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The urinary system is very amenable to imaging with ultrasound. The liver and spleen provide windows through which the kidneys can be visualized. The bladder is located directly behind the pubic symphysis and is readily seen from the suprapubic approach, particularly when full. Ultrasound of the kidneys and bladder can play a vital role in the effective management of patients in the emergent and critical care setting. Focused renal and urinary ultrasound can determine the presence of hydronephrosis, directly visualize stones, and measure bladder volume. Ultrasound easily identifies renal cysts, and may identify masses or clots in the kidneys or bladder. Ultrasound may guide urinary procedures such as ensuring correct Foley catheter placement, and suprapubic bladder aspiration.
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Focused renal ultrasound should be incorporated into the examination of any patient with undifferentiated flank or abdominal pain. Ureteral stones can be difficult to visualize because they are retroperitoneal, but the presence of hydronephrosis on the side of the pain is very specific for ureterolithiasis as the cause of the patient's symptoms. Patients with abdominal, back, or flank pain may also have more serious diagnoses such as abdominal aortic aneurysm or dissection, cholecystitis, ovarian torsion, ruptured ectopic pregnancy, and others. Bedside ultrasound may be helpful in diagnosing other causes of pain although other imaging modalities such as consultant-performed ultrasound or CT may be required if the diagnosis remains uncertain.
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Bedside ultrasound evaluation of the urinary system should be performed in patients presenting with the following:
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- Acute renal failure
- Abdominal pain or flank pain suspicious for renal colic
- Back, abdominal, or pelvic pain of uncertain cause
- Urinary retention
- Palpable abdominal or flank mass
- Renal trauma
- Gross hematuria
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Curvilinear Probe with a Frequency of 3.0–5.0 MHz
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When imaging the kidneys and bladder, the curvilinear probe with a frequency of 3.0–5.0 MHz should be utilized. Alternatively, a phased-array probe with a frequency of 2.0–4.0 MHz can be used to visualize the kidneys through the intercostal spaces. Lower frequencies, which improve penetration, can provide better visualization in obese patients.
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Tissue Harmonic Imaging
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Another ultrasound machine setting that can be of particular use in renal imaging is tissue harmonics. Renal stones, which are often made up of calcium, block the penetration of the ultrasound beam and produce an acoustic shadow. Tissue harmonics enhance acoustic shadowing and may improve the identification of nephrolithiasis.
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The sonographer should start with adequate depth in order to not miss abnormalities in the far field. This is important for findings such as free fluid around the bladder, which is easily missed if subtle. Once the far field is fully interrogated, the depth can be decreased in order to make sure the kidneys or bladder are taking up most of the screen.
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Gain and Time-Gain Compensation
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The overall gain can be ...