ABPS Diagnostic Radiology cheat sheet

Silhouette sign: loss of a normal border indicates the adjacent structure and the opacity share a radiographic plane (e.g., right middle lobe obscures the right heart border).

Air bronchograms indicate alveolar airspace disease; their absence with volume loss favors atelectasis or a mass.

Kerley B lines are 1–2 cm peripheral horizontal septal lines at the lung bases, marking interlobular septal thickening (interstitial edema).

The PA chest radiograph is taken at 1.8–2 m with a 72-inch SID; a lordotic AP film falsely elevates the apparent cardiac silhouette.

Normal cardiothoracic ratio on a PA erect film is ≤0.50; >0.50 suggests cardiomegaly or pericardial effusion.

The azygoesophageal recess is bounded by the azygos vein and esophagus; its displacement helps localize posterior mediastinal masses.

Hampton hump = peripheral wedge-shaped opacity in pulmonary infarction; Westermark sign = focal oligemia distal to a pulmonary embolus.

Mosaic attenuation on HRCT: check inspiratory/expiratory and prone/supine series to distinguish small airways disease from vascular disease or infiltrative lung disease.

Bowel obstruction: small bowel >3 cm, colon >6 cm, cecum >9 cm define abnormal dilation; valvulae conniventes traverse the full lumen of small bowel.

Portal venous gas is peripheral and extends to the liver capsule; pneumobilia is central and does not reach the capsule.

Rigler sign (air on both sides of bowel wall) and pneumoperitoneum on upright chest film indicate perforation.

The normal appendix is ≤6 mm in diameter; non-visualization of a normal appendix in suspected appendicitis warrants further evaluation.

On CT, the 'target sign' of intussusception is a bowel-within-bowel appearance; in adults, a lead point (often tumor) must be excluded.

The CT 'whirl sign' and 'barber pole sign' suggest midgut volvulus; coronal reformats increase diagnostic sensitivity.

Acute cholecystitis: gallbladder wall thickening >3 mm, pericholecystic fluid, and a sonographic Murphy sign support the diagnosis.

Hepatic steatosis reduces liver attenuation relative to spleen on unenhanced CT; a liver-to-spleen attenuation difference >10 HU is a common threshold.

Renal colic: non-contrast CT has high sensitivity for calculi; ureteral stones at the UPJ, iliac vessel crossing, and UVJ are common sites of impaction.

A ureteral stone with a 'tissue rim sign' suggests a stone rather than a phlebolith, which may show a comet-tail sign.

The Bosniak classification stratifies renal cysts (I–IV) by septa, calcification, and enhancement; higher categories warrant surgical consideration.

Renal cell carcinoma subtypes: clear cell is most common and hypovascular papillary and chromophobe types have different enhancement patterns.

Prostate MRI uses PI-RADS v2.1; a score of 3–5 in the peripheral zone requires DWI, with T2-weighted imaging dominant in the transition zone.

Hydronephrosis grading (mild/moderate/severe) reflects pelvicalyceal dilation and cortical thinning; it does not by itself establish obstruction.

Adrenal adenomas often show signal loss on opposed-phase MRI or washout >60% on delayed CT, distinguishing them from metastases.

Bladder cancer staging: T1 invades lamina propria, T2 invades muscularis propria, T3 extends into perivesical fat.

A posterior fat pad sign on a lateral elbow radiograph in an adult indicates an occult intra-articular fracture (e.g., radial head).

The anterior humeral line should intersect the middle third of the capitellum; displacement suggests a supracondylar fracture.

The scaphoid view requires ulnar deviation and a dedicated view; a negative initial radiograph with snuffbox tenderness warrants immobilization and follow-up.

On MRI, a complete rotator cuff tear shows full-thickness tendon discontinuity with fluid signal; partial tears may show intrasubstance signal without full-thickness defect.

Bone marrow edema is low signal on T1-weighted images and high signal on fluid-sensitive sequences.

The ABCS approach (Alignment, Bones, Cartilage/joint space, Soft tissues) is a systematic method for radiograph interpretation.

Osteosarcoma typically shows aggressive periosteal reaction (Codman triangle, sunburst) and mineralized tumor matrix.

The Salter-Harris classification (I–V) describes physeal fractures; type II is the most common.

The ASPECTS score quantifies early ischemic changes on non-contrast CT; lower scores correlate with worse outcome in MCA stroke.

