Journal of Postgraduate Medicine Medknow Publications and Staff Society of Seth GS Medical College and KEM Hospital, Mumbai, India ISSN: 0022-3859 EISSN: 0972-2823 Vol. 56, Num. 2, 2010, pp. 131-139

Journal of Postgraduate Medicine, Vol. 56, No. 2, April-June, 2010, pp. 131-139

Symposium

Genitourinary imaging: Current and emerging applications

O' Donoghue PM, McSweeney SE, Jhaveri K

Joint Department of Medical Imaging, University Health Network and Mount Sinai Hospital, University of Toronto, 610 University Ave, Toronto, ON, M5G 2M9, Canada

Correspondence Address: Dr. Kartik Jhaveri, Joint Department of Medical Imaging, University Health Network and Mount Sinai Hospital, University of Toronto, 610 University Ave, Toronto, ON, M5G 2M9, Canada

kartik.jhaveri@uhn.on.ca

Date of Submission: 16-Dec-2008

Date of Decision: 30-Aug-2009

Date of Acceptance: 17-Feb-2010

Code Number: jp10036

PMID: 20622393

DOI: 10.4103/0022-3859.65291

Abstract

Keywords: Computed tomography, magnetic resonance imaging, ultrasound

This review discusses the current and emerging techniques in urinary tract imaging. Recent technical advances and novel discoveries combine to make this an exciting but challenging time for urinary tract imaging. The review is divided into four sections: 1) adrenal imaging, 2) renal, urinary tract and bladder imaging, 3) prostate imaging and 4) finally the recently encountered adverse reaction of nephrogenic systemic fibrosis (NSF).

Imaging of the Adrenals

Medical imaging plays a critical role in the characterization of adrenal lesions. This section outlines the role of imaging of the adrenal gland, which has made great strides in the last decade. The current major adrenal imaging modalities as well as new applications are discussed with particular attention to incidentally detected adrenal lesion.

Incidental adrenal lesions are detected in approximately 0.2% of computed tomography (CT) scans performed on patients aged 20 to 29 years and these increase to 7-10% of scans in older patients. ^[1] The role of imaging these lesions is to distinguish benign from malignant lesions. The main modalities used are CT, magnetic resonance imaging (MRI) and positron emission tomography-CT (PET-CT).

CT

Up to 70% of adrenal adenomas contain intracellular fat with remaining 30% termed lipid-poor adenomas. Detection of this fat on cross-sectional imaging (CT and MR) is used in characterizing the incidentally detected adrenal lesion as a benign adenoma. ^[1],[2] Thin-section non-enhanced multidetector computed tomography (MDCT) can be performed to assess an adrenal lesion. Subsequent CT attenuation/density values of the lesion can be calculated and previous meta-analysis has shown that lesions with average CT attenuation values < 10 hounsfield units (HU) can be accurately characterized as an adenoma with specificity of 98% and sensitivity of 78%. ^[1],[3] However, lesions above 10 HU on an unenhanced CT are considered indeterminate and other tests are required to further characterize them. Another CT technique to assess adrenal lesions is dynamic CT washout studies. The basis of this being adenomas tended to wash out or de-enhance faster than non-adenomatous or malignant lesions. ^[4] This is calculated using the percentage of the dynamic enhancement that is washed out at delayed scanning and can be absolute or relative depending on whether non-enhanced or contrast-enhanced CT of the adrenals are available for calculation. ^[1],[4] An adrenal lesion that demonstrates greater than 40% relative or 60% absolute washout on delayed imaging indicates a benign lesion and correspondingly malignant lesions show less than 40% relative or 60% absolute washout with high sensitivity and specificity. ^[5],[6] CT histogram analysis can also be used in the diagnosis of lipid-poor adenoma and can reduce the need to perform adrenal washout CT. ^[7]

MRI

The majority (>70%) of malignant lesions detected on CT or MRI are > 4 cm so this can also be useful when a large lesion is detected incidentally. ^[8] The most important component of the adrenal MRI protocol is chemical shift imaging which also characterizes adenomas by detection of intracellular fat by exploiting the different resonant frequencies of fat and water protons. ^[1] Chemical shift imaging is performed with in-phase and out-of-phase spoiled gradient-recalled-echo (GRE) sequences, which, with some imaging units, can be combined into a single sequence [Figure - 1]. ^[9] CT washout is probably the examination of choice in patients with adrenal lesion with average attenuation > 10 but chemical shift MRI can also be useful in this group and has a high sensitivity for hyperattenuating adrenal adenoma with attenuation values of 10-30 HU. ^[10]

