many years, but CT has replaced this technique for that purpose. Renal arteriography revolutionized the evaluation of renal masses, especially renal cell carcinoma, and renovascular hypertension until the availability of CT.
Cystography and urethrography were developed early and were modified over the years to evaluate injuries, tumors and urethral strictures. Retrograde urethrograms and retrograde cystograms are used today to evaluate strictures and injuries of the lower urinary tract.
Vesico-ureteral reflux was shown in animals in the early 20th century and its relationship with urinary tract infections was suspected. However, it was not until 1952 that Hutch reported reflux diagnosed by cystography in adult patients with neurogenic bladders, urinary tract infections and radiographic evidence of chronic pyelonephritis.11 This report led to the investigation of children with urinary tract infections. The frequent positive findings in these children led to the development of the subspecialty of pediatric urology during the 1950s and 1960s. The technique of voiding cystography in children using fluoroscopic monitoring was described by Schofner in 1970.12 Radionuclide cystography, reported in 1972, had advantages over a conventional voiding cystogram of less radiation exposure, greater sensitivity and reliability, and more quantitative data.13 However, a disadvantage is its inability to define male urethral anatomy. Therefore, a radionuclide cystogram is now recommended as the initial study to evaluate girls with urinary tract infections and for follow-up evaluation in both sexes.
Lymphography is discussed briefly for historical interest only since it has been replaced by cross-sectional imaging studies. Roentgenographic visualization of the lymphatic system was attempted in animals and humans as early as the 1920s, but a clinically useful technique was first described in the 1950s.14,15 This technique was used by many for approximately 20 years to evaluate urological malignancies, but was discontinued when computed tomography became available with less toxicity and greater accuracy.
Radionuclides were reported to measure renal function by external scintillation counting in 1956 (Taplin).16 Bone scans were first described in 1959 and became widely used with the introduction of bone-seeking complexes of technecium-99m in the early 1970s. Radionuclides were first reported for cystography in 1972 and testis scans in 1975.17,18
Renal scans are able to measure total renal function and differential renal function better than any other imaging study. Most of the development of techniques and radiopharmaceuticals occurred during the first two decades after Taplin's report.19,20 Practical applications include urinary tract dilatation, urinary tract infections including those with vesico-ureteral reflux, pyelonephritis associated with neuropathic bladder, renovascular hypertension, potential renal donor evaluation and renal transplants.21,22 Special techniques, such as diuretic renography and evaluation of renal transplants, are described in the references.
A bone scan was often used for the initial evaluation of prostate cancer until around 1990, because bone metastases were previously present in approximately 40% of cases. Introduction of prostate-specific antigen (PSA) in the late 1980s led to more frequent diagnosis of early stage prostate cancer with fewer bone metastases. Therefore, a bone scan is used now for prostate cancer only if other criteria suggest bone metastasis, such as elevated PSA, a high Gleason Score (8 or greater), a high volume of cancer or symptoms. An analysis of 23 studies examining the usefulness of bone scans found metastases in 2.3% of men with PSA levels <10.0 ng/mL, 5.3% in men with PSA levels from 10.1 to 19.9 ng/mL, and 16.2% in men with PSA levels >20 ng/mL.23 A bone scan is used to evaluate other urological tumors in selected situations, such as bone pain or elevated alkaline phosphatase or poor performance status with renal cell cancer, clear cell sarcoma, Wilms' tumor or if symptoms suggest bone metastasis with bladder cancer. A bone scan is also helpful in the evaluation of neuroblastoma.
Medical applications of ultrasound were first described in the 1960s. Ultrasound technology has improved greatly in the past few decades, making the current advantages of ultrasound to include lack of invasiveness, portability, low cost compared with other imaging studies, lack of nephrotoxic contrast media, lack of ionizing radiation, and it is easy to carry out in the pediatric population. Performance of sonographic examinations in accredited ultrasound laboratories with good quality control is of benefit, because the results of ultrasound are frequently operator-dependent.
The use of ultrasound has increased dramatically in the past two decades and is often the initial examination of choice. The expanded role of abdominal ultrasound to evaluate abdominal symptoms in adults has led to the not uncommon finding of incidental solid renal masses. This will be discussed in the section on computed tomography, which is often required for characterization of these silent lesions. Ultrasound is also requested commonly in the setting of acute renal failure to differentiate medical renal disease from mechanical urinary obstruction. It can rapidly detect or exclude the diagnosis of hydronephrosis. Urolithiasis can be easily visualized in the setting of hematuria.
