My DDU point form notes on renal ultrasound…
The rest of my DDU notes are here.
Indications
oliguria / anuria
UTI
trauma
haematuria
post transplant (early / late)
Aim
rule out issue needing urgent intervention
Anatomy
kidney
bean shaped
echogenic capsule = Gerota’s fascia + perinephric fat
convex laterally, convex medially at hilum
9-12 cm long, 4-5 cm wide by ultrasound
regions
parenchyma
normally 1.0-1.8 cm thick from surface to hyperechoic sinus
outer “echogenic” cortex
isoechoic / hypoechoic to liver, hypoechoic to spleen
cf hyperechoic means diffuse parenchymal pathology
— well demarcated in children and young from —
inner hypoechoic medulla with renal pyramids
sinus
hyperechoic, fatty
renal pelvis, calyces (both usually collapsed)
arteries, veins, lymphatics
ureters
normal 8mm wide, not well seen
if seen, consider hydroureter
bladder
if empty → lies behind symphysis pubis, thick irregular walls
if full
teardrop shaped, anechoic in longitudinal
rectangular in transverse
thin (<4-5 mm) smooth walls
Imaging
equipment
sector 3-5 MHz – or 5-7 MHz to evaluate children, thin patients, transplanted kidneys
assess
transverse + longitudinal views – must assess cortex and renal pelvis
also look perirenally for abnormalities, assess bladder (only possible if distended)
positioning
R kidney
supine or L lateral decubitus, through liver
R lateral subcostal margin in anterior axillary line
optimise until kidney is longest
transverse: upper, mid and lower sections
L kidney
higher, and spleen smaller than liver ∴ harder to see
R lateral decubitus ideal, with probe in posterior axillary line or left costovertebral angle
bladder
supine
probe 1 cm above symphysis
usually transverse first
symmetric, smooth, no inner surface irregularities
longitudinal
oriented toward umbilicus
Colour and spectral Doppler
classically low vascular resistance “ski slope” appearance
resistive index
measured in arcuate corticomedullary junction / interlobular border of medullary pyramids arteries
(systolic peak velocity – diastolic peak velocity) / systolic peak velocity
normal ~0.6, helps suggest lack of obstruction
> 0.70 → consider sepsis, prerenal ARF, (possibly) acute rejection, ?obstruction without dilatation
ureteral jets
in transverse bladder scan, colour from lateral posterior border coursing superiorly and medially
1-12 jets per minute
can’t assess in collapsed bladder!
Limitations
supine, lack of cooperation, monitoring devices, dressings, bowel gas, oedema, ascites, postsurgical changes
kidneys move 2-3 cm with respiration on a ventilator
Applications
renal failure
kidney in normal position?
size, echogenicity, RI
normal / enlarged kidneys with ↑ echoes, ? ↑ RI → ARF
small (5-8 cm) kidneys with ↑ echoes → CRF
obstruction
risk factors
urolithiasis, retroperitoneal cancer, solitary kidney
collecting system – hydronephrosis is key
renal pelvis, calyces dilated, posterior enhancement
categories
mild (grade I): splaying
moderate (grade II): rounding of calyces (like a 3 on its side), obliteration of papillae
severe (grade III): massive dilatation with cortical thinning
often more dilated + rounded with thin cortex if chronic
cf acute less dilated, concave – so acute severe obstruction, particularly in critically ill with renal failure, may have only minimal hydronephrosis
causes of hydronephrosis
obstruction, infection, persistent diuresis, reflux
other helpful features
RI > 0.7 controversial
ureteric jets particularly if unilateral loss cf bilateral could be same specific gravity in bladder and ureter
mimics of hydronephrosis
cysts spherical, anechoic, well defined walls, no wall thickness, posterior acoustic enhancement especially parapelvic cysts, extrarenal pelvis calyces not usually dilated, polycystic renal disease
stones may be seen, easier if >5 mm Ø
ureter
if seen in flank view, nearly always pathological → obstruction
ureteral jets suggest no obstruction
bladder
distension → d/dx IDC blocked, bladder haematoma
useful in monitoring of renal recovery to avoid in-out catheters
kidney transplant
use 5-7 MHz transducer
again, size, shape, echogenicity, hydronephrosis, colour / spectral Doppler, and bladder scan
bladder usually kept empty
goals
obstruction (vs rejection / ATN)
nil specific for rejection or ATN – kidney may be enlarged with ↑ cortex echogenicity / distorted outline
obstruction may be due to
anastomotic failure
collection
urinoma / seroma (lymph) / haematoma
anechoic (haematoma may by echogenic)
fluid collections (see above)
Doppler assessment
↑ RI, without obstruction or infection, suggests rejection
trauma
USS has poor sensitivity, limited capacity to grade renal trauma (contrast CT better)
subcapsular / perinephric fluid collections may be blood, urine, other free fluid – but USS useful for monitoring haematomas with conservative Mx
Doppler useful for perfusion
Urosepsis
common in ICU
due to IDC / immunocompromise / comorbidities
pyelonephritis
no specific sonographic features, can be focal and mass like
useful for complications
hydronephrosis
pyonephrosis
suspect in UTI + hydronephrosis with low level echoes / layering in dependent portion of collecting system
abscesses
intraparenchymal
complex hypoechoic mass ± debris level, irregular thick walls
perinephric
heterogeneous crescent shaped fluid collection surrounding kidney
emphysematous pyelonephritis
female diabetics mainly
see high amplitude echoes in parenchyma and sinus with posterior acoustic shadowing → need CT to confirm it’s air
Renal mass
can tell mass vs simple cyst
mass needs CT, urology r/v (malignant until proven otherwise)
Nephrolithiasis
intensely hyperechoic, linear / arching, posterior acoustic shadowing
occasionally can see obstructing stones
Bladder ultrasound
IDC → usually not possible to image bladder
but if unexpectedly not collapsed → IDC may be malfunctioning
Foley catheter can be pulled into prostate
stones
hyperechoic, posterior shadowing, gravitational
clots
polypoid hyperechoic projection from bladder wall
d/dx bladder tumor
diverticuli
sonolucent masses adjacent to bladder
prostate
round or polypoid protrusion into bladder at base
Renal parenchyma
length
9-12 cm normal
↓ in CRF, potential ↑ in ARF
thickness
normal 1.5-1.8 cm
↓ in CRF
margins
should be sharp and regular
V-indentation “persistent foetal lobulation”
renal lobulation / irregularity / rounded angles → chronic inflammation, ischaemia
echogenicity
↑ with age, pathology
parenchyma (hypoechoic) vs renal sinus (hyperechoic) clearer in young
perfusion status
Doppler
transverse scanning plane
resistive index
measured in arcuate or interlobular arteries
(systolic peak velocity – diastolic peak velocity) / systolic peak velocity
> 0.70 in sepsis, prerenal ARF, acute rejection possible
can assess for arterial / venous thrombosis post transplantation
Cover image: “Kidney” by Pöllö – Own work. Licensed under GFDL via Wikimedia Commons
Tags: DDU DDU Notes Renal