PET

PET-CT combines the complementary modalities CT and PET using 18F-fluorodeoxyglucose (FDG), as lesion morphology on CT can be fused with the metabolic (functional) activity from PET, allowing for accurate anatomic localization of any FDG abnormalities. PET-CT allows the detection and characterization of a variety of adrenal conditions including benign lesions (adenoma, myelolipoma), malignant lesions [Figure - 2] (metastatic disease, lymphoma, collision tumors and adrenalocortical carcinoma), functional lesions (pheochromocytoma and Cushing′s syndrome) and benign mimics of neoplasia (brown fat, adrenal hemorrhage). ^[1],[11] Adrenal masses that are not FDG avid are likely to be benign with a high negative predictive value especially in cancer patients undergoing therapy, however, there is a small but statistically significant false-negative rate. Unfortunately, a considerable proportion of benign nodules also have increased FDG activity. ^[12] PET imaging is unreliable for lesions less than 1 cm in size, as metastatic lesions of this size may not demonstrate increased FDG uptake. ^[13]

Imaging of Kidneys, Ureter and Bladder

The renal tract is mainly investigated for abnormal renal function and to identify causes of renal colic and hematuria. ^[14] The increasing use of ultrasound (US), MDCT, MR and MR urography (MRU) has led to a limited role for plain film studies and intravenous urography (IVU).

Ultrasound

The universal availability and absence of exposure to radiation have promoted widespread use of US in the assessment and follow-up of renal disease with urgent US examination indicated in the assessment of new-onset renal failure to exclude urinary obstruction. Ultrasound is also valuable in detection of renal size, depth of renal cortex, renal calculi, renal masses and the assessment of renal parenchyma and vasculature. ^[14] Ultrasound is very useful in determining the internal architecture of renal lesions detected on other modalities such as hyperattenuating lesions detected on MDCT, to determine whether they represent hyperdense cysts or solid lesions.

Ultrasound is also very valuable in the evaluation of the post-renal-transplant patient and has the advantage of being able to safely image the structure of the graft with grey scale imaging and its perfusion with Doppler imaging without need for intravenous contrast or ionizing radiation. Ultrasound can be used to assess for immediate (first week), early (first month) and late (>one month) complications post transplant. ^[15] Ultrasound, and specifically color flow Doppler when utilized in a monitoring capacity is very helpful to the transplant physician in detecting graft dysfunction and in the delineation of peri-transplant collections, some of which can be drained under ultrasound guidance. Ultrasound is also extremely helpful in the diagnosis of chronic vascular complications including transplant artery stenosis and arteriovenous fistula, although its use in the diagnosis of chronic rejection is limited. ^[15]

MDCT and MDCT urography

Unenhanced CT is accepted as the primary imaging investigation to detect urinary tract calculi with sensitivity and specificity of 97% and 96% and this compares with much poorer sensitivities of 60% for plain radiography and 48% for IVU [Figure - 3]. ^[16]

The advent of MDCT has enabled evaluation of the kidneys, urinary tract and bladder during a single breath-hold, with a concomitant reduction in respiratory misregistration and partial-volume effect. In addition, the acquisition of multiple, thin, overlapping slices of an optimally distended and opacified urinary tract potentially provides excellent two-dimensional (2-D) and three-dimensional (3-D) reformations of the urinary tract and bladder. ^[17] The concept of multidetector CT urography (MDCTU) has emerged from these technical improvements. MDCTU may be defined as MDCT examination of the urinary tract in the excretory phase following intravenous contrast administration. ^[18]

One of the main advantages of dynamic or multiphase MDCT in the evaluation of causes of renal disease is its ability to evaluate the entire urinary tract using a single imaging test, renal calculi/masses on non-contrast CT [Figure - 3], renal parenchyma and masses at nephrographic phase and pelvicaliceal systems, ureters and the bladder at delayed excretory phase [Figure - 4]. ^[16] Alternative imaging studies, i.e. ultrasonography, IVU and nuclear medicine alone do not offer equivalent coverage. ^[19]

Dedicated renal mass protocol CT comprises two or three-phase CT scans (unenhanced, arterial and delayed phase) to assess for definitive enhancement or de-enhancement when arterial phase images are compared with unenhanced and delayed phase images (5 min). Density measurements in HU, can be made before and after contrast, and an assessment can be made for unequivocal enhancement, which is indicative of solid mass and neoplasm. ^[16]

MDCT is also valuable in assessing renal vasculature with new multiplanar and 3-D volume-rendered techniques and particularly valuable in preoperative planning in patients undergoing living related kidney donation [Figure - 5].