Color Doppler ultrasound was reported in 1990 and has many uses in urological imaging. Doppler diagnosis of renal artery stenosis has become commonplace. Color and spectral Doppler has largely replaced radionuclide studies to assess vascular flow of the testis in equivocal testicular torsion. Color Doppler ultrasound of the testis has extremely high sensitivity, specificity and accuracy for the diagnosis of acute ischemia. The application of power Doppler improves sensitivity for slow flow even further. Many other indications for scrotal ultrasonography have also been developed. It can be used to distinguish testis cancer from epididymitis and can identify occult testis cancer, even in the presence of hydroceles, which might interfere with a physical examination. Finally, non-palpable testis masses found on ultrasound are often benign. An intraoperative ultrasound can help localize such a mass. Diagnosis and treatment of etiologies for erectile dysfunction relies on spectral Doppler investigation of the cavernosal arteries after a physiological response to a prostaglandin injection.
Transrectal ultrasound techniques were described in the 1980s and have been used widely for prostate biopsy and prostate brachytherapy since the 1990s.
The shift towards cross-sectional techniques
Urological imaging and urology advanced greatly during the first seven decades of the 20th century. In 1970, urological imaging included chest radiography, chest tomography, retrograde urography, excretory urography with tomography, renal arteriography, renal mass puncture and ultrasonography. Ultrasound techniques have continued to evolve as described in the previous section on ultrasound. Nuclear medicine technology was evolving as well. Subsequently, many imaging advances have occurred leading to many changes in diagnosis and management of urological patients. Cross-sectional imaging technologies are increasingly applied in urological evaluation, treatment and surveillance.
The first to publish the concept of CT was Allan M Cormack and the first CT device was created in 1972 by Sir Godfrey N Hounsfield, a physicist working in a research branch of a music company in London, UK. Cormack and Hounsfield shared the 1979 Nobel Prize in Medicine and Physiology for their contributions to the development of CT. The advantages of modern CT over excretory urography include greater sensitivity for urinary stones, better visualization of renal parenchyma, ability to show other abdominal abnormalities, speed and accessibility of exams, and capability for angiographic evaluation by CT. In most situations, the use of intravenous contrast media enhances the diagnostic quality of a CT examination. Urological indications for CT include evaluation of renal masses, staging urological cancer, congenital anomalies, vascular abnormalities, urolithiasis, renal donor evaluation and characterization of incidental adrenal lesions.24,25 A few of the uses for CT that have altered the management of urological diseases will be discussed.
The frequent use of imaging in the setting of trauma and to evaluate unexplained abdominal symptoms has identified many solid asymptomatic renal masses that can be assessed with abdominal CT. These incidental renal masses now represent approximately 50% of all solid renal masses and are more likely to be organ confined than symptomatic lesions. Except for fat-containing angiomyolipoma, no standard imaging study can distinguish between localized benign and malignant solid tumors or between indolent and aggressive tumor biology. However, a CT scan is able to characterize the renal mass, show contralateral renal morphology, and assess veins, lymph nodes, adrenal glands and the liver. Fat in an enhancing renal mass is pathognomonic for angiomyolipoma. Other characteristics of solid renal masses that make them more likely to be benign are a diameter of <3 cm and lesions in young women or older patients. Approximately 20% of small, solid CT-enhancing renal masses prove to be benign oncocytoma or atypical, fat-poor angiomyolipoma after surgical excision.
A CT-guided renal mass biopsy or fine needle aspiration (FNA) traditionally has been used selectively for metastatic evaluation or suspicion of abscess or lymphoma. A recent report showed improved results with FNA, in vitro, with agar microbiopsy.26 The use of renal mass biopsy is now being reconsidered for the following reasons: (i) 20% of T1 masses are benign; (ii) false-negative rate is only 1%; (iii) significant complications occur in <2%; and (iv) molecular analysis shows great promise for treatment. A renal mass biopsy is also used selectively for observation candidates to aid in treatment planning.
CT has revolutionized the evaluation of urolithiasis. Historically, excretory urography had been the preferred radiographic test for evaluation of acute renal colic since the late 1920s. However, the development of the helical CT in 1994 modified the diagnostic work-up of urinary diseases. Compared with excretory urography, unenhanced helical CT was better in a prospective study of 53 patients with acute signs of renal colic.27 Unenhanced CT was more accurate, was much quicker to carry out, eliminated the need for contrast, and showed signs of obstruction with ureteral dilatation and perinephric and periureteral fat stranding. Currently, the unenhanced helical CT is the preferred test for assessment of acute renal colic. However, repeated use and resultant radiation risks in young patients with urolithiasis have recently caused concern among the imaging community.
