Magnetic Resonance Imaging: Hardware and Data Acquisition
A system for performing magnetic resonance imaging requires several fundamental hardware components to generate, encode, and receive the signals that are transformed into an image. First, a strong magnet generates a uniform static magnetic œeld over a large volume of space in which the imaged object is placed. Second, a magnetic œeld gradient system (composed of gradient coils and gradient ampliœers) creates local variations in the magnetic œeld to encode spatial information. Finally, radiofrequency (RF) transmit hardware (composed of an RF ampliœer and RF transmit coil) injects energy into the resonating nuclei within the imaged sample. A£er excitation, RF energy is emitted back from the imaged object and detected by RF receive hardware. še received analog RF signal is digitally sampled and stored by a computer-controlled spectrometer until enough data are acquired to reconstruct an image. In addition to the magnet, gradients, and RF components, a modern conventional MRI scanner includes other hardware components such as those shown schematically in Figure 3.1. Typically, three separate interconnected computers control the scanner (shown in gray boxes in Figure 3.1). še operator uses a high-powered computer workstation and viewing console for image display, scan planning, and other operator input of scanning control parameters that are sent/ downloaded to the spectrometer. A modern spectrometer usually is itself a high-speed digital computer running a realtime operating system to allow precise pulse and waveform
generation to output to the gradient and RF hardware, and to enable precise data acquisition timing. Acquired data samples are received by a high-speed image reconstruction computer with a large memory capacity to enable fast image generation even with large data matrices. Physiological monitoring hardware is frequently used to observe, record, or synchronize cardiac, respiratory, or other physiological signals coming from the imaged subject. Not only are the physiology signals displayed on the operator’s console, but they may also be used by the spectrometer to trigger or synchronize the MRI scanning process . Most scanners employ a mechanical patient table and a precise landmarking system, e.g., laser alignment cross hairs for patient positioning. Figure 3.1 also shows the components of an active magnetic shimming system, including shim ampliœers and shim coils, needed to improve the homogeneity of the static magnetic œeld on most high œeld scanners. For longer-term storage of images, the viewing console is typically connected to a mass storage device, archiving mechanism for burning of CD/DVD media, and a local area network (LAN) to reach a picture archiving and communication system (PACS). Other typical hardware components not depicted in Figure 3.1 include a cryogen cooling system that is required when using a superconducting magnet, magnetic shielding to decrease the e›ect of the fringe magnetic œelds in the area surrounding the MRI magnet, and RF shielding of the MR scanner suite to isolate the system from external sources of RF interference.