MR kidneys

Ultrasound and CT are the modalities of first choice in renal imaging with MRI mainly used as a problem-solving technique. It is however a useful tool in case of compromised renal function, severe contrast allergy, or in case radiation exposure is a problem, such as in children and pregnant women. MRI has the advantage of superior soft-tissue contrast, which provides a powerful tool in the detection and characterization of renal lesions. ^[20] Once a lesion is shown to enhance on MRI, renal cell carcinoma is the primary consideration, and staging of the tumor can be performed with the images acquired during the MR examination. ^[21] MRI outperforms CT when evaluating tumor invasion into the renal vein or IVC, with negative predictive values of nearly 100% for the absence of tumor involvement. ^[22]

MRU

Magnetic resonance urography (MRU) like MDCTU allows all the anatomic components of the urinary tract to be thoroughly imaged in a single test. ^[23] MRU, using either heavily T2-weighted pulse sequences or gadolinium-enhanced T1-weighted sequences is clinically useful in the evaluation of suspected urinary tract obstruction, hematuria and congenital anomalies, as well as surgically altered anatomy. It is particularly beneficial in pediatric or pregnant patients or when ionizing radiation is to be avoided. ^[24],[25] The main disadvantages of MR urography, which have hindered its widespread usage in the evaluation of the urinary tract, is its limited ability to reliably detect urinary tract calcifications, calculi and air; limited availability in comparison to MDCTU; and limited experience in interpretation of images. ^[16],[23]

Imaging of Prostate

This section reviews the role of imaging in the diagnosis and management of prostate cancer. Transrectal ultrasonography (TRUS), which can be used to guide biopsy, is the most commonly used imaging modality in evaluation of a patient with prostate cancer. For staging of disease, CT and MRI are the most frequently used modalities with bone scintigraphy and PET having roles in advanced disease. Currently, the imaging modalities, especially MR are evolving to improve disease detection and staging.

TRUS

TRUS provides good-quality images of the prostate gland because a high-frequency (5- to 7.5-MHz) probe can be placed in the rectum close to the prostate. Initially TRUS of prostate was performed to evaluate for prostatic disease including prostate cancer, benign prostatic hyperplasia (BPH), prostatitis, prostatic abscess, and prostatic calculi.

Since the introduction of the prostate-specific antigen (PSA) screening test and early detection of prostate cancer, the role of TRUS has changed; it is now mainly used to visualize the prostate and to aid in ultrasound-guided needle biopsy. It is the most frequently used modality to evaluate the prostate in patients with prostate cancer. ^[26] Prostate cancer is most often seen as a hypoechoic or rarely hyperechoic area within the peripheral zone but up to 40% are isoechoic, thus limiting their detection with TRUS. ^[26] The finding of a hypoechoic area within the peripheral zone is not specific for prostate carcinoma and can also be seen in benign processes. Therefore, TRUS has a limited role in the detection of prostate cancer due to its poor sensitivity and specificity. ^[26],[27]

Its role in local staging is also limited by its limited soft tissue resolution; the addition of Doppler imaging, showing vascular changes in tissues, can be used to improve the detection of prostate cancer on TRUS but accuracy remains limited. ^[27] Contrast-enhanced TRUS is a new technique that is under investigation for the assessment of prostate cancer.

Volume assessment of the prostate is an important and integral part of TRUS and is helpful in planning treatment with brachtherapy, cryotherapy, or minimally invasive BPH therapy (e.g. radiofrequency, microwave.

CT

CT has traditionally been used to evaluate the extent of local disease but is of little value in detection or local staging because even with contrast enhancement, CT lacks the soft tissue resolution necessary for a)visualization of the margins of the prostate, b) depiction of intraprostatic anatomy and (c) detection of prostate cancer.

