MRI Atlas of the Abdomen (a self-guided tutorial)

MRI Atlas of the Abdomen(a self-guided tutorial) Jeff Velez HMS3 Eric Chiang, MD Gillian Lieberman, MD

Goals The purpose of this atlas is to provide students with; • an outline of the anatomy of the abdomen via MR imaging. • an introduction to how an MR image is created. • a basic understanding of how the manipulation of various parameters (TR,TE, pulse sequence) of an MR scan yield desired tissue differentiation. • a list of some basic sequences used in abdominal MR. By coupling this review of how an MR image is created and manipulated with a thorough tour of abdominal anatomy seen through MRI, this tutorial can serve as an instructive tool in preparing students for their likely future clinical encounters with abdominal MRI in evaluating and managing abdominal disease.

MRI Introduction Magnetic resonance (MR) imaging has been in widespread clinical use for well over a decade. Its use was primarily localized to the evaluation of the central nervous system and then more recently, the musculoskeletal system. Motion during the cardiac cycle , respiration, and peristalsis made MR imaging of the thorax and abdomen a major challenge. MR imaging of the abdomen started with the evaluation of solid visceral organs such as the liver and kidney. With technologic developments in MR hardware and software occurring at a swift and steady pace, MR imaging of the abdomen is beginning to expand beyond the solid viscera into the entire abdomen, including the hollow viscus of the GI tract.

MRI Basics of MRI • In order to read and understand an MR image, one must gain a basic understanding of the principles underlying its production. • MR imaging is based on the naturally occurring magnetic moment that exists within the nuclei of a hydrogen atom, as well as its ubiquitous presence in organic tissue. When an external magnetic field is applied to organic tissue, protons within hydrogen nuclei align themselves in parallel with this field and also begin to resonate. When a radiofrequency (RF) pulse is applied to these aligned protons, it provides enough energy to dislodge (or excite) them from this orientation. However, this is a temporary phenomenon, and the nuclei relax back into realignment with the external magnetic field. Upon relaxation, energy is released in the form of RF waves. This “echo” is detected and a signal of variable intensity for a given location is produced. • Tissue contrast is created because different tissues have different relaxation times. This is attributable to the different microenvironments surrounding the magnetized nuclei.

MRI 4 Key Parameters of MRI • T1 • T2 • Echo Time (TE) • Repetition Time (TR) • The relaxation times of protons shifting from a higher to lower energy level, are referred to as T1 and T2 and are tissue specific. • The TE and TR are variables that can be controlled by an MR scanner operator.

MRI T1 and T2 • T1 and T2 represent relaxation time constants. • Each tissue has a specific, inherent T1 and T2 value. • For example: fat has a short T1 and T2, whereas fluid has a long T1 and T2. • These values are measured in milliseconds. • T1 – the time it takes nuclei in a particular tissue that has been excited or “dislodged” from its parallel orientation to return to its nonexcited state. (The time when about 63% of the original longitudinal magnetization is reached). • T2 – the time it takes nuclei in a particular tissue that has been excited into a (phase coherent) transverse or perpendicular orientation to return to its non excited (non phase coherent) state. (The time when transverse magnitization decreases to 37% of the original value).

MRI TR and TE • These are two major parameters that can be adjusted (unlike T1 and T2) to create the desired tissue differentiation. • When an MR image is taken, it begins with a magnetic field being established that is parallel with the bore of the scanner. This field has a strength on the order of 1-2 Teslas, depending on the scanner. Once this is established, and protons have aligned with the field, a sequence of radiofrequency (RF) pulses are administered. This excites the protons to a higher energy level. This is then followed by relaxation back into a low energy state. This relaxation time is constant (T1 and T2). What can be changed however is the repetition time (TR) or time between administered RF pulses. What also can be manipulated is the time that the RF “echo” is received by the RF detector. This time is referred to as TE, or echo time. • By adjusting TE and TR, according to a tissue’s T1 and T2, the various tissues in a region of interest can be differentiated.

