"Kidney" by Pöllö - Own work. Licensed under GFDL via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Kidney.JPG#mediaviewer/File:Kidney.JPG

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

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