The major role of CT is in the advanced disease and nodal staging of prostate cancer, for which it is also limited. Today, CT remains the method of choice for staging distant disease, i.e., metastases to lymph nodes that lie outside the confines of the true pelvis; typically, with short-axis measurement > 1.0 cm. CT has also been used to monitor bone metastases, but bone scans and MRI have been reported to be superior to CT in the diagnosis of bone metastases. ^[28]

MRI

As with all radiological modalities, MRI continues to change rapidly with resultant improved evaluation of the prostate. ^[29] At present, MRI with use of flexible endorectal coil within an inflatable balloon along with the conventional pelvic phased array coils is used to evaluate prostatic disease. ^[27]

T2-weighted fast spin-echo imaging is the optimum sequence for depicting the anatomy of the prostate because the prostate has uniform intermediate signal intensity at T1-weighted imaging with the zonal anatomy not clearly identified on T1-weighted images. On T2-weighted images, the peripheral zone has high signal intensity and is composed of mainly glandular tissue, in contrast to the low signal intensity of the central and transitional zones, which consist of compactly arranged smooth muscle and loose or little glandular tissue. ^[30]

MRI is widely used in pre-treatment staging of prostate cancer with T2-weighted imaging being the mainstay, but its accuracy for the detection and localization of prostate cancer can be unsatisfactory. On T2-weighted imaging, prostate cancer shows as a low-signal lesion within the peripheral zone of the prostate [Figure - 6]. To improve the utility of MRI for detection and localization, various other techniques may be used.

Dynamic contrast-enhanced MRI helps in allowing differentiation of cancer from normal prostate tissue. The advantage of this technique includes the direct depiction of tumor vascularity, but has the disadvantage of limited visibility of cancer in the transitional zone of the prostate. ^[30] Diffusion-weighted imaging demonstrates restricted diffusion in cancerous tissue and allows short acquisition time with high contrast resolution between cancer and normal tissue, but the individual variability in apparent diffusion coefficient values may erode diagnostic performance [Figure - 6]. ^[30] MR spectroscopy, depicts a higher ratio of choline and creatine to citrate in cancerous tissue than in normal tissue and is generally accepted to improve accuracy of prostate cancer detection and localization [Figure - 6]e. ^[27]

Bone Scintigraphy and PET

The role of PET is still under investigation in the staging of patients who have prostate cancer. Fluorodeoxyglucose (FDG), the most commonly used PET tracer, was reported to be ineffective for the initial staging of prostate cancer because most primary prostate cancer lesions were not FDG avid and as a result there has been work on developing specific PET tracers for prostatic cancer. FDG-PET, however, could have a role in the detection of local recurrence or distant metastases in patients with continuingly increasing PSA after initial therapy. ^[29]

Nephrogenic Systemic Fibrosis

This final section discusses the recently encountered delayed adverse reaction of Nephrogenic Systemic Fibrosis (NSF) in patients who received gadolinium-based contrast agents.

For many years gadolinium-based contrast-enhanced MRI was believed to be safe and was the preferred contrast instead of iodine-based contrast material in patients with renal impairment. Since early 2006 evidence has been growing that some gadolinium-based contrast agents may potentially cause the fibrosing scleroderma-like condition NSF in patients with renal failure. ^[31]

Patients at highest risk of NSF are 1) patients who have severe acute or chronic renal impairment with glomerular filtration rate (GFR)< 30 mL/min/1.73m2), 2) patients on dialysis (hemo or peritoneal) and 3) patients with reduced renal function awaiting liver transplantation. Patients with GFR between 30 and 60 mL/min/1.73m2 are at lower risk and there in no reported case of NSF in patients with GFR > 60. ^[32]

From the data available in the literature it is apparent that the prevalence of NSF is significantly higher after the unstable linear gadolinium agents are used than after any other gadolinium-based agent (3-7% versus 0-1% per injection) in patients with reduced renal function. ^[33]

The recognition of this adverse reaction to gadolinium-based agents in renal-impaired patients emphasizes the need for appropriate clinical indication for gadolinium-enhanced MR in these patients. If the examination is indicated, the use of the lowest dose of gadolinium-based contrast possible and the agent that leaves the smallest amount of gadolinium in the body post administration is recommended.

Conclusion

This review on genitourinary tract imaging shows the current and emerging state-of-the-art imaging modalities for evaluation of the genitourinary system. ^[34]

References

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