MRI T1 weighted images vs. T2 weighted images • The following 2 slides offer graphs to help explain tissue contrast on T1 vs. T2 weighted images. • These graphs are depictions of the signal intensity as function of time for two tissues types (fat and fluid) in an external magnetic field. • A helpful way to analyze these graphs is to identify which curve provides the higher signal intensity (red or blue) at the time point indicated by the dashed vertical line (detection time). That point represents the tissue that will appear brighter on the MR image. • Keep in mind that the TR and TE (along with the sequence of RF pulses) are what we can manipulate, while T1 and T2 are constant and tissue dependent. They are represented by the degree of line curvature (exponential relationship) on the graphs to follow.

MRI T1 Weighted Image—short TR and TE — fat — fluid TR = repetition time TE = echo time Signal Intensity TR TE T1 Weighted Image In this graph fat has a greater signal intensity than fluid. Tissues with short T1 and T2 (fat) will appear brighter than those with longer T1 and T2 (fluid). Although this is a gross oversimplification, when an image is T1 weighted, this means that the protocol used to scan a patient involves adjusting the TE and TR (shortening their times) in a manner that will cause tissues with fast T1 and T2 relaxation times (e.g. fat) to appear brighter.

MRI T2 Weighted Image—long TR and TE — fat — fluid TR = repetition time TE = echo time Signal Intensity TR TE T2 Weighted Image In this graph fluid has a greater signal intensity than fat. Tissues with long T1 and T2 (fluid) will appear brighter than those with short T1 and T2 (fat). • On a T2 weighted image the protocol used is one that will result in tissue with long T1 and T2 (fluid) having a higher signal intensity. This is illustrated in the following slides. • This protocol involves using a TR and TE that are relatively longer than the T1 weighted sequence.

MRI Beyond T1 and T2—Abdominal MRI • Along with the advancements in MR scanner hardware technology, developments in the pulse sequences used have led to the growing role of MRI in abdominal imaging. • The fundamental principle behind these sequences is to maximize contrast, resolution, speed, and coverage while keeping motion and noise (relative to signal) at a minimum. • A list of commonly used sequences (acronyms provided) that capture abdominal anatomy and pathology include: VIBE, HASTE, STIR, TSE, and GRE sequences. • Although a description of all of these sequences is beyond the scope of this atlas, a brief discussion of the VIBE sequence can provide an introduction to the MR parameters that are manipulated to achieve maximal contrast, resolution, speed, and coverage.

MRI Volumetric Interpolated Breath-hold Examination (VIBE) • The VIBE Sequence is T1 based (short TR and TE). • It is a complex 3D Fourier transform sequence that allows for fast acquisition time, thus reducing motion artifact and allowing for adequate coverage of the abdomen. • In a given amount of time the VIBE sequence can provide better tissue contrast by utilizing a technique known as fat saturation. • Given the relatively high resolution and coverage, VIBE sequences can be reconstructed and used for angiographic examinations. • The axial, coronal, sagittal, and selected 3D reconstructions of the abdomen to follow were performed using the VIBE sequence.

Anatomy Anatomy of the Abdomen Throughout this atlas, in axial, coronal, sagittal, and oblique 3D planes, we will highlight; • Liver • Biliary System • Pancreas • Spleen • Gastrointestinal Tract • Kidneys • Retroperitoneum • Peritoneum

Anatomy We have used images from 3 different patients: • Patient A - 32 year old female MR settings: VIBE sequence, MR abdomen Planes: Axial, coronal, and sagittal; coronal MRCP image • Patient B - 54 year old female MR settings: VIBE sequence, MRA abdomen (focused on celiac/SMA) Planes: Maximum intensity projection (MIP) 3D reconstruction • Patient C - 27 year old male MR Settings: VIBE sequence, MRA abdomen (focused on renal arteries) Planes: Maximum intensity projection 3D reconstruction

Anatomy Pt A - Axial VIBE Plate 1

Anatomy Pt A - Axial VIBE - Dome of the Liver Liver R.Ventricle Inferior Vena Cava L.Ventricle Esophagus Aorta L. Lower lobe of lung R. Lower lobe of lung Azygos v. Plate 3

Anatomy Pt A - Axial VIBE - Hepatic Veins L. Lobe of liver (lateral segment) Gastric fundus L. hepatic v. L. Lobe of liver (medial segment) M. hepatic v. Inferior vena cava R. lobe of liver (anterior segment) R. hepatic v. Aorta R. lobe of liver (posterior segment) Plate 8 Azygos v. Gastroesophageal junction Hemiazygos v. L. lower lobe of lung Spleen

Anatomy Pt A - Axial VIBE - Hepatic Divisions Plate 10 LMS LLS L. hepatic vein M. hepatic vein RAS Inferior vena cava R. hepatic vein RPS LLS—Lateral segment of left lobe LMS—Medial segment of left lobe RAS—Anterior segment of right lobe RPS—Posterior segment of right lobe The superior aspect of the liver serves as a good reference point when inspecting axial images of the liver. It can be divided into 4 segments based on the alignment of the hepatic veins draining into the inferior vena cava. The dashed line indicates the respective course of the three hepatic veins. These segments can be further divided into superior and inferior segments.

