Pulse travels at constant speed. When it encounters a reflective surface, a portion of the sound reflects back toward the source. The farther the pulse travels, the longer it takes for the reflection to come back (deeper tissue). The more reflective a surface is, the more percentage of the pulse bounces back and the more intense the reflection back. Pulsed ultrasound: sends out wave, then listens. Listens probably 99% of the time. Pulsed means short pulse of wound, then listen. Then pulse again.
Frequency: number of times per second the wave is repeated. Typical range 2-15Mhz. If transducer frequency is increased, resolution and image quality improve. While resolution may increase, penetration will decrease.
Type and density of tissue combined with its degree of homogeneity or heterogeneity contributes to the rate of attenutation (how weak or strong the signal is). This is why distended bladder transabdominally helps visual posterior structures like uterus.
Echogenicity: amplitude display of the returning echoes. If its hyperechoic, it is more echogenic (of increased amplitude (brighter/lighter)) than the surrounding anatomy. Anechoic refers to the absence of echoes (usually fluid filled structures).
Acoustic Shadowing: usually occurs when the sound encounters highly reflective surface – the reflected energy is returned to the transducer with little acoustic energy available to continue traveling to deeper structures. Ex. gallstones, kidneystones, ribs, tissue calcifications.
Gain: increases or decreases the amount of returned echoes displayed on the viewing screen. Brightens/darkens the display. Gain does not affect acoustic output.
TGC (time-gain compensation): the gain changes the deeper the tissue is. This is automatically adjusted internally, but can sometimes be adjusted by the user (ex. if looking at uterus and appears very bright underneath the hyperechoic bladder.)
M-mode: motion mode. tissue movement depicted along a single narrow ultrasound beam on the vertical axis while time is depicted on the horizontal axis. Used primarily in cardiac US to depict motion of cardiac valves. Also for fetal cardiac activity.
Color Doppler: machine listens for frequency shift in echoes returned from the tissue. Can detect movement toward or away from the transducer. The shift is converted to color, movement toward transducer is red and movement away from transducer being blue.
Higher frequency means less penetration, but better detail to superficial structures.
Convex probe: crystals embedded in curved, convex array. Works well for trauma, gallbladder, kidneys, aorta, transabdominal scanning of pelvis. Transducer frequency of 3-4Mhz.
Phased Array probe: small heads, heads are flat. Linear array. The crystals are fired at different time causing the machine to alter the composite ultrasound beam. Gives sector-shaped image much like convex probes. Helpful between intracostal spaces like with echo. Frequencies 2.5-3.5Mhz.
Linear probe: crystals embedded in straight falt head. Beams from crystals are directed straight ahead, parallel to one another, leading to rectangle image rather than sector-shaped. Great at view smaller structures. Good for DVT, peripheral vascular access, testicles, fracture reduction, evaluating tendons/ligaments. Frequency typically 5-7.5Mhz.
Endocavitary probe: Basically a microconvex probe on long handle. Usually tinted ventrally to help visualize posterior fornix. Usually higher frequency than convex probe given structures of interest are closer to the probe than say an aorta. 5-7.5Mhz.
FAST: (focused assessment with sonography for trauma). screening tool for identifying free intrathoracic or intraperitoneal fluid. One major advantage of US compared to peritoneal lavage: free pericardial and pleural fluid. Unable to find source for bleeding; hard to assess on obese patients; unable to sort out blood vs ascites. Can’t assess retroperitoneal space as well compared to CT.
Good for: unstable patients who can’t go to CT, unreliable secondary exam due to drug ingestion, distracting injury, CNS injury, unexplained hypotension. Sensivity of 90%, Specificity of 99%, accuracy 99%.
Morison’s pouch: potential space between liver and right kidney – common site for blood to accumulate.
Can also detect hemothorax. Look superior to diaphgram on both sides.
EFAST: (extended FAST): adds lung exams to rule out pneumothorax.
During normal respiration, there is physiologic sliding of the two pleural surfaces upon one another. On ultrasound, you look for lung sliding. Use doppler to detect motion (Power Slide). M-mode looks for ‘seashore’ sign or its lung sliding abscence ‘stratosphere sign.’ because pleural movement will create homogenous granular pattern in this mode.
Comet tail artifacts are from distended, water-filled interlobular septae under the visceral pleura. US equivalent of Kerley B lines on CXR. If both lung sliding and comet tail artifact seen, can be fairly confident in excluding pneumothorax. Supine pneumothoraces are most common located anteriorly (84%), apical (57%), basal (41%), and medial.
Looking for the ‘lung point’ (with lung sliding, on ultrasound changes from between pneumothorax and regular lung. This correlates with size of pneumothorax.
Looks for mechanical cardiac contractions during PEA. Better marker of very low BP compared to carotid or femoral pulses. If during PEA arrest, if there is not a carotid pulse but there is mechanical cardiac contraction, continue resuscitation. If there is cardiac standstill, it is reasonable to stop resuscitative efforts.
Screening for patients > 50yo with abdomen, back, flank, groin, or patients with dizziness, syncope, unexplained hypotension, cardiac arrest. Mortality of ruptured AAA is 80%. Transverse view: looking for > 3cm diameter. Usually infrarenal and terminates before proximal bifurcation. 90% occur infrarenal.
First Trimester Pregnancy
Urine qualitatitive HCG can detect > 20 IU/L.
Progesterone levels: produced by corpus luteum in early pregnancy. Generally lower in abnormal pregnancies such as ectopic. Level > 11ng/mL more likely early pregnancy over ectopic or abortion (91% sensitive, 84% spec). Level < 5 88% sensitive in detecting ectopic pregnancy in setting on indeterminate US (non-specific free fluid or empty uterus).