CT angiography (CTA) has impacted on the diagnosis of vascular disease as greatly as CT has influenced the work-up of stone disease. CTA differs from routine CT because of its rapid contrast injection rate, thin slice acquisition and optimization of the contrast bolus for the vascular region in question. It has enabled direct visualization of vascular stenosis without the invasiveness of catheter angiography. Detection and characterization of renal artery aneurysm (Fig. 2), arteriovenous malformation and renal venous disease is unsurpassed in anatomic detail. Three-dimensional reconstructions are now commonly carried out and can aid the surgeon in preoperative planning for complex cases.
Magnetic resonance imaging
Magnetic resonance imaging (MRI) was first reported in 1973 and the first studies reported in humans were published in 1977.28 The basic physical principle of this technology is that nuclear property of the free water protons (hydrogen ions) are positioned by the application and withdrawal of a strong magnetic force, with their resultant signals producing images. Advantages of MRI are absence of radiation, no iodinated contrast is used and soft tissue contrast resolution is superior to CT. The major disadvantages are that it is more expensive than CT, requires more time to carry out and is frequently affected by motion artifacts. Also, some patients find the confined space and duration of the study to be uncomfortable. Major contraindications to using MRI are the presence of an implant, such as a pacemaker, aneurysmal clips, retained foreign bodies, and metallic prostheses and pacemakers or defibrillators.
MRI can detect renal lesions of 1 cm as well as CT. Renal lesions smaller than 1 cm are difficult to detect by either modality, but especially by MRI because of its lower spatial resolution, but treatment is rarely recommended for lesions this small. CT remains the cross-sectional imaging study that is used more commonly than MRI to evaluate renal masses, but practice patterns vary significantly.
MRI is the preferred examination for the evaluation of renal tumor thrombus in the inferior vena cava (IVC), pheochromocytoma, complications of pregnancy to avoid radiation exposure when ultrasound is inconclusive, indeterminate lesional enhancement on CT and allergy to iodinated contrast. MRI is not useful for the evaluation of urolithiasis, because stones cannot be distinguished from a tumor or blood clots.
MRI is the premier study for evaluation of IVC tumor thrombus with nearly 100% sensitivity of detecting venous invasion (Fig. 3). MRI assesses both the cephalad and caudal extent of the thrombus. Venacavography is now reserved for patients with equivocal MRI findings or those who cannot tolerate MRI. Sensitivities for CT detection of renal venous and IVC tumor involvement are 78% and 96%, respectively.29 Suggestive findings are venous enlargement, abrupt change in the caliber of the vein and intraluminal areas of decreased density or filling defects. Most false negative studies are on the right side when a short length of vein and mass effect from the tumor make detection difficult when there is thrombus enhancement as a result of tumor vascularity. However, the initial CT is often sufficient for evaluation of venous extension, especially on the left side.
For years, MRI was the imaging modality of choice for evaluation of renal mass when a patient could not undergo contrast enhanced CT as a result of renal insufficiency. However, a new entity was described in 1997 and now it appears to be related to intravascular exposure to gadolinium based contrast agents (GBCA), which have been used frequently with MRI.30,31 The disease, first described in patients with severe renal failure, is a severe and potentially devastating disorder characterized by progressive fibrosis of the skin and other body tissues. It was first called “scleromyxedema-like cutaneous diseases in renal dialysis patients” and was later called nephrogenic systemic fibrosis (NSF). An important review article was published by Natalin in 2010.32 Urologists and radiologists must understand the risks, disease nature and treatment strategies. Gadolinium exposure seems to be the most significant component of NSF. Use of GBCA in patients with a glomerular filtration rate of less than 30 mL/min should be limited and all patients with renal insufficiency in this range should have an informed consent. The risk is extremely low in patients with chronic estimated glomerular filtration rate of 30–60 mL/min with a single dose of gadolinium. The risk is also exceedingly low in the administration of cyclic gadolinium agents, such as Prohance. Contrast dosing should be carried out within the limits detailed in package insert guidelines.
The current role of CT urography and MR urography in the evaluation of the urinary tract was reviewed thoroughly by Silverman et al. in 2009.33 CT urography or MR urography were concluded to be superior compared with the previous standard of excretory urography. CT urography has emerged as the favorite in most cases to evaluate urolithiasis, renal masses, urinary tract infections, trauma and obstructive uropathy. CT urography provides a detailed anatomic picture of the entire urinary tract, providing a good evaluation for hematuria. MR urography has the ability to assess the entire urinary tract without using ionizing radiation and provides more functional information than CT. Both tests can be used to evaluate the urinary tract with remaining concerns of cost containment, radiation exposure and renal insufficiency (iodinated contrast material in CT and gadolinium-based contrast material in MR). Current research is directed to identify technical issues, such as timing, diuretic use, compression and other issues, that will improve urinary distention and examination quality.
