ARRT Sonography cheat sheet

ALARA: keep acoustic output and exposure time at the lowest level that still yields diagnostic information; the technique applies to every exam, including normal pregnancies.

Thermal index (TI) estimates the temperature rise from absorbed energy (TI = 1 approximates a 1 degree C rise); mechanical index (MI) estimates cavitation risk from peak rarefactional pressure.

FDA upper limits: MI 1.9 and TI 1.0 for general diagnostic imaging; ophthalmic imaging is limited to MI 0.23 and TI 1.0.

High-level disinfection is required for endocavitary, transesophageal, and intraoperative probes; low-level disinfection or an approved wipe is acceptable for probes contacting only intact skin.

Late-pregnancy hypotension: place the patient in left lateral decubitus (or right-side elevation) to relieve inferior vena cava compression and restore venous return.

For vasovagal syncope, place the patient supine with legs elevated or in Trendelenburg; use cautiously in cardiac or respiratory compromise.

Two patient identifiers (name plus date of birth or medical record number) must be verified before scanning; standard precautions apply to all patients.

Ultrasound contrast agents are contraindicated in patients with known right-to-left intracardiac shunts because microbubbles can bypass the pulmonary filter.

Operator ergonomics: keep the wrist neutral, support the transducer cable, and adjust the monitor and bed height to reduce repetitive strain injury.

Range equation (soft tissue, 1540 m/s): depth in cm = round-trip time in microseconds x 0.077; wavelength in mm = 1.54 / frequency in MHz.

Axial resolution = spatial pulse length / 2; improve it with higher frequency, fewer cycles per pulse, and damping. It is always better than lateral resolution.

Lateral resolution is determined by beam width at the focal zone; it improves with higher frequency, larger apertures, and electronic focusing.

Elevational (slice-thickness) resolution is fixed by the transducer element height and is the thinnest at the focal zone.

Attenuation in soft tissue averages about 0.5 dB/cm/MHz; doubling frequency doubles attenuation and halves penetration depth.

Nyquist limit: the Doppler shift must be less than half the PRF to avoid aliasing; fix aliasing with a higher PRF, lower frequency, shallower sample gate, or a shifted baseline.

Time gain compensation selectively amplifies later-arriving (deeper) echoes to offset attenuation and produce uniform echogenicity across depth.

Frame rate = PRF / number of pulses per frame; increasing color box size, depth, or line density lowers frame rate.

Reflection occurs at boundaries with an acoustic impedance mismatch; a 90-degree angle of incidence maximizes the returning echo.

Normal liver echogenicity is slightly greater than or equal to renal cortex; a liver that is darker than the cortex suggests fatty infiltration or edema.

Portal veins have echogenic walls (fibrous sheath) and hepatopetal flow; hepatic veins have imperceptible walls and pulsatile flow toward the IVC.

Normal gallbladder wall is 3 mm or less in a fasting patient; thickening with pericholecystic fluid and a positive sonographic Murphy sign supports acute cholecystitis.

Common bile duct: up to 6 mm is normal, add about 1 mm per decade after 60, and up to 10 mm is accepted after cholecystectomy.

Normal adult kidney length is 9-12 cm; cortex is hypoechoic to liver and spleen, and the central sinus is hyperechoic from fat and collecting structures.

Aortic diameter greater than 3 cm defines an aneurysm; measure outer wall to outer wall, and 5 cm or greater or rapid enlargement prompts surgical or endovascular referral.

Spleen: normal length up to 13 cm and thickness up to 5 cm; it is homogeneous and slightly more echogenic than the liver.

Hydronephrosis grading: mild shows renal pelvic dilation, moderate adds blunted calyces, and severe shows cortical thinning with a dilated, tortuous collecting system.

Right upper quadrant pain workup includes liver, gallbladder with wall thickness and stones, CBD caliber, pancreas, and both kidneys to exclude referred renal causes.

Transvaginal landmarks: gestational sac at about 4.5-5 weeks, yolk sac at 5-6 weeks, and fetal pole with cardiac activity at 6-6.5 weeks.

Crown-rump length is the most accurate dating parameter from 7 to 13 weeks (about plus or minus 5 days); measure in mid-sagittal with neutral fetal posture.

Nuchal translucency is measured at 11-14 weeks with CRL 45-84 mm; increased NT (typically above 3.0 mm) raises aneuploidy and cardiac defect risk.

Second-trimester dating: BPD and HC are most reliable at 14-26 weeks; after that, composite biometry with emphasis on head and femur is used.

Abdominal circumference is the best single marker of fetal growth and nutrition in the third trimester; measure at the stomach bubble and portal sinus level.

Estimated fetal weight is derived from BPD, HC, AC, and FL; AC and FL are the most sensitive parameters in growth restriction.

Amniotic fluid index sums the deepest pocket in four quadrants: normal 5-24 cm, oligohydramnios under 5 cm, polyhydramnios over 24 cm.

Placenta previa is placental tissue covering or within 2 cm of the internal os, confirmed transvaginally; low-lying placenta may resolve as the uterus grows.

Ectopic pregnancy: empty uterus with a complex adnexal mass, free pelvic fluid, or an extrauterine gestational sac with a yolk sac or embryo.

Normal thyroid is homogeneous and hyperechoic relative to strap muscles and measures roughly 4-6 cm long with each lobe 1.5-2 cm in AP dimension.

Suspicious thyroid nodule features: solid hypoechoic, taller-than-wide, irregular margins, microcalcifications, and extrathyroidal extension.

Simple cyst criteria (breast or elsewhere): anechoic, thin imperceptible wall, smooth margins, posterior acoustic enhancement, and no internal vascularity.

Breast malignancy features: irregular shape, non-parallel orientation, spiculated margins, shadowing, and internal vascularity.

Normal testis is homogeneous, mid-level echogenicity, and 3-5 cm long; a dilated pampiniform plexus over 2-3 mm that enlarges with Valsalva indicates varicocele, most often left-sided.

Lower-extremity DVT: a normal vein fully compresses; a noncompressible vein with intraluminal echoes and absent phasic flow indicates acute thrombus.

Anisotropy: tendons and nerves become falsely hypoechoic when not perpendicular to the beam; keep the transducer at 90 degrees to the structure.

Normal lymph node: oval, thin cortex, and an echogenic fatty hilum; loss of the hilum, round shape, and increased short axis raise concern for malignancy.

Foreign bodies: most are hyperechoic with posterior shadowing or reverberation; a hypoechoic halo of granulation tissue may surround chronic objects.