Diagnostic Ultrasound - Abdomen and Pelvis

Kidneys
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Anatomy: Abdomen
GROSS ANATOMY
Overview
• Kidneys are paired, bean-shaped, retroperitoneal organs
○ Function: Remove excess water, salts, and wastes of
protein metabolism from blood
Anatomic Relationships
• Lie in retroperitoneum, within perirenal space, surrounded
by renal fascia (of Gerota)
• Each adult kidney is ~ 9-14 cm in length, 5 cm in width
• Both kidneys lie on quadratus lumborum muscles, lateral to
psoas muscles
Internal Structures
• Kidneys can be considered hollow with renal sinus occupied
by fat, renal pelvis, calyces, vessels, and nerves
• Renal hilum: Where artery enters, vein and ureter leave
renal sinus
• Renal pelvis: Funnel-shaped expansion of upper end of
ureter
○ Receives major calyces (infundibula) (2 or 3), each of
which receives minor calyces (2-4)
• Renal papilla: Pointed apex of renal pyramid of collecting
tubules that excrete urine
○ Each papilla indents a minor calyx
• Renal cortex: Outer part, contains renal corpuscles
(glomeruli, vessels), proximal portions of collecting tubules
and loop of Henle
• Renal medulla: Inner part, contains renal pyramids, distal
parts of collecting tubules, and loops of Henle
• Vessels, nerves, and lymphatics
○ Artery
– Usually 1 for each kidney
– Arise from aorta at about L1-L2 vertebral level
○ Vein
– Usually 1 for each kidney
– Lies in front of renal artery and renal pelvis
○ Nerves
– Autonomic from renal and aorticorenal ganglia and
plexus
○ Lymphatics
– To lumbar (aortic and caval) nodes
IMAGING ANATOMY
Overview
• Well-defined retroperitoneal bean-shaped structures,
which move with respiration
Internal Contents
• Renal capsule
○ Normal kidneys are well-defined due to presence of
renal capsule and are less reflective than surrounding fat
• Renal cortex
○ Renal cortex has reflectivity that is less than adjacent
liver or spleen
○ If renal cortex brighter than normal liver (echogenic),
then high suspicion of renal parenchymal disease
• Medullary pyramids
○ Medullary pyramids are less reflective than renal cortex
• Corticomedullary differentiation
○ Margin between cortex and pyramids is usually welldefined
in normal kidneys
○ Margin between cortex and pyramids may be lost in
presence of generalized parenchymal inflammation or
edema
• Renal sinus
○ Echogenic due to fat that surrounds blood vessels and
collecting systems
○ Outline of renal sinus is variable, from smooth to
irregular
○ Renal sinus fat may increase in obesity, steroid use, and
sinus lipomatosis
○ Renal sinus fat may decrease in cachectic patients and
neonates
○ If sinus echoes are indistinct in noncachectic patient,
tumor infiltration or edema should be considered
• Collecting system (renal pelvis and calyces)
○ Not usually visible in dehydrated patient
○ May be seen as physiological "splitting" of renal sinus
echoes in patients with a full bladder undergoing diuresis
○ Physiological "splitting" of renal sinus echoes is common
in pregnancy
– Cause of dilatation of pelvicalyceal system may be due
to mechanical obstruction by enlarging uterus,
hormonal factors, increased blood flow, and
parenchymal hypertrophy
– May occur as early as 12 weeks into pregnancy
– Seen in up to 75% of right kidneys at 20 weeks into
pregnancy, less common on left side, thought to be
due to cushioning of ureter from gravid uterus by
sigmoid colon
– Obvious dilatation of pelvicalyceal system can be seen
in 2/3 of patients at 36 weeks
– Changes usually resolve within 48 hours after delivery
○ Possible obstruction can be excluded by performing post
micturition images of collecting system
– AP diameter of renal pelvis in adults should be < 10
mm
• Renal arteries
○ Normal caliber 5-8 mm
○ 2/3 of kidneys are supplied by single renal artery arising
from aorta
○ 1/3 of kidneys are supplied by 2 or more renal arteries
arising from aorta
– Main renal artery may be duplicated
– Accessory renal arteries may arise from aorta superior
or inferior to main renal artery
– Accessory renal arteries may enter kidney either in
hilum or at poles
– Extrahilar accessory renal arteries may arise from
ipsilateral renal artery, ipsilateral iliac artery, aorta, or
retroperitoneal arteries
○ Spectral Doppler
– Open systolic window, rapid systolic upstroke
occasionally followed by secondary slower rise to peak
systole with subsequent diastolic delay but persistent
forward flow in diastole
– Continuous diastolic flow is present due to low
resistance in renal vascular bed
– Low resistance flow pattern is also present in
intrarenal branches