Genitourinary MRI

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Genitourinary MRI Powered By Docstoc
					MRI of the Adrenals, Kidneys,
Bladder, and Prostate

                                                                                 James P. Earls, M.D.
                                                             Director of Cardiovascular and Body MRI
                                                            INOVA Fairfax Hospital, Falls Church, VA
                                                                      Assistant Professor of Radiology
                                                     Johns Hopkins Medical Institutions, Baltimore MD

Outline                                                signal adds, when out of phase (180 degrees
• MR techniques                                        opposite each other) their signal destructively
• Contrast agents                                      interferes. For this reason fat-water mixtures lose
• Clinical applications                                signal when out of phase. This signal loss occurs
  - Adrenal, Kidney, Bladder, Prostate
                                                       with adrenal adenomas, myelolipomas, renal
                                                       angiomyolipomas, and adnexal lesions.
MR Techniques
          A phased-array torso coil is optimally
used to perform GU MRI because it significantly
increases signal and reduces noise in the abdomen.
A combination of routine T1 and T2 weighted
sequences are used for most GU purposes. In
selected cases newer sequences, including ultra-
fast T2 (HASTE, SSFSE) are employed.
Opposed-phase imaging techniques are commonly
used for adrenal lesion characterization. 3D
gadolinium-enhanced MR angiography is useful
for angiographic evaluation of the GU system,
including renal donor and recipients, renal cell
carcinoma evaluation, and other selected cases.
          Ultra-fast T2 sequences are heavily T2
weighted and fluid is bright. These provide for
very rapid “MR urograms” that can be performed
during suspended respiration. These can be used
for rapid screening of the bladder, ureters, and
other fluid filled structures.
          Opposed-phase imaging is a useful tool
for characterization of certain fat-water
admixtures, such as that which occurs in adrenal
adenomas. Fat and water protons spin at slightly
different frequencies. At predictable time points,     Fig 1. An obstructed right ureter is depicted on this coronal
they will be completely “in-phase” or “out of          ultrafast T2 weighted image (SSFSE single shot fast spin echo)
                                                       acquired in less than 1 second.
phase” with each other. When in-phase, their

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         Another technique that is useful as                        Gadolinium is safe to use in these
indicated above is 3D Gad MR Angiography. This            patients. In one study of 342 patients with renal
provides for a very rapid and accurate                    insufficiency who received both nonionic contrast
angiographic analysis of the arteries and veins of        exam and gadolinium (on separate days), a
the abdomen and pelvis. Most scanners can                 significant rise in serum creatinine was noted after
perform this sequence in approximately 20-30              iodinated contrast, but not after gadolinium.
second. Please refer to the Gadolinium-enhanced           (Prince et al, JMRI 1996; 6:162-166). In this study
MR angiography chapter for further information.           nephrotoxicity = 5mg/dl serum creatinine increase.

                                                          Gad vs. Nonionic contrast; Nephrotoxicity
MR Contrast Agents
         Several contrast agents are currently                                  Creatinine          Nephrotoxicity
available for MRI. As a whole these agents are                                    change
significantly safer than iodinated agents used for         Nonionic            + 0.35 mg/dl                17%
CT and other radiographic exams. Gadolinium is
used most frequently for both evaluations of               Gadolinium          - 0.07 mg/dl                0%
tumor, infection, and angiography. The other two
new agents, ferumoxides and Mn-DpDp, are                                   (Prince et al, JMRI 1996; 6:162-166)
usually used for hepatic imaging.
AGENT             MECHANISM         USE
                                                                    MR evaluation of the adrenal glands is
                                                          rapid and accurate. MR is more specific than
Gadolinium        Extracellular     General,
                                                          spiral CT for characterization of adrenal
                  fluid             3D MRA
                                                          adenomas. Adrenal MR studies are performed
                                                          rapidly and are very specific.
Ferumoxides       RES specific      Liver lesion
                                                                    Use of opposed-phase technique with a
                                    detection, FNH
                                                          single in-phase followed by an out of phase series
                                                          takes less than 5 minutes of “table time” and can
Mn-DpDp           Hepatocyte        HCC, liver
                                                          characterize adenomas with a greater than 95%
                  specific          lesions,
                                                          specificity and sensitivity. Using opposed-phase
                                                          techniques, adenomas lose signal intensity by 10%
                                                          or more between in and out of phase images. All
          Gadolinium can be used safely in patients       imaging parameters, except for the TE, must be
with renal insufficiency. It has no reported renal        identical (i.e. the same TR, fov, slice thickness,
toxicity. Both ionic and nonionic iodinated               gap, bandwidth, flip angle etc).
contrast media have a real risk of producing
          In one study (Schwab et al, NEJM 1989;
302:149-153), nephrotoxicity, defined as a 44
mumol/l serum creatinine increase, was evaluated
in 443 patients s/p cardiac catheritization. They
were randomized to ionic or nonionic contrast.
Nephrotoxicity was seen in a substantial number
of patients who received both nonionic and ionic
iodinated contrast agents (see table).