Anatomy Pt A - Axial VIBE - Splenic Hilum Plate 12 Splenic flexure The spleen is an intraperitoneal structure, enclosed by peritoneum except at its hilum where the splenic vessels enter and leave. It can be readily differentiated from the kidney by its location adjacent to the posterolateral chest wall. Important relationships of the spleen include abutment of the posterior aspect of the stomach as well as the tail of the pancreas Posterior aspect of stomach Tail of pancreas Splenic vein Splenic artery Posterior chest wall

Anatomy Pt A - Axial VIBE - Adrenal Gland and Spleen Plate 14 Gastric fundus L. portal vein Inferior vena cava Aorta R. portal vein R. adrenal gland Body of pancreas L. adrenal gland R. crus of diaphragm Spleen L. crus of diaphragm Ascending lumbar veins Vertebral body Spinal cord Ascending lumbar veins

Anatomy Pt A - Axial VIBE - Adrenal Glands Plate 15 This image illustrates the characteristic “inverted Y” appearance of the adrenal glands. The adrenal glands reside on the anteromedial and superior aspect of the kidneys.

Anatomy Pt A - Axial VIBE - Celiac Trunk Common hepatic a. Celiac Trunk Ligamentum teres Plate 17 Gastric body Hepatic a. fossa Splenic flexure Caudate lobe Body of Pancreas Portal vein L. adrenal gland Inferior vena cava Desc. colon Spleen L. kidney R. kidney Aorta

Anatomy Pt A - Axial VIBE Plate 18 Hepatic artery Portal vein Caudate lobe Inferior vena cava R. Adrenal gland (see plates 20-24) A notable anatomic relationship exists at the level of the right adrenal gland that involves a posterior to anterior sequence of structures that line up in a relatively linear fashion. These include, from posterior to anterior—R. adrenal gland, IVC, caudate lobe, portal vein, and hepatic artery.

Anatomy Pt A - Axial VIBE - Body of Pancreas Gastric body Small bowel Splenic vein Pancreatic duct L. lobe (lateral) Ligamentum teres L. lobe (medial) Neck of gallbladder Porta hepatis Portal vein Hepatic artery Inferior vena cava R. kidney Superior mesenteric artery Plate 19 Aorta Body of pancreas L. kidney Descending colon Spleen

Anatomy Pt A - Axial VIBE - Origin of SMA Gastric body Small bowel Descending colon Ligamentum teres Body of pancreas Gastric antrum Hepatic artery Neck of gallbladder Porto-splenic confluence Portal vein Neck of pancreas Splenic vein R. kidney Plate 21 Inferior vena cava R. renal vein Superior mesenteric artery Aorta L. kidney

Anatomy Pt A - Axial VIBE - Relationships of the Superior Mesenteric Artery Plate 22 Superior mesenteric artery (SMA) Body of pancreas This slide shows another important relationship that exists surrounding the SMA. There are four structure to be aware of. These include the body of the pancreas and splenic artery, which pass over the SMA anteriorly. Posteriorly, the duodenum and left renal vein cross behind the SMA. In this particular image, the transverse aspect of the duodenum is out of plane leaving a small distal portion visible. Splenic vein Distal duodenum L. Renal vein Aorta

Anatomy Pt A - Axial VIBE - Origin of the Renal Arteries Falciform ligament Ligamentum teres fissure Plate 24 Gastric antrum Gastric body Superior mesenteric vein Hepatic flexure Superior mesenteric artery Body of gallbladder Small bowel Duodenum (1st part) L. renal vein L. renal artery Head of pancreas Hilum of left kidney Duodenum (2nd part) Hilum of right kidney Inferior vena cava

MRI Atlas of the Abdomen (a self-guided tutorial)