Urological imaging in 2010
So far, this report has described the evolution of urological imaging during the past 110 years and the close association of the evolution of urology. The diagnosis and management of many urological diseases have been altered by the development of urological imaging, Table 1. The remainder of the review will present current developments and anticipated possibilities in the near future.
Table 1. Urological disease management altered by imaging
Renal mass evaluation
Kidney cancer staging
Dual energy CT
Early work has shown that different materials absorb ionizing radiation differently, depending on the energy level of the beam. This concept has only recently been successfully implemented in clinical CT technology. In the past 5 years, dual energy CT has become available in clinical care. There are many opportunities in urological imaging; the greatest center on the retrospective removal of iodinated contrast from enhanced CT images. In these patients, a single phase study can obtain images in the nephrographic phase. Conventional CT with contrast has been shown to obscure small renal stones. However, these stones can now be retrospectively detected by removing the contrast from the images (Fig. 4), allowing detection without returning the patient to the radiology department at a later date for stone CT with an additional radiation dosage.34 Another application of material decomposition is the characterization of renal stones.35 Dual energy CT can identify uric acid and a program then can encode uric acid stones in a different color from other stone types to aid in characterization and guide therapy.
CT positron emission tomography
Until the past decade, positron emission tomography (PET) and PET-CT have had limited roles in urological imaging. Conceptually, PET-CT involves the injection of radioactive material within the bloodstream and subsequent imaging by a dual head detector system. The imaging characteristics differ based on which molecule the radiotracer is attached to. The most commonly used agent is a glucose analog, 18F-FDG. This molecule is processed by cells and therefore remains intracellular in metabolically active tissues that use glucose, such as tumors. Other analogs might be used for specific tumors, such as 18F-choline analog for the evaluation of prostate cancer metastases.
Clinical applications include detection of metastases from urological tumors (Fig. 5) and the early evaluation of metastatic disease response to systemic therapy. A recent Consensus Conference from Germany summarizes the PET-CT literature for urological imaging well.36 In general, PET-CT might be more sensitive for osseous metastases, but data has not conclusively shown increased sensitivity for liver, lung and nodal metastases. For tumor response assessment, the best mode of surveillance and timing is still controversial, but PET-CT shows promise to detect response to therapy after tyrosine kinase inhibitor therapy, even when tumor size is unchanged. However, visualization of tumors within the urinary tract itself is limited, because the tracer is normally excreted by the kidneys.
Staging of bladder cancer is one of the potential applications for PET-CT. For invasive tumors, it is critical to know whether there is nodal involvement or distant metastatic disease; the presence of either significantly worsens the prognosis. In a study of 40 patients by Drieskens et al., PET provided additional information beyond conventional CT in 15% of patients by identifying additional malignant lesions or showing that enlarged nodes were not metabolically active.37 However, overall sensitivity for small nodal metastases was limited. In a more recent study of 42 subjects staged by FDG PET-CT before radical cystectomy for muscle-invasive bladder cancer, the sensitivity and specificity of PET-CT was 70% and 94%, respectively.38
Staging of treated testicular carcinoma for recurrent disease can also be carried out by PET-CT. A recent prospective multicenter study of 72 patients showed only a mild benefit of PET-CT over CT in the initial staging.39 In a study that evaluated 50 patients with testicular cancer before and after chemotherapy, there was no clear benefit of PET-CT over CT in the initial work-up. However, PET-CT was more useful in post-chemotherapy follow up.40 In this clinical scenario, PET-CT can potentially detect metastases when CT is negative, and in other cases it can confirm lack of metabolic activity in a lesion, suggesting a fibrotic mass without residual germ cell tumor.40,41 In a different study, FDG PET-CT successfully changed management in 57% of patients with elevated tumor markers but a normal CT scan.42 For prostate cancer, PET currently has limited utility. It has potentially better detection of bone metastasis response to therapy, but there is limited published research available.