Hyperdense MCA sign indicates acute thrombus; it is a non-contrast CT sign of large-vessel occlusion.

The Alberta Stroke Program and NASCET criteria use percent stenosis; NASCET compares the narrowest diameter to the normal distal internal carotid artery diameter.

On MRI, restricted diffusion (high DWI, low ADC) indicates cytotoxic edema from acute infarction or high cellularity.

The 'eye of the tiger' sign on T2-weighted MRI in the globus pallidus is associated with pantothenate kinase-associated neurodegeneration.

Spinal cord compression: epidural mass, disc herniation, or fracture; urgent MRI is indicated when neurologic deficit is present.

In head and neck imaging, the 'danger space' lies between the alar and prevertebral fascia and extends from skull base to the mediastinum.

Hounsfield unit thresholds: acute hemorrhage on non-contrast CT is typically 40–60 HU and hyperdense relative to brain parenchyma.

BI-RADS categories: 0 incomplete, 1 negative, 2 benign, 3 probably benign (<2% malignancy), 4 suspicious, 5 highly suggestive of malignancy, 6 known biopsy-proven malignancy.

BI-RADS 3 lesions are typically followed at short interval (often 6 months) rather than biopsied.

Mammographic breast density categories A–D (BI-RADS) reflect fibroglandular tissue amount; dense tissue lowers sensitivity.

Screening mammography typically uses two views per breast (craniocaudal and mediolateral oblique); additional views include spot compression and magnification.

Microcalcification distribution descriptors include diffuse, regional, grouped, linear, and segmental; linear and segmental patterns raise suspicion.

Breast MRI is most sensitive for invasive lobular carcinoma and for evaluation of silicone implant integrity.

Stereotactic or MRI-guided biopsy is used for lesions not visible on ultrasound; clip placement is standard after biopsy.

Male breast cancer most often presents as a subareolar mass; gynecomastia is typically flame-shaped and bilateral.

Coronary calcium score is derived from unenhanced CT and is a marker of atherosclerotic burden; it is not a stenosis measure.

Coronary CTA is limited by motion and calcification; a high calcium score reduces specificity for stenosis.

Acute aortic dissection: Stanford type A involves the ascending aorta and is a surgical emergency; type B does not.

Pulmonary embolism CTA: filling defect within the pulmonary artery; right heart strain signs include RV/LV ratio >1 and septal bowing.

Cardiac MRI late gadolinium enhancement patterns help distinguish ischemic (subendocardial/transmural) from nonischemic (midwall/epicardial) disease.

Aortic stenosis severity by echocardiography uses peak velocity, mean gradient, and valve area; severe is typically area <1.0 cm².

Myocardial perfusion imaging with SPECT or PET detects ischemia as a reversible perfusion defect.

Aortic aneurysm measurement should be perpendicular to the vessel axis; a diameter of ≥5.5 cm in the ascending aorta is a common surgical threshold.

Pediatric bone age is estimated from hand/wrist radiographs using the Greulich and Pyle atlas.

In children, the thymus may appear as a sail sign or normal mediastinal widening; it should not be mistaken for pathology.

Necrotizing enterocolitis often presents with pneumatosis intestinalis and portal venous gas in premature infants.

The pediatric cervical spine has pseudosubluxation of C2 on C3 and a normal predental space up to 5 mm in young children.

Hypertrophic pyloric stenosis: pyloric muscle thickness >3 mm and canal length >15 mm on ultrasound.

Intussusception in children is often ileocolic and treated with air or contrast enema in the absence of perforation.

Developmental dysplasia of the hip: ultrasound is used in infants <6 months; the Graf classification describes acetabular morphology.

Radiation dose in children is reduced by using lower kVp/mAs, avoiding unnecessary CT, and using the 'as low as reasonably achievable' (ALARA) principle.

FDG-PET uptake is quantified by SUV; high SUVs are nonspecific and can occur in infection, inflammation, and benign tumors.

A normal FDG distribution includes brain, myocardium (variable), liver, spleen, kidneys, and urinary tract.

V/Q scan interpretation uses the PIOPED criteria; a high-probability scan shows multiple mismatched perfusion defects.

Bone scan with Tc-99m MDP shows increased uptake in metastases, fracture, infection, and arthritis; it is nonspecific.

I-131 therapy for thyroid cancer uses the sodium-iodide symporter; thyroid stunning can reduce subsequent uptake.

