Mr Imaging of the Pancreas

Abstract

Magnetic resonance (MR) imaging has been a clinically useful tool since the mid-1980s. MR imaging technology offers the possibility of obtaining images that demonstrate exceptional soft tissue contrast resolution without the use of ionizing radiation. Image contrast in MR imaging results from the varied behavior of tissues in a magnetic environment and capitalizes on the abundance of hydrogen atoms inherent to human tissues.

The natural motion of hydrogen protons produces tiny magnetic fields. After being placed in the large magnetic field generated by the MR instrument (typically 1.5 T), the hydrogen atoms within the tissues of an individual are converted to an equilibrium state that is amenable to an energy exchange. That energy exchange occurs as the phenomenon of magnetic resonance after the application of appropriate radiofrequency pulses.

In MR imaging, tissues can be distinguished based on two distinct properties related to the manner in which they return to the equilibrium state established by the large external magnetic field. The rates of return to equilibrium, or relaxation times, characterize a given tissue and are measured in the form of signal intensities. These signal intensities are then mapped into a brightness scale for image display.

The two main relaxation times that characterize tissues are called T1 and T2. While expressing different features of the tissue, these relaxation phenomena occur simultaneously and at different rates. Radiologists are able to influence the image contrast that is expressed from the tissues by manipulating certain image acquisition parameters. With certain manipulations, T1 phenomena can be emphasized to yield an image that is T1 weighted. Images with T1 contrast generally show simple fluid with a low signal intensity. Alternatively, the T2 features of tissues can be emphasized to yield an image that is T2 weighted. Images with T2 contrast generally show simple fluid with a very high signal intensity. An MR examination sent to a referring physician may contain images obtained with various pulse sequences to accentuate these and other properties. Furthermore, each pulse sequence can be acquired in multiple planes. Thus, it is not uncommon for referring physicians to receive large sets of films with a wide range of image features. These different perspectives are complementary, and each provides unique diagnostic information.

Initial MR applications largely focused on the relatively static anatomy of the CNS and musculoskeletal system because high-quality images required more time to complete than could be achieved during a single breath hold. Abdominal imaging remained the purview of CT scanning, in which imaging could be performed in seconds.

Advances in MR technology, such as high-performance gradient systems, faster computers, and the design of efficient signal detectors (also called surface coils),9, 18 have resulted in much faster acquisition times for T1- and T2-weighted images. It is now possible to obtain large-volume data sets in the time frame of a breathhold or alternatively, other MR imaging strategies can be used to acquire each image in 1 second or less, much like CT scanning.

MR acquisitions can be classified as spin echo (SE) or gradient-recalled echo (GRE). SE imaging is usually the more time-consuming of the two, whereas GRE imaging often can be accomplished in the time frame of a breath hold. Rapid SE techniques achieve marked reduction in imaging times and are particularly useful for obtaining T2-weighted images.

In addition to improved imaging speed, the diagnostic capacity of MR imaging has expanded with the development and implementation of various MR contrast media. Augmented soft tissue contrast can be achieved and tissue perfusion data can be obtained with exogenous contrast media in MR imaging. Gadolinium (Gd) chelates (diethylenetriamine-penta-acetic acid [DTPA] being most commonly used) have had the widest clinical application.

The pharmacokinetics and value derived from using Gd chelates are analogous to those obtained from the use of iodinated contrast during CT scanning. Gd is freely filtered by the glomeruli and can enhance parenchymal organs and vessels, increasing tissue contrast and signal from vascular structures. Importantly, Gd is used in relatively small doses (0.1 mmol/kg) without concern for contrast medium–induced nephrotoxicity.31, 37 From the perspective of other adverse reactions, there is an extraordinary safety profile.³¹ The introduction of MR-compatible power injectors bestows precision to the administration of contrast media. When coupled with rapid imaging, such precision yields MR angiograms with outstanding results.