Imaging Contrast Media and Nuclear Medicine Review

Comprehensive practice flashcards covering DOTANOC PET/CT, MRI contrast agents, Ultrasound microbubbles, and Contrast-Induced Nephrotoxicity (CI-AKI) based on the lecture transcript.

What are the three main components of the radiotracer used in $^{68}Ga-DOTANOC$ PET/CT?

1. Gallium-68 (positron-emitting radionuclide), 2. DOTA (chelating agent), and 3. NOC (NaI3-octreotide, a somatostatin analogue).

What is the half-life of Gallium-68 ($^{68}Ga$)?

Approximately $68$ minutes.

Which somatostatin receptors (SSTRs) does $^{68}Ga-DOTANOC$ primarily bind to?

It binds mainly to $SSTR2$, $SSTR3$, and $SSTR5$, with high affinity for $SSTR3$ and $SSTR5$.

Explain the physical process that occurs after the $^{68}Ga-DOTANOC$ tracer is internalized into tumor cells.

Gallium-68 emits a positron which combines with an electron (annihilation), producing two gamma photons of $511\,keV$ in opposite directions ($180^{\circ}$) detected by the PET scanner.

What organ shows the maximum physiological uptake in a $^{68}Ga-DOTANOC$ PET scan?

The spleen.

Describe the inverse relationship between tumor grade and tracer avidity in NETs.

Well-differentiated low-grade NETs are DOTA-avid and FDG-negative, while poorly differentiated high-grade NETs are less DOTA-avid and more FDG-avid.

What are the three phases of the 'Theranostic' approach?

1. The Diagnostic Phase (low-dose tracer identification), 2. The Patient Selection Step (measuring absorption and location), and 3. The Therapeutic Phase (using a potent radioactive payload to destroy targeted cells).

Why is Gadolinium considered a 'positive' contrast agent in MRI?

It is a paramagnetic agent that shortens T1 relaxation time, which increases signal intensity on T1-weighted images, making tissues appear brighter.

How do superparamagnetic iron oxide (SPIO) particles affect MRI images?

They shorten T2 and T2* relaxation times, causing signal loss and making tissues appear darker (negative contrast agents).

What is the clinical difference between linear and macrocyclic chelates in Gadolinium-Based Contrast Agents (GBCAs)?

Macrocyclic chelates enclose gadolinium in a cage-like ring and are more stable, carrying a lower risk of gadolinium dissociation, Nephrogenic Systemic Fibrosis (NSF), and tissue deposition compared to open-chain linear chelates.

Which specific MRI contrast agent is used for hepatocyte-specific imaging?

Gadoxetate disodium (Eovist/Primovist) or Gadobenate dimeglumine (MultiHance).

What is Nephrogenic Systemic Fibrosis (NSF) and what is its primary risk factor?

A rare, serious fibrosing disorder associated with gadolinium exposure; the primary risk factor is severe renal dysfunction (eGFR $< 30\,mL/min/1.73\,m^2$).

Where in the brain has gadolinium retention been most frequently noted?

The dentate nucleus and the globus pallidus.

What is the standard dose for extracellular gadolinium contrast agents?

$0.1\,mmol/kg$ body weight.

What are the two major components of ultrasound contrast agents?

1. A gas core (e.g., sulfur hexafluoride or perflutren) and 2. An outer shell (e.g., phospholipids, albumin, or polymers).

What Mechanical Index (MI) is typically used in Contrast Enhanced Ultrasound (CEUS)?

A low mechanical index, usually around $0.05\text{--}0.2$, to avoid premature destruction of the microbubbles.

How is the gas component of ultrasound contrast media eliminated from the body?

It is exhaled through the lungs.

What are the three imaging phases of CEUS following injection?

1. Arterial Phase ($10\text{--}30$ seconds), 2. Portal Venous Phase ($30\text{--}120$ seconds), and 3. Late Phase (up to $4\text{--}6$ minutes).

Describe the typical CEUS enhancement pattern for a Hemangioma in the liver.

Peripheral nodular enhancement followed by progressive centripetal fill-in.

According to current criteria, how is Contrast-Induced Acute Kidney Injury (CI-AKI) defined?

An increase in serum creatinine $\ge 0.3\,mg/dL$ within $48$ hours, or an increase to $\ge 1.5$ times baseline ($\ge 50\%$ rise).

What is the Most Important Preventive Measure for Contrast-Induced Nephrotoxicity?

Adequate hydration with intravenous isotonic saline ($0.9\%\,NS$) started $6\text{--}12$ hours before and continued $4\text{--}12$ hours after the procedure.

What are the recommendations for Metformin use in patients undergoing contrast studies?

Hold Metformin for $48$ hours in patients with eGFR $< 30\,mL/min/1.73\,m^2$ or those undergoing intra-arterial studies, and restart only after reassessing renal function.

How are acute adverse reactions to contrast media classified by time of occurrence?

Acute reactions occur within $1$ hour of administration (most within the first $20$ minutes).

What is the first-line drug and dose for treating anaphylaxis in adults caused by contrast media?

Epinephrine (Adrenaline) $1:1000$ intramuscularly (IM) at a dose of $500\,micrograms$ ($0.5\,mL$).

What is the characteristic clinical presentation of a vasovagal reaction during contrast administration?

Hypotension associated with bradycardia.