Magnetic resonance imaging in lung cancer.

Magnetic resonance (MR) is based on the principle that nuclei of elements with an odd number of protons and neutrons spin and behave as magnets. When they are placed in a strong magnetic field, these nuclei align themselves in relation to the field. Ifradiowaves with a frequency specific to the nucleus and strength of the magnetic field (resonant frequency) is applied, some of the nuclei will absorb the energy of the radiowaves and change the direction of their spins to align against the field. In a fixed magnetic field, as the nuclei spontaneously return to their original direction of spin, they emit radiowaves ofthe same frequency as the one originally absorbed.’ MRI is achieved by using a nonuniform magnetic field, ie, with a known gradient, the frequency of the MRI signals emitted will enable them to be localized within the field. The strength ofthe signal is proportional to the proton density in each location. In most situations at present, clinical imaging uses hydrogen nuclei; they are most numerous and thus their signal is strongest.2’3 Two additional properties ofthe MRI signal, the T, and T2 relaxation times, depend on the local physicochemical environment ofthe hydrogen nuclei. These factors influence the rate at which nuclei will realign themselves within the magnetic field (T,) and the rate at which the emitted radiosignal decreases or decays following radiofrequency stimulation (T2). The T, and ‘F2 magnetic relaxation times vary independently in different tissues and thus affect the intensity of the MR image.’3 An MR image is a transformation of the MR intensity (strength of the emitted radiofrequency signal) at different points in the body. In spin-echo images, the pixel intensity at any given point depends on the local hydrogen density, the T, and T2 values ofthe tissue, bulk movement ofprotons within the imaging plane, and flow offluids into or out ofthe plane. Paramagnetic substances also influence the image intensity. Although the hydrogen density of most soft tissues varies over a range ofonly 20%, the T, and T2 values can vary by over 500%. In general, on spin-echo images, fat is the most intense (brightest) tissue, followed by brain and spinal cord, viscera, and muscle, in descending order of intensity.3 Fluid-filled cavities tend to be of variable intensity. Blood vessels containing rapidly flowing blood, bone, and air-filled lung are lowest in intensity.