Question

In relation to specialized MRI techniques:

a. Perfusion brain imaging is based on the differences between the magnetic properties of oxy- and deoxyhaemoglobin

b. Brain perfusion imaging and functional imaging are usually performed with fast T1-weighted sequences

c. Areas of increased brain activity in functional BOLD imaging demonstrate a reduction in magnetic resonance (MR) signal

d. MR spectroscopy requires a strong magnet that provides a uniform magnetic field

e. MR spectroscopic images of 1 H, 31P and 13C can easily be obtained in vivo

Answer 1

a. False. This forms the basis of brain oxygenation level dependent (BOLD) functional imaging (see below). Perfusion imaging is based on a susceptibility effect (shortening of T2 time upon first passage of an exogenous contrast agent) or on arterial spin labelling (inversion of the longitudinal magnetization of blood upstream of the investigated organ).

b. False. Sequences sensitive to magnetic susceptibility, i.e. T2 weighting, are used.

c. False. The BOLD effect relies on different magnetic characteristics of oxyhaemoglobin (diamagnetic) and deoxyhaemoglobin (paramagnetic). When the metabolic activity of the brain increases, there is a slight increase in deoxyhaemoglobin production but a disproportionately larger increase in blood flow (due to neurovascular coupling) carrying oxyhaemoglobin. The net effect is an increase in MR signal on T2 -weighted sequences due to the increase in diamagnetic oxyhaemoglobin over the resting-state level of paramagnetic deoxyhaemoglobin.

d. True. It is based on measuring chemical shift (changes in resonance frequency of protons in different compounds), which is proportional to the strength of the external magnetic field. The field must be uniform to better than 1 ppm.

e. False. While 1 H and 31P spectroscopy is possible, spectroscopic imaging of 13C is impractical due to the low signal-to-noise ratio and natural abundance. In the future, imaging of 13C may be possible using hyperpolarization techniques.