Incidence of Iodinated Contrast Induced
              All Patients  High Risk

Nonionic       8.2%               15%
                                                                      Fig 2. An adrenal adenoma is depicted as a low
Ionic          10.2%              17%                     signal intensity mass on this out of phase T1 weighted GRE
                                                          image (FLASH TR 150, TE 2.5, and FA 80).
           (Schwab et al, NEJM 1989; 302:149-153)

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Opposed-phase Imaging Technique                          the histologic composition of masses and other
•   Use standard T1-weighted GRE images.                           The one disadvantage of MR as compared
                                                         to CT is the relative insensitivity of MR to small
•   Incremental TE change makes the sequence             calcified stones in the collecting system or ureters.
    in-phase or out-of-phase.                            Therefore in the evaluation of a potential renal
•   Fat and water have different resonance               donor, we obtain a plain radiograph to evaluate for
    frequencies.                                         nephrolithiasis.
•   When imaged “out of phase” fat and water                       Most commonly, renal MR is doe to
    signals interfere destructively.                     evaluate an indeterminate renal mass seen on
•   Used to depict areas of hepatic steatosis and        another examination. Patients may be renal
    adrenal adenomas.                                    insufficient or have iodinated dye allergies, and
                                                         therefore cannot have a CT. Occasionally, a mass
           Because the adrenal glands are thin           seen on renal-protocol CT is sent for MR
structures, and adrenal lesions may be quite small,      evaluation because of an indeterminate CT
thin sections are optimal when performing adrenal        reading. Protocol for these studies is pretty
MRI. We normally use 4-mm slices for imaging.            simple; we do a T2 FSE in one or occasionally 2
Fat saturation is both useful and can be a               planes (axial, or axial and coronal). This is
hindrance. The retroperitoneal fat provides for          followed by a pre and dynamic post-contrast t1
good internal contrast and makes the adrenals easy       GRE series performed as a breathhold. It is best to
to see, so suppression of the fat with fat saturation    use thin slice; 4-5 mm is optimal depending on the
may actually make it more difficult to visualize         length of time the patient is able to hold their
subtle abnormalities. However, if one wants to           breath. The pre-contrast images can be subtracted
prove that a lesion in the adrenals contains fat,        from the post-contrasted images to produce a
then it is best to perform a T1 GRE series with and      “subtraction” series. This is excellent for
without fat saturation, any bulk fat containing          determination of mass enhancement.
lesions, such as a myolipoma, will lose signal on                  MR is an excellent tool for evaluating the
the fat-saturated series.                                renal vascular system, especially the main renal
           T2-weighted images are useful when            arteries in case of suspected renovascular
evaluating other adrenal lesions, such as cysts,         hypertension and for evaluating the renal veins in
hemorrhage, and pheocromcytomas. Therefore as            cases of suspected thrombosis or tumor extension.
a standard adrenal protocol, we include T2-              Please see the chapter on renal and aortic MRI in
weighted images, even though they are not helpful        this syllabus.
for the most common indication for adrenal MRI;
characterization of adrenal adenomas                     Sample Renal MR Protocol
                                                         Sequence                            Plane
                                                         T2 FSE                              axial
Sample Adrenal MR Protocol                               T1 GRE -                            axial
                                                         Pre and Post Gadolinium
Sequence                      Plane
GRE T1                        axial                      Optional
In-phase & Out of phase                                  1. GRE T1 - Opposed phase           axial
Optional-                                                2. 3D MRA                           coronal
T2 FSE fat sat                axial
GRE T1 fat sat                axial

Kidneys                                                           Bladder imaging is performed less
          Renal MR can be thought of in a similar        frequently than adrenal or renal MR, but it can still
manner to renal CT, i.e., pre and post contrast          provide useful diagnostic information in a number
images are essential for evaluation of the               of diagnoses. In our practice, we use dynamic
enhancement of masses to differentiate cysts from        contrast enhanced bladder MRI for staging of
solid masses. In addition, T2 pulse sequences and        invasive bladder cancer. Cancers typically
fat saturation techniques can be used to evaluate        enhance earlier than normal bladder muscle, so an

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arterial phase study is helpful to fully delineate the                accuracy of MRI in the local staging of PCa. The
tumor.                                                                reported accuracy of MRI in staging prostate
          Other bladder applications include the                      cancer ranges from 54% to 90%. These results
evaluation of uretheral diverticulum in women.                        have raised concerns about interobserver
We perform T2 weighted sequences in multiple                          variability and lack of reproducibility. Over the
planes using thin slices with a high matrix to                        past three years, however, more encouraging
achieve high spatial resolution.                                      results have been obtained for endorectal MRI.
          Occasionally we image the bladder with                      Diagnostic performance has improved with
MR to evaluate for other tumors such as extra-                        reported accuracy consistently between 75% and
adrenal pheochromocytomas.                                            90%. Another strength of MRI that has been
                                                                      identified is its high specificity (> 90%) in
                                                                      excluding extraprostatic tumor.

