Magnetic resonance imaging of the head and neck

Computed tomography (CT) and magnetic resonance imaging (MRI) are used to detect pathology of the head and neck. Although CT still has a superior role in detecting calcifications, bony pathology, fractures, and acute hemorrhage [1], MRI is now the preferred imaging method for head and neck pathology. MRI allows superior tissue discrimination and fat- and water-suppression imaging. Fat-suppression imaging has greatly improved the ability to detect abnormal contrast enhancement in the head and neck region. Without this technique, abnormal contrast enhancement appearing as a bright area may be hidden by the very bright signal of fat. Very rapid MRI techniques now allow angiographic images and kinescopic (real-time) depiction of movement. Injectable contrast agents (gadopentetate) aid in the differentiation of certain tumor types and vascular lesions. Albumin-tagged agents which allow direct blood-volume calculations have been developed and are now pending FDA approval. Injectable MR agents, specifically for head and neck imaging, are also currently under development. MRI, which uses nonionizing radiation from the radio-frequency (RF) band of the electromagnetic spectrum, functions by taking advantage of the weak magnetic properties of human tissue. The patient is placed inside a large magnet with a uniform magnetic field. When the hydrogen nuclei in the patient’s body are exposed to this magnetic field, they align with the field. A sequence of RF pulses is then applied to the area of the body being evaluated; this signal must be at the resonant frequency so that the body’s proteins can absorb it. These pulses create a transient magnetic field that is perpendicular to the main field; when exposed to this field, the hydrogen nuclei in the body absorb energy and change the direction of their axis of rotation. When the RF pulse is terminated, the nuclei again realign themselves with the external magnetic field. Energy is released from the tissue as weak RF signals, which are received by coils in the MRI device. The signals are rendered detectable by the application of an RF pulse that tips the energized proton down by 90 to be detected as a current by the receiving coil [2–4]. Subjecting these signals to a series of computer operations translates the signal into an image. The signals emitted from the excited proton have two components, which are referred to as T1 and T2 relaxation times. T1 is defined as the spin-lattice relaxation time, reflecting the longitudinal axis, and decays at a faster rate than the T2 relaxation time. This is the energy expended by the excited proton to the lattice that contains the proton. As the excited proton is deflected by the 90 RF pulse toward the receiving coil and partially recovers to its free steady state during the early phase of read-out, the signal is dominated by the T1 relaxation time. T2 is defined as the spin-spin relaxation time and reflects the vertical axis. This is the energy expended between the various protons that have been energized.