4.3 Abdomen Protocols

Abdomen Protocols

MRI of the abdomen is a highly specialized clinical area focusing on the liver, biliary tree, pancreas, kidneys, and adrenal glands. Imaging this region is challenging due to respiratory motion, cardiac motion, and bowel peristalsis. Achieving diagnostic images requires advanced motion-compensation techniques, precise patient preparation, and specific sequence and contrast timing.

Liver Dynamic Phases

Dynamic contrast-enhanced MRI is the gold standard for detecting and characterizing focal liver lesions, such as hepatocellular carcinoma (HCC), hemangiomas, and metastases. The protocol utilizes a 3D T1-weighted gradient-recalled echo sequence with fat suppression (e.g., LAVA, VIBE, or THRIVE).

• Pre-contrast Phase: Baseline scan. Includes in-phase (TE ~4.4 ms at 1.5T) and out-of-phase (TE ~2.2 ms at 1.5T) T1-weighted chemical shift imaging to detect intracellular lipid content. In patients with hepatic steatosis (fatty liver), a significant signal drop is observed on the out-of-phase images.
• Arterial Phase (Early/Late): Acquired 15–20 seconds (early) and 25–35 seconds (late) post-injection. The hepatic artery is enhanced, but liver parenchyma is unenhanced. HCC, which receives its blood supply from the hepatic artery, shows rapid arterial wash-in (hyperenhancement).
• Portal Venous Phase: Acquired 60–70 seconds post-injection. The portal and hepatic veins are enhanced, and normal liver parenchyma reaches peak enhancement. Hypovascular metastases stand out as dark (hypointense) lesions against the bright background of the enhanced liver.
• Equilibrium/Delayed Phase: Acquired 3–5 minutes post-injection. Normal hepatocytes and HCC wash out, while fibrous tissue (such as cholangiocarcinoma or the capsule of a hemangioma) retains contrast, showing delayed enhancement.
• Hepatobiliary Phase (Gd-EOB-DTPA / Eovist): Acquired 20 minutes post-contrast. Functioning hepatocytes actively take up the agent and excrete it into the bile ducts, making normal liver tissue highly hyperintense (bright). Non-functioning tissues (metastases, most HCCs, cysts) do not take up the contrast and appear dark (hypointense), increasing lesion conspicuity.

MR Cholangiopancreatography (MRCP)

MRCP is a non-contrast technique used to visualize the biliary tree and pancreatic ducts.

• Physics of MRCP: Utilizes heavily T2-weighted pulse sequences (e.g., 3D RARE, Single-Shot Fast Spin Echo/SSFSE, or HASTE) with extremely long echo times (TE of 600–1000 ms). According to T2 relaxation physics, a very long TE allows the transverse magnetization of tissues with short T2 times (like solid liver tissue, muscle, fat) to decay completely, rendering them dark (signal-less). Only fluids with extremely long T2 times (such as bile and pancreatic juice) retain their transverse magnetization, appearing highly hyperintense (bright) against a black background.
• Patient Prep: NPO for 4–6 hours to prevent gallbladder contraction and minimize stomach/duodenal fluid secretions. Administering an oral negative contrast agent (such as black tea, pineapple juice, or blueberry juice, which contain manganese or iron that shortens T2 relaxation time) before the scan helps suppress the bright signal of fluid in the stomach and duodenum, preventing it from overlapping and obscuring the biliary ducts.

Kidneys and Adrenal Glands

• Kidneys: Evaluated using coronal and axial T2-weighted and multiphase dynamic contrast-enhanced T1-weighted sequences. The key phases are the corticomedullary phase (30–40s, highlighting the renal cortex), nephrographic phase (80–120s, highlighting the renal parenchyma), and excretory phase (5–10 min, highlighting the collecting system) to evaluate renal cell carcinoma (RCC).
• Adrenal Glands: High-resolution T1-weighted chemical shift imaging (in-phase and out-of-phase) is the gold standard to differentiate benign adrenal adenomas from malignant adrenal lesions. Benign adrenal adenomas contain abundant intracellular lipid (fat). On out-of-phase T1-weighted images, water and fat protons within the same voxel precess out of phase, causing their signals to cancel out. This results in a significant drop in signal intensity of the adenoma relative to the spleen or liver on the out-of-phase images compared to the in-phase images. Malignant lesions do not contain intracellular fat and do not lose signal on out-of-phase images.

Respiratory Gating and Motion Compensation

Because of diaphragmatic movement during breathing, abdominal MRI is highly susceptible to respiratory motion artifacts.

• Respiratory Bellows: A physical pressure-sensitive belt placed around the patient's upper abdomen to monitor the respiratory cycle. Data acquisition is triggered only during the end-expiration phase, when the diaphragm is at its most stable position.
• Navigator Echoes: An electronic tracking beam placed at the dome of the right diaphragm. It dynamically measures the liver-lung interface and gates data acquisition to occur only when the diaphragm is within a pre-defined displacement window (typically 1–2 mm).
• Breath-Holding: Fast sequences (e.g., single-shot FSE, 3D gradient echo) are acquired within a single breath-hold (usually 15–20 seconds) during expiration.

Summary of Abdomen Protocols