                                                                      MR Imaging
                                                                                 The signal intensity and the detection of
                                                                      PCa depend on the type of imaging sequence used.
                                                                      On Tl-weighted images, the prostate is of
                                                                      homogeneous medium signal intensity. On T2-
                                                                      weighted MR images, prostatic carcinoma most
                                                                      commonly is shown with decreased signal
                                                                      intensity within the high-signal-intensity normal
                                                                      peripheral zone. The detection of PCa on MRI
                                                                      (similar to TRUS) is applicable only to the tumors
                                                                      located in the peripheral zone. Even in the
                                                                      peripheral zone, tumor detection may be hampered
                                                                      by post biopsy changes. Depending on the time
                                                                      interval between biopsy and MRI scan, the biopsy
                                                                      changes may cause either under- or over staging of
Fig 3. Gadolinium is safe to use in patients with renal               tumor presence and extent. It has been
insufficiency. Here it enhances a bladder transitional cell CA        demonstrated that MRI study should be performed
in a patient with a serum Cr of 3.0 and chronic renal
                                                                      at least three weeks after biopsy. While PCa
                                                                      detection rates as high as 92% have been reported,
Prostate                                                              the results of large multicenter studies are
                                                                      disappointingly low, with only 60% of lesions
                                                                      greater than 5 mm in any one dimension being
     The use of diagnostic imaging in staging
                                                                      detected on MRI scans.
evaluation of Prostate Cancer (PCa) is a subject of
controversy. The recommendations range from
denial to strong advocacy for imaging prior to any
therapy decision. Considering the disagreements
about PCa detection and choice of treatment, the
debate concerning imaging is not surprising.
There are evidence-based guidelines on the use of
imaging in assessing the risk of distant spread of
PCa. The radionuclide bone scan and computed
tomography (CT) supplement clinical and
biochemical evaluation (PSA, prostatic acid
phosphates) for suspected metastatic disease to
bones and lymph nodes. Guidelines for the use of
bone scans (in-patients with PSA > 10 ng/ml) and
CT (in patients with PSA > 20 ng/ml) have been
reported and are in clinical use.
     No such consensus exists at the current time
                                                                      Fig 4. A low-T2 signal intensity area of prostate cancer
for the use of imaging in evaluating PCa local                        (arrow) is depicted in the peripheral zone of this gland..
tumor extent, specifically on the use of MRI.
Variable results have been found for the diagnostic

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                                                                 page 4
          The role of MRI is in the evaluation of        depends on the equipment and local expertise
tumor extracapsular and seminal vesicle invasion.        available. None of the modalities are perfect, and a
MRI findings of extracapsular extension on               judicious combination provides the best results. In
endorectal coil MRI include a) irregular bulge of        the staging of PCa, each modality - transrectal US,
the prostate margin, b) contour deformity with           MRI and CT - has advantages and disadvantages.
step-off or angulated margin, c) breech of the           Evaluation by TRUS is restricted to local staging
capsule with direct tumor extension, d) obliteration     only, while both CT and MRI allow detection of
of rectoprostatic angle, and e) asymmetry of             local, nodal, and distant metastatic invasion. The
neurovascular bundles. The seminal vesicle               role of CT in staging PCa is reserved for the search
invasion is diagnosed when there is a)                   for lymph node metastasis, evaluation of advanced
demonstration of contiguous low-signal-intensity         disease, and planning radiation therapy. MRI
tumor extension into and around seminal vesicles,        offers the most complete evaluation of PCa
and/or b) tumor extension along the ejaculatory          assessing local, regional and nodal disease. The
duct resulting in nonvisualization of the                endorectal coil provides higher staging accuracy
ejaculatory duct, decreased signal intensity of          than the body coil. Discrepancies in the opinion on
seminal vesicles, and loss of seminal vesicle wall       the value of the endorectal coil attest to the
on T2-weighted images. While transaxial planes           immaturity and still developing field of MR
of section are essential in the evaluation of            imaging. The combination of MR Imaging and
extracapsular invasion, the invasion of the seminal      Spectroscopic imaging offers anatomic and
vesicles is facilitated by the evaluation of             metabolic information and appears to be the
transaxial and coronal plane of section.                 method of the future.
          In a recently reported study by Yu et al„
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