Atlas MRI Abdomen

This photo gallery presents the anatomy of the abdomen by means of MRI.

Axial T1-weighted images
Axial T1-weighted fat-suppressed images after injection of contrast
Axial T2-weighted fat-suppressed images

Radiologists routinely use magnetic resonance imaging (MRI) to diagnose many upper abdominal tumors(1).

MRI is non-ionizing, making the modality more advantageous over other imaging sequences that involve higher radiation exposure. MRI can provide practical information and better soft tissue resolution than computed tomography (CT)(2-3).

The primary advantage of MRI in treating upper abdominal malignancies, particularly liver cancers, lies in CT’s difficulty to detect and characterize tumors. Additionally, MRI can better characterize some organs at risk, such as the common bile duct and distal biliary tree(4).

Upper abdominal organs are also subject to respiratory motion, bowel peristalsis, and daily positional variation. These conditions are suitable for online MRI guidance and adaptive radiation therapy(5-8).

The commercial availability of MRI-guided treatment units has increased. This boost in availability amplified the focus on online MRI-guided radiation therapy (MRgRT) for treating pancreatic cancer, liver cancer, kidney cancer, adrenal cancer, and upper abdominal metastases(9-11).

MRI simulation and online MRgRT are becoming more commonly used to identify gross tumor volume (GTV) and organs at risk (OAR). These modalities are useful in upper abdominal radiation planning and treatment (especially for pancreas and liver cancer)(12).

Experts have used two commercially integrated online MRgRT systems for treating patients with upper abdominal tumors:

  1. A 0.35T MRI scanner combined with 3 Cobalt-60 teletherapy heads or a 6X FFF linear accelerator and multileaf collimators (MRIdian, ViewRay)
  2. A 1.5T MRI scanner combined with a 6X linear accelerator (Unity MRLinac, Elekta AB)(13-14).

Other online MRgRT systems are in development. Among these systems are the 1.0T Australian MRI LINAC System (Australian MRI-LINAC Program, Sydney, Australia) and the 0.6T Aurora-RT System (MagnetTx, Edmonton, Alberta, Canada)(15-16).

The accurate definition of the GTV and OARs ensures safe radiation treatment. Precise definitions can realize the potential of real-time MRI-based image guidance and adaptive treatment planning(17).

Meanwhile, several standard tissue contouring atlases for radiation planning exist. However, they are CT-based(18-19).

Professionals expect online MRgRT to increase the accuracy and precision of radiation treatment delivery. The method also improves adaptive radiation therapy and allows response assessment throughout treatment(20).

With the increasing use of MRI, standardization of target volume and OAR definition via MRI have become necessary. 

Standardization improves the consistency of radiation treatment plans. Having standard definitions also facilitates high-quality collaborative studies and OAR dose-volume-toxicity analyses(21).

Many radiation oncologists have not routinely used MRI for simulation or guidance. Most radiation therapists also lack training in MRI anatomy(22).

An MRI-based atlas is useful in defining commonly used vocabulary (such as liver segments). An atlas can also define OARs that CT-based radiation therapy may not have consistently visualized or contoured.

Liver

Unenhanced MRI-defined GTV connects better with the pathologic GTV than other imaging modalities among patients with primary and secondary liver tumors. Other modalities include CT or positron emission tomography(23).

MRI in liver imaging has superior soft-tissue contrast. This advantage allows better detection and characterization of malignant and benign focal liver lesions(24).

The development of liver-specific MRI contrast agents (substances that make images clearer) has further improved MRI diagnostic yield in detecting and characterizing lesions.

It is necessary to detect focal liver lesions early, particularly malignant ones. Patients’ survival rates improve upon the resection of liver metastases of some malignancies, such as colorectal cancer(25).

Spleen

The spleen is an intraperitoneal (within the abdomen) organ located in the abdomen’s upper-left quadrant. The gastrosplenic and splenorenal ligaments, two of the spleen’s ligaments, maintain the spleen’s normal anatomic position(26).

CT and ultrasound are the primary modalities that radiologists use for imaging the spleen. In some instances, MRI can be helpful.

The adult spleen’s average signal on MRI is hyperintense on T2-weighted imaging. On T1-weighted imaging, the spleen is hypointense relative to the liver(27). Deviations from this pattern can reflect iron overload disease processes, including hemochromatosis and hemosiderosis.

Pancreas

Using MRI for target volume delineation in pancreatic cancer has also resulted in smaller target volumes and reduced the interobserver variation. This occurrence is likely due to the improved soft-tissue contrast(28-29).

Recently, a research group published specific MRI-based GTV and OAR contouring recommendations in pancreatic cancer(30-31).

Pancreatic ductal adenocarcinoma is a typically malignant tumor. This condition, which accounts for 85% to 90% of all malignant pancreatic tumors, is the fourth most common cause of cancer death worldwide(32).

The remaining tumors of the pancreas are a diverse group of pancreatic neoplasms. These abnormal tissue masses comprise endocrine tumors, cystic pancreatic neoplasms, and other uncommon pancreatic tumors(33).


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