Indeterminate transvaginal US with B-HCG > 1000 have presumed ecoptic pregnancy or embryonic demise and requires immediate OB consult. If indeterminate US and HCG < 1000, patient hemodynamically stable and unremarkable exam can be discharged with repeat US and HCG in 2 days with clear ectopic pregnancy discharge instructions.
Serum HCG levels increase by 1.6x every 48 hours, may even doube between 36-48 hours. Most ectopic pregnancies have HCG < 1000, 50% less than 500.
Tubal ring is nearly diagnostic of ectopic pregnancy – concentric hypoechoic structure found in the adnexa – equivalent to gestational sac – can differentiate it from ovarian cyst by relatively thick, brightly echogenic round symmetric wall.
The acoustic output is the amount of energy that is transmitted from the US probe. Increasing output can hypothetically increase risk of tissue heating and cavitation. Thermal injury is particular concern when a stationary acoustic beam is used, as in doppler or M-mode scanning.
Peripheral IVs: basilic and cephalic veins are superficial veins that are readily cannulated using US.
Ultrasound has highest sensitivity for gallstones. HIDA has highest sensitivity for cholecystitis. CT can miss 25% of gallstones.
Over 40yo, at least 20% of women and 8% of men have gallstones.
Acute Cholecystitis: ultrasound 94% sensitive / 84% specific. 95-99% of patients with cholecystitis have gallstones. No single ultrasound finding predicts cholecystitis. Combination helps.
HIDA scan: Cholescintigraphy using 99mTc-hepatic iminodiacetic acid. Injected thru IV, absorbed thru hepatic cells and excreted into biliary system. HIDA scan positive for cholecystitis if gallbladder does not uptake the enhanced bile. Caused by cystic duct obstruction from stone or edema.
2% of cholecystitis will have absence of gallstones (acalculous cholecystitis) (UtD states 10%). Usually occurs in chronically debilitated patient with DM, immunocompromise, recovering from recent trauma.
Gallstones and gallbladder wall thickening had PPV of 95.2%. That on top of exam and lab values increase the likelihood.
Jaundice and Biliary Duct Dilation: Can detect intrahepatic duct dilation, but less sensitive at defining exact cause of obstruction.
Gallbladder anatomy: Fundus / Body / Neck. Spiral Valves of Heister are spiral valves within the neck that can be confused with impacted stones.
Depends on the patient. If you can see the gallbladder under the ribs in the recumbent position; then great. If not, having the patient lay in the left lateral decubitus position sometimes brings the gallbladder out. Also in extreme cases can have the patient almost prone with the ultrasound probe underneath the patient.
Start viewing gallbladder with curvilinear probe with probe indicator toward the patient’s head (probe sagittal).
4 Must-Have Views
Gallbladder viewed in two planes
Gallbladder traced from funds to neck
Anterior gallbladder wall should be measured at clearest point. Focal thickening should be compared to other parts of the wall.
Common bile duct should be measured and traced as far medially as possible if dilated.
Locating the gallbladder: With probe indicator pointed toward patient head, will likely find the gallbladder in the transverse plane. If having difficulty, have patient take deep breath and hold. Also look for main portal vein. This connects with gallbladder with a thick fibrous band called the main lobar fissure. Follow the fissure to the gallbladder.
Short-axis views or transverse view help with differentiating shadowing from gallstones vs something else.
If you find gallstones, move the patient around to see if they will move (they should).
Measure anterior wall thickness. Anything > 3mm is abnormal (UtD states > 4-5mm).
Finding portal vein with gallbladder in longitudinal view. Portal vein usually has bright echogenic wall compared to hepatic veins. Rotating the portal vein 90 degrees will bring the portal triad more into vein (portal vein, hepatic artery and common bile duct). “Mickey Mouse sign.” Use color doppler to identify vascular structures (no flow in CBD).
Common bile duct measurement: inner wall to inner wall: Less than 7mm is normal. If dilated, keep tracking it until its not visualized to look for stone.
Common bile duct diameter: Usually also less than 1/10 the age of the person in millimeters. After cholecystetomy, CBD can get up to 1 cm in all age groups.
Cholecystitis Findings: wall thickness > 4mm, pericholecystic fluid, presence of sonographic Murphy’s sign. 92% of cholecystitis will have wall thickness > 3mm. Wall thickening IS NOT pathognomonic. Pericholecystitic fluid is very specific for cholecystitis. Small percentage of patients may present with no sonographic evidence of disease.
Hepatomegaly: If liver parenchyma of right lobe extends beyond inferior pole of kidney, enlargement is probable. Splenomegaly: length measured in long axis exceeds 12-14cm.
Phrygian cap: folds of the fundus.
Biliary sludge is nonshadowing. Seen in states of biliary stasis.
Contracted gallbladder occurring in postprandial patients can have non pathologic thickened wall.
Mirizzi syndrome: a gallstone impacted in the distal cystic duct causing extrinsic compression on common bile duct – elevated bilirubin).
Determine presence/abscence of hydronephrosis. Mild/moderate hydro can usually be managed in the outpatient setting. Severe hydro needs likely CT, urgent consultation.
Severe usually occurs with some degree of cortical thinning.
Can also use for bladder scan/volume.
Inability to collapse a vein on US indiciated thrombus within the vein. The lumen of the vein must disappear completely to exclude presence of clot.
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Emergency Ultrasound, Second Edition, Ma/Mateer/Blaivas.
UptoDate, Acute cholecystitis: Pathogenesis, clinical features, and diagnosis.
Emergency Ultrasonography, Chapter 299.4, Tintinalli Seventh Edition