Jeffrey C. Weinreb

SOURCES OF CONTRAST AND PULSE SEQUENCES

To use magnetic resonance imaging (MRI) as a clinical tool, it is necessary to understand why the various tissues appear as they do on the MR image and why the cortical bone is always black while the bone marrow usually appears white. This chapter discusses a multiplicity of factors on which the appearance of tissue on the MR image depends. The greater the amplitude or strength of the signal, the whiter or brighter the pixel; the lower or weaker the amplitude, the blacker the pixel. In the MR vernacular, a bright white area on an image is said to demonstrate high signal intensity, whereas a dark area is referred to as an area of low signal intensity. In biologic tissues, the concentration of mobile protons is related primarily to water content and secondarily to lipid content. Images in which the signal intensity primarily reflects the proton density of various tissues are called “proton density” or “spin density” images. Because the signal intensity is directly proportional to the proton density, the gas and cortical bones demonstrate extremely low signal intensity and appear black on all MR images. The flow dramatically influences the appearance of blood vessels on MR images. It can also influence the appearance of other moving fluids in the body—such as cerebrospinal fluid and ascites.

BASIC IMAGE INTERPRETATION

This chapter presents an approach to magnetic resonance (MR) image interpretation and discusses the evaluation of signal intensity of different tissues on spin echo images. With MR imaging, an operator has to make many choices before performing a study. The selection of the appropriate RF coil, slice thickness, field of view, matrix size, and number of signal acquisitions are important decisions to be taken in this regard. All of these decisions are interrelated, the most important being which pulse sequence to use. The selection of a pulse sequence depends not only on the part of the body being imaged but also on the type of disorder being evaluated. Before the selection of the appropriate pulse sequence, the presumed abnormality must be identified from the patients history and physical exam. Once this is accomplished, a pulse sequence that optimizes the contrast between the suspected abnormality and contiguous tissues can be selected. For example, the pulse sequence that is perfectly appropriate for detecting prostatic carcinoma may be different from the best pulse sequence to evaluate spread of the same prostatic cancer into pelvic fat.

ADRENALS AND KIDNEYS

This chapter discusses the role of magnetic resonance imaging (MRI) in adrenals and kidneys. MRI can depict normal and abnormal adrenal glands at a rate comparable to that of computed tomography (CT). As there is natural contrast between signal intensity of the gland and lack of signal from blood vessels, intravenous contrast medium is not needed to differentiate between the gland and adjacent veins. Although the spatial resolution of MRI is inferior to that of CT, and tiny adrenal lesions can go undetected, most significant adrenal masses are large enough to be readily detected with MRI. MRI is rarely the primary imaging modality for the evaluation of the kidneys. Intravenous urography and CT scanning with intravenous contrast medium have been the best methods for the evaluation of the collecting systems, for detection of renal masses, and for determination of renal functions. MRI has many such capabilities. It graphically displays the kidney and surrounding structures. Simple cysts can be differentiated from other masses because of smooth outline, thin uniform margins, sharp interface with renal parenchyma, homogeneous internal content, and prolonged relaxation times of a pure fluid. However, MRI has several potential advantages. No iodinated contrast medium is necessary to identify the main renal arteries and veins. This means that one can evaluate vascular patency in patients with poor renal function or contrast medium allergies, who would not ordinarily be candidates for contrast-enhanced CT.

MRI OF THE LIVER AND SPLEEN

This chapter discusses the limited role of magnetic resonance imaging (MRI) in splenic diagnosis. As the liver moves during respiration, MR imaging of the liver and spleen is subject to respiratory ghost artifacts and as image unsharpness. Ideally, the imaging sequence used for the liver and spleen would eliminate both the ghosts and image unsharpness. To eliminate both these problems, these images must be obtained within one breath hold, that is, with rapid MR imaging. Most hepatic lesions demonstrate relatively prolonged Tl and T2 relaxation times, and they cannot be accurately differentiated with MRI. There are, however, several exceptions to this rule. With simple proton MRI, fatty infiltration of the liver cannot usually be detected. However, with a technique called chemical shift imaging, separate images of the fat and water protons can be obtained, and MRI then becomes a very sensitive method for detection of hepatic fat.

CHAPTER 12 – MUSCULOSKELETAL SYSTEM

Publisher Summary This chapter discusses the role of magnetic resonance imaging (MRI) in the evaluation of the musculoskeletal system. A major advantage of MRI as compared to computed tomography (CT) is a remarkable degree of soft-tissue contrast. Densities of adjacent soft tissues—such as tumor and muscle—are very similar on CT, but their intensities may differ by a lot on MRI. This feature allows for better detection and delineation of soft-tissue abnormalities with MRI. As MRI is performed without ionizing radiation or injection of contrast material, it may be distinctly advantageous in patients who are young, who require sequential examinations to follow the course of disease, or who cannot tolerate contrast material. MRI shows the presence or absence of vascular involvement even without the injection of the intravenous contrast material. Although sagittal and coronal reformation can be accomplished with CT, the information content is always reduced compared to that provided by the direct transaxial images. In contrast, MRI can generate images of equal resolution in any plane. This capability permits a distinct definition of the craniocaudad extent of the lesion, which is critical when the extent of a tumor determines the level of amputation. It also gives a more accurate assessment than transaxial images of epiphyseal and joint involvement.

