A 36-year-old male is taken to the emergency room after jumping from a building. Bilateral fractures to the femur were stabilized at the scene by emergency medical technicians. The patient is lucid upon questioning and his vitals are stable. Pain only at his hips was elicited. Cervical exam was not performed. What is the best imaging study for this patient?

Computed tomography (CT) scan of cervical spine, hips, and lumbar area

AP and lateral radiographs of hips

Lateral radiograph (x-ray) of hips

Magnetic resonance imaging (MRI) of hips, knees, lumbar, and cervical area

Anterior-posterior (AP) and lateral radiographs of hips, knees, lumbar, and cervical area

A 65-year-old man is referred by his primary care provider to a neurologist for leg pain. He reports a 6-month history of progressive bilateral lower extremity pain that is worse in his left leg. The pain is 5/10 in severity at its worst and is described as a "burning" pain. He has noticed that the pain is acutely worse when he walks downhill. He has started riding his stationary bike more often as it relieves his pain. His past medical history is notable for hypertension, diabetes mellitus, and a prior myocardial infarction. He also sustained a distal radius fracture the previous year after falling on his outstretched hand. He takes aspirin, atorvastatin, metformin, glyburide, enalapril, and metoprolol. He has a 30-pack-year smoking history and drinks 2-3 glasses of wine with dinner every night. His temperature is 99°F (37.2°C), blood pressure is 145/85 mmHg, pulse is 91/min, and respirations are 18/min. On exam, he is well-appearing and in no acute distress. A straight leg raise is negative. A valsalva maneuver does not worsen his pain. Which of the following is the most appropriate test to confirm this patient's diagnosis?

Computerized tomography myelography

A 60-year-old male is admitted to the ICU for severe hypertension complicated by a headache. The patient has a past medical history of insulin-controlled diabetes, hypertension, and hyperlipidemia. He smokes 2 packs of cigarettes per day. He states that he forgot to take his medications yesterday and started getting a headache about one hour ago. His vitals on admission are the following: blood pressure of 160/110 mmHg, pulse 95/min, temperature 98.6 deg F (37.2 deg C), and respirations 20/min. On exam, the patient has an audible abdominal bruit. After administration of antihypertensive medications, the patient has a blood pressure of 178/120 mmHg. The patient reports his headache has increased to a 10/10 pain level, that he has trouble seeing, and he can't move his extremities. After stabilizing the patient, what is the best next step to diagnose the patient's condition?

CT head without intravenous contrast

Doppler ultrasound of the carotids

CT head with intravenous contrast

MRI head without intravenous contrast

MRI head with intravenous contrast

MRI principles and interpretation for USMLE Step 1: High-Yield Anatomy Lessons

MRI Physics - Magnets at Work

Core Principle: Utilizes strong magnetic fields and radiofrequency (RF) pulses to manipulate hydrogen protons (¹H) in water and fat.
Process:
- Alignment: A powerful primary magnet ($B_0$) aligns ¹H protons in the body along its field.
- Excitation: An RF pulse ($B_1$) is applied, knocking the aligned protons out of their low-energy state.
- Relaxation: The RF pulse is turned off. Protons "relax" back to their baseline alignment, releasing energy detected by the scanner.
- Signal: Different tissues relax at different rates, creating signal contrast.

Proton alignment and excitation in MRI

⭐ The strength of the main magnetic field ($B_0$) is measured in Tesla (T). Most clinical scanners are 1.5T or 3T, affecting signal quality and scan time.

T1 vs. T2 - Fat & Water

Characteristic	T1-Weighted ("Anatomy Scan")	T2-Weighted ("Pathology Scan")
Water / CSF	Dark (hypointense)	Bright (hyperintense)
Fat	Bright (hyperintense)	Also bright (hyperintense)
Core Use	Defining normal anatomy	Highlighting fluid, edema, inflammation

*   T**1**: Time for **1** thing: Anatomy.
*   T**2**: H**2**O is bright (WW2: Water is White).

⭐ Gadolinium contrast is exclusively used with T1-weighted sequences. It shortens the T1 relaxation time of tissues where it accumulates (e.g., tumors, areas with blood-brain barrier breakdown), causing them to appear bright. This enhancement is crucial for identifying active disease.

Advanced Sequences - Special Ops MRI

Diffusion-Weighted Imaging (DWI): Maps random water motion. Essential for detecting acute ischemia (cytotoxic edema) within minutes.
- Paired with ADC (Apparent Diffusion Coefficient) maps to differentiate true restriction.
- Acute Ischemia: Bright on DWI, Dark on ADC.
- T2 Shine-through: Bright on DWI, Bright on ADC.
Diffusion Tensor Imaging (DTI): Visualizes white matter tracts by measuring water diffusion directionality (fractional anisotropy). Key for traumatic axonal injury & pre-op mapping.
Perfusion-Weighted Imaging (PWI): Assesses cerebral blood flow (CBF) and volume (CBV). Defines ischemic penumbra and grades tumor neovascularity.
MR Spectroscopy (MRS): Measures brain metabolite concentrations. Key peaks: ↓NAA (neuronal health), ↑Choline (membrane turnover), ↑Lactate (anaerobic metabolism).

⭐ The DWI/PWI mismatch highlights the ischemic penumbra: viable but at-risk tissue. This is the target for acute stroke intervention.

Contrast & Safety - Gadolinium Rules

Mechanism: Paramagnetic agent; shortens T1 relaxation time, causing ↑ signal (brightness) on T1-weighted images.
Indications: Enhances vessels, inflammation, and helps characterize tumors by assessing vascularity.
Safety Profile:
- ⚠️ Nephrogenic Systemic Fibrosis (NSF): Rare, severe fibrosing disease in patients with renal dysfunction.
- Allergic Reactions: Uncommon; much less frequent than with iodinated contrast.
- Deposition: Trace amounts can be retained in the brain (globus pallidus) and body; long-term significance is under investigation.

⭐ Avoid gadolinium in patients with an estimated Glomerular Filtration Rate (eGFR) < 30 mL/min/1.73m² due to the high risk of NSF.

High‑Yield Points - ⚡ Biggest Takeaways

T1-weighted: Provides excellent anatomical detail. Fat appears bright, while water is dark.

T2-weighted: Superior for detecting pathology (edema). Water and fat both appear bright.

Gadolinium contrast: Shortens T1 relaxation, causing bright enhancement in vascular or inflamed tissues.

FLAIR: A modified T2 sequence that suppresses CSF signal, ideal for periventricular lesions (e.g., MS).

DWI: The key sequence for acute stroke, detecting restricted water diffusion within minutes.

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