Renal transplant imaging
In previous years, the evaluation of a potential renal donor patient before renal transplantation involved IVP, renal scan and conventional angiography. In recent years, all has been replaced by a single CT technique. The basic concepts are well described in a review by Kawamoto et al.,43 but the specific timing and imaging techniques vary by institution. For instance, unenhanced images to exclude stone disease are commonly carried out, although not recommended in the review. Contrast enhancement evaluates the arteries first, followed by renal parenchymal phase that also shows venous structures well. The ureters can usually be visualized with a CT urographic phase, but renal donors are typically without hematuria to suspect a urothelial lesion. In donors, the number of ureters is more commonly the question answered by excreted contrast. This multiphase CT approach has allowed “one stop shopping” as a unified imaging evaluation in an outpatient setting. Recent developments have evaluated the ability of CT to quantify split renal function in renal donors (Fig. 6) and patients with renal vascular disease, based mainly on renal volumes and enhancement calculations.44–46
For transplant recipients, duplex ultrasound is the primary imaging modality. Concerns about nephrotoxicity and gadolinium-related complications limit the usefulness of CT and MRI in these patients. The major benefits of ultrasound in transplanted kidneys are to exclude or confirm urinary obstruction or vascular abnormality, such as renal vein thrombosis or renal artery stenosis. Doppler criteria have been developed for renal artery stenosis and renal vein thrombosis, based on flow velocities and waveform morphology. Whereas Doppler is critical for vascular evaluation, grayscale is highly sensitive for hydronephrosis. Grayscale ultrasound can also detect abnormal echogenicity to suggest acute tubular necrosis, rejection or cyclosporine toxicity. However, it is unable to differentiate the specific underlying etiology. Doppler evaluation can detect high intrarenal resistance, but is again unable to differentiate between the underlying etiologies.47
The effectiveness of prostate biopsy has transformed the diagnosis and treatment of localized prostate cancer. Still, there are many cases where imaging (mostly by MRI) is beneficial. Growing literature supports the use of MRI when there is rising PSA and a lack of a cancer diagnosis by routine biopsy. MRI with an endorectal coil has high spatial resolution and can well visualize the peripheral zone. On T2-weighted images, a low signal intensity tumor stands out among the high signal normal tissue. Recent research has capitalized on the high spatial resolution of 3-tesla MRI with an endorectal coil to improve the detection of localized disease.48 In more challenging cases, dynamic contrast enhancement is used, because tumors might enhance more rapidly than the rest of the tissue (Fig. 7). Newer techniques, such as diffusion weighted imaging and choline specific MR spectroscopy show promise in further improving the detection of localized prostate disease.
Microbubble contrast enhanced ultrasound
Although available in Europe since the early 1990s and Canada since 2001, ultrasound contrast agents are poorly understood by physicians in the USA. Approval of these agents has been slow to obtain from the Food and Drug Administration, limiting research and clinical applications. A recent review describes the uses and obstacles to approval in the USA in detail.49 There are many potential clinical indications, with the benefits of ultrasound including absence of radiation or nephrotoxicity. Also, the safety profile for microbubble contrast agents greatly exceeds those for iodine or gadolinium based agents.
Contrast enhanced ultrasound begins with standard grayscale imaging. Next, intravenous access allows injection of a small bolus or drip infusion of bubbles that contain a phospholipid shell. The bubbles flow through the bloodstream to the point of interest. As ultrasound waves contact the bubbles, they transiently contract the bubble. As the bubble contracts and expands, the signal that returns to the ultrasound transducer is different to normal reflections; these harmonics are detected by the ultrasound for image creation. This microbubble reverberation greatly improves the ultrasound signal for visualization of flow. At higher ultrasound energy levels, the microbubbles actually burst and this microscopic explosion generates a strong signal to the ultrasound machine.
Many potential uses of ultrasound contrast agents should be considered. The injection of microbubbles can detect flow within a renal mass when CT and MR contrast is contraindicated (Fig. 8). They can evaluate the renal vasculature. In the future, applications might routinely include exclusion of testicular torsion. It has been suggested that instillation of microbubbles into the bladder could reliably diagnose vesico-ureteral reflux in infants without radiation.
Most radiologists and urologists now understand that radiation dose issues will be a significant topic of discussion and research in the upcoming decade. National endeavors, such as the “Image Gently” campaign within the USA, have raised the awareness of the public to radiation risks.50 Medical imaging is the single highest artificial contributor to population radiation exposure. Ionizing radiation results from radiography, such as chest radiographs and IVP, but much higher doses result from fluoroscopic procedures and most importantly, CT. Vendors are making efforts to reduce the radiation dose while maintaining diagnostic quality. These include automated dose reduction setting and calibrations in the acquisition of images. New reconstruction algorithms are providing better images with less radiation. However, these improvements face the ever increasing volume of CT scans ordered as physicians become more and more dependent on cross-sectional imaging in their daily clinical practice.