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How to Scan the Renal Tract
KIDNEYS
The kidneys are easy to scan because we're familiar with viewing them during and EFAST. I start by finding the kidneys in the RUQ and LUQ as I would for an EFAST: abdominal transducer, probe marker cranial, longitudinal in the lateral chest posterior to the midaxillary line. I then rotate and fan posteriorly until I get the kidneys in their longest and widest dimension in long axis.
In order to get a true longitudinal image of the kidneys, remember that the upper pole of the kidney is medial and the lower pole sits lateral. Thus when rotating the transducer, the probe marker end should point towards the nipple slightly.
If you're having difficulty, sweep the transducer posteriorly and roll the patient to a lateral decubitus position.
The normal kidney in long axis has a bean shape with recognisable thick cortex (approx 7-10mm). Renal cortical thickness decreases with age and renal impairment and has been shown to correlate with eGFR (1). The pelvis is normally hyperechoic due to pelvic fat. The ureter is usually not visualised. The hyperechoic line around the kidneys is Gerota's fascia. The normal kidney is 10-12cm (2).
The normal kidney is 10-12cm with a 7-10mm renal cortex
Anechoic structures within the pelvis are usually blood vessels and are better visualised with colour doppler. Always assess with colour doppler if you suspect hydronphrosis because what looks like a dilated collecting system could just be a vessel.
Now rotate the transducer 90 degrees and evaluate the kidneys from superior to inferior. Renal cysts >3cm, with septations or internal echogenicity should be referred for a formal US/CT to evaluate for malignancy (3).
URETERS
The ureters are only visualised if they are dilated. To image the renal pelvis and PUJ have the transducer longitudinal, probe marker cranial at the posterior axillar line. Fan anteriorly. Now slide the transducer caudally keeping the dilated ureter in view. It runs anterior to the psoas muscle. Bowel gas may obscure the middle third of the ureter.
BLADDER
Imaging the bladder requires peering into the pelvis like you're looking over the rim of a bucket. Begin with the transducer transverse, in the midline just above the symphysis pubis with the probe marker to the patient's right. Fan into the pelvis.
Fanning anteriorly will visualise the trigone of the bladder. This is where you will see the VUJ and any associated stones. At the trigone, you may see evidence of prostatic hyperotrophy as a hypoechoic non mobile mass growing into the bladder.
Bladder trigone in transverse, Note the central hypoechoic mass within the anechoic bladder cavity: this is the enlarged prostate.
BLADDER VOLUME
Fanning posteriorly, freeze the image of the bladder in its largest transverse dimension and measure the vertical and transverse diameters. Now rotate the transducer 90 degrees and measure the depth of the bladder. Most machines provide an automatic volume calculation from these three measurements.
Bladder vol (ml) = HT x W x D
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REFERENCES
1, Beland MD, Walle NL, Machan JT, Cronan JJ. Renal cortical thickness measured at ultrasound: is it better than renal length as an indicator of renal function in chronic kidney disease? AJR Am J Roentgenol. 2010 Aug;195(2):W146-9
2. Hansen KL, Nielsen MB, Ewertsen C. Ultrasonography of the Kidney: A Pictorial Review. Diagnostics (Basel). 2015 Dec 23;6(1):2.
3. Israel GM, Bosniak MA. An update of the Bosniak renal cyst classification system. Urology. 2005 Sep;66(3):484-8.