Tc-99m sestamibi is used for parathyroid imaging and myocardial perfusion; it is taken up by mitochondria.

Sentinel lymph node mapping uses a radiotracer and blue dye; the first draining node is identified for biopsy.

PET/CT improves anatomic localization and attenuation correction compared with PET alone.

First-trimester dating: crown-rump length is most accurate up to 13+6 weeks; later, biparietal diameter and femur length are used.

Ectopic pregnancy: empty uterus with an adnexal mass and free fluid; a gestational sac with yolk sac outside the uterus is diagnostic.

Ovarian torsion: enlarged ovary with peripherally displaced follicles and decreased or absent Doppler flow; Doppler may be normal.

Endometrial thickness >4–5 mm on transvaginal ultrasound in postmenopausal bleeding warrants tissue sampling.

Fetal biometry includes biparietal diameter, head circumference, abdominal circumference, and femur length; estimated fetal weight is derived from these.

Amniotic fluid index (AFI) is the sum of the deepest vertical pockets in four quadrants; oligohydramnios is typically AFI ≤5 cm.

The nuchal translucency measurement is performed at 11–14 weeks and is associated with aneuploidy risk.

Doppler ultrasound of the umbilical artery assesses placental resistance; absent or reversed end-diastolic flow indicates fetal compromise.

Seldinger technique: needle puncture, guidewire insertion, catheter placement over the wire, and removal of the wire.

The common femoral artery is accessed at the level of the femoral head; too-high puncture risks retroperitoneal hemorrhage.

Post-embolization syndrome includes pain, fever, and nausea after transarterial embolization; it is usually self-limited.

TIPS creates a portosystemic shunt between the hepatic vein and portal vein for portal hypertension complications.

Radiofrequency and microwave ablation are options for small hepatic, renal, and pulmonary tumors; microwave achieves higher temperatures and larger ablation zones.

Complications of central venous access include pneumothorax, arterial puncture, and catheter-related infection.

Angioplasty and stenting are used for arterial stenosis; restenosis remains a limitation, especially in small vessels.

IVC filter placement is considered when anticoagulation is contraindicated in patients with venous thromboembolism.

The photoelectric effect dominates at low photon energies and in high-atomic-number materials; it produces high subject contrast.

Compton scattering dominates at higher energies and in soft tissue; it reduces contrast and contributes to scatter.

Beer-Lambert law: I = I0 e^(-μx), where μ is the linear attenuation coefficient and x is thickness.

The half-value layer (HVL) is the thickness of material that reduces beam intensity by half; it reflects beam quality.

CT Hounsfield units are defined as HU = 1000 × (μ_tissue − μ_water) / μ_water.

Spatial resolution is the ability to distinguish small objects; it is often expressed in line pairs per millimeter.

The Nyquist theorem requires sampling at least twice the highest spatial frequency to avoid aliasing.

MRI signal depends on T1 and T2 relaxation; gadolinium shortens T1 and increases signal on T1-weighted images.

The SI unit of absorbed dose is the gray (Gy); 1 Gy = 100 rad. The sievert (Sv) is the unit of equivalent and effective dose.

Effective dose accounts for tissue radiosensitivity and is used to compare stochastic risk across modalities.

ALARA (as low as reasonably achievable) is a guiding principle for radiation protection.

Occupational dose limits: 50 mSv/year effective dose; lens of eye 150 mSv/year; extremities 500 mSv/year (ICRP recommendations).

The inverse square law: dose rate decreases with the square of distance from a point source.

Shielding for diagnostic x-rays includes lead aprons and thyroid shields; lead equivalence is typically 0.25–0.5 mm Pb.

Pregnancy: fetal dose from most diagnostic studies is low; the decision to image should weigh clinical benefit against risk.

Time, distance, and shielding are the three basic principles of radiation protection.

Quality assurance programs include equipment testing, image quality assessment, and radiation dose monitoring.

The ACR accreditation program requires phantom image testing, clinical image review, and personnel qualifications.

Repeat/reject analysis identifies common causes of image rejection and guides process improvement.

Daily, weekly, and annual QC tests differ by modality; mammography has specific MQSA requirements.

Diagnostic reference levels (DRLs) are used to benchmark radiation dose and identify outliers.

Peer review and double reading improve diagnostic accuracy and are part of quality improvement.

Incident reporting and root cause analysis are used to investigate errors and prevent recurrence.

Maintenance of equipment and calibration of dose meters are essential for reliable QC data.