CHAPTER 11 – PELVIS

Publisher Summary This chapter discusses the role of computed tomography (CT) and magnetic resonance imaging (MRI) in pelvis. MRI is extremely useful in the evaluation of the pelvis because of its ability to do many projections, its soft tissue resolution, and the fact that it does not require ionizing radiation. A thorough knowledge of pelvic anatomy and anatomic relationships of the pelvic viscera is required. Motion is not a major problem in the pelvis, but the bowel can simulate or hide masses and cause motion artifacts just as it does in the abdomen. CT has been the most useful noninvasive technique for evaluating abnormal lymph nodes in the pelvis. However, this usually requires injection of the intravenous contrast medium to differentiate between adenopathy and blood vessels. MRI is comparable to CT in its ability to depict lymphadenopathy; however, unlike CT, it does not require use of the intravenous contrast medium. MRI offers a noninvasive technique for an evaluation of the prostate. Because of its excellent contrast resolution and multiplanar capabilities, it may be more accurate than CT and ultrasound for the delineation of neoplastic extension. MRI probably has the same accuracy as CT for the detection of lymph node involvement, and it appears that MRI, like CT and ultrasound, will not permit a specific diagnosis of benign or malignant prostatic disease.

CHAPTER 8 – ABDOMEN

Publisher Summary This chapter discusses some factors affecting the quality of abdominal magnetic resonance imaging (MRI) and describes the uses of MRI in relation to some specific abdominal organs—such as the pancreas and bowel. The main factor that limits the application of MRI in the abdomen is physiologic motion. Unlike the computed tomography (CT) scan, in which each section is scanned in just a few seconds with standard spin echo pulse sequences, it takes about two to five minutes to obtain an MR image of acceptable quality. This prolonged data acquisition time is usually not detrimental to MR scans of stationary body parts, such as the extremities, but it significantly degrades images of the abdomen. Motion causes two types of problems on an MR image: (1) image unsharpness and (2) ghost images. Image unsharpness is because of structures being in different places at different times during data acquisition. The gastrointestinal tract is also difficult to evaluate with MRI, owing to motion and the lack of a good gastrointestinal tract contrast agent. Bowel contractions are less of a problem if intravenous glucagon is administered to relax the bowel muscle during the examination, but it is not likely that MRI can supplant radiographic contrast studies and CT for evaluation of the GI tract.

CHAPTER 2 – BASIC PRINCIPLES

Publisher Summary This chapter discusses how a magnetic resonance (MR) image is made. MR image is a two-dimensional pictorial representation of a slice through the body. Each slice is made up of a large number of two-dimensional picture elements called pixels. There are as many as 262,144 pixels in each image, and each pixel typically measures about 1 mm × 1 mm. The key to forming an MR image is the introduction of weak gradient magnetic fields. A magnetic field gradient is a variation of field strength along a given direction, which is superimposed on the powerful magnetic field of the MRI system. A gradient is designed so that it adds to the main magnetic field at one side, subtracts from it at the other side, and varies smoothly in between. In MR imaging devices, there are three sets of gradients that create magnetic field gradients in the X, Y, and Z axes. If the three gradients are all applied simultaneously, the magnetic field experienced by a specific proton would be the sum of the three gradients, and this sum might be the same at different points in the patient.

CHAPTER 6 – VASCULAR SYSTEM

Publisher Summary The signal from flowing blood can be bright, dark, or anywhere in between, and its appearance depends upon a multitude of flow factors and magnetic resonance imaging (MRI) variables. The flow factors include the direction of flow, the motion characteristics of flow, and the spatial distribution of velocities and accelerations across the lumen of the vessel. The relevant MRI factors include the type of pulse sequence, the pulse repetition time and the echo delay time, the position of a section in a multi-section sequence, and the spatial direction and type of magnetic imaging gradients. This chapter discusses a few rules of thumb to identify different flow effects on MR images. Flowing blood is not always dark on MR images; it can also have an increased signal. It is particularly important to learn to recognize normal-flow-induced signal in blood vessels so that it is not mistaken for a thrombus or a pathologic flow state. The chapter describes several causes for paradoxical enhancement, including flow-related enhancement, even echo rephasing, and diastolic pseudogating.

CHAPTER 7 – CHEST

Publisher Summary Computed tomography (CT) is an accurate practical imaging technique for the assessment of the chest. It can detect abnormal mediastinal and pulmonary masses and can differentiate between vascular, fatty, fluid, and other soft tissue masses. CT does have some limitations in the evaluation of diseases in the chest. Frequently, a large volume of iodinated contrast medium must be injected to differentiate between the hilar vessels and adenopathy. Some aspects of the mediastinum are difficult to study with the transaxial images. This chapter discusses the anatomy of the chest and describes the techniques for magnetic resonance imaging (MRI) of the mediastinum and hili. For this purpose, it is often advantageous to gate image acquisition to the electrocardiogram (ECG). Cardiac gating is accomplished by monitoring a patients ECG and initiating the MR imaging sequence based on the occurrence of the R wave. This significantly increases resolution of the heart, mediastinal great vessels, pulmonary hili, and mediastinal or lung masses abutting the heart by eliminating the image degradation caused by cardiac motion.