BIO Magnetic resonance imaging (MRI) In magnetic resonance imaging (MRI), a patient lies...

BIO Magnetic resonance imaging (MRI) In magnetic resonance imaging (MRI), a patient lies in a strong 1- to 2-T magnetic field u B oriented parallel to the body. This field is produced by a large superconducting solenoid. The MRI measurements depend on the magnetic dipole moment um of a proton, the nucleus of a hydrogen atom. The proton magnetic dipoles can have only two orientations: either with the field or against the field. The energy needed to reverse this orientation (“flip” the protons) from with the field to against the field is exactly ΔU = 2µB (like the energy needed to turn a compass needle from north to south). The pulse of a radio frequency probe field irradiates the patient’s body in the region to be imaged. If this probe field is tuned correctly so that its energy equals the ΔU = 2µB needed to reverse the orientation of the protons from with the external B field to against it, a reasonable number of protons will flip. When the protons return to their initial orientation and a lower energy state, they emit this same radio frequency radiation in different directions. This radiation is detected and provides a measure of the concentration of protons in the region irradiated by the probe field. The proton concentration differs in fat, muscle, and bone tissue, and in healthy and diseased tissue. Thus, the probe signal makes an image of the tissue type in each local region. The MRI image of an internal body part is made by adjusting an auxiliary magnetic field, which varies the external B field over the region being examined so that the probe field energy equals the flipping energy ΔU = 2µB in only a small area of the body. A measurement is made at that point. The external magnetic field is then adjusted to flip protons in a neighboring small area of the body. Continual shifts in the magnetic field and detection of proton concentrations at different tiny locations produce a map of proton concentration in the body. The MRI image of the lower back in Figure 17.37 indicates an L45 disk that has partially collapsed— it has lost water and because it contains fewer protons produces a darker MRI image.

Why might a herniated disk projecting slightly out from between two vertebrae look different in an MRI image than a non herniated disk?

(a) The vertebrae adjacent to a herniated disk are closer than vertebrae beside a nonherniated disk.

(b) There is a different concentration of hydrogen atoms in bone and in disks.

(c) Protons in the herniation produce an image that can be seen.

(d) b and c

(e) a, b, and c