Magnetic Resonance Imaging

Physics

MRI is based on the magnetization properties of atomic nuclei. A powerful, uniform, external magnetic field is employed to align the protons that are normally randomly oriented within the water nuclei of the tissue being examined. This alignment (or magnetization) is next perturbed or disrupted by introduction of an external Radio Frequency (RF) energy. The nuclei return to their resting alignment through various relaxation processes and in so doing emit RF energy.

Sequences

• high signal intensity = white
• intermediate signal intensity = grey
• low signal intensity = black

T1

”Anatomical sequence”

The dominant signal intensities of different tissues are:

• fluid (e.g. urine, CSF): low signal intensity (black)
• muscle: intermediate signal intensity (grey)
• fat: high signal intensity (white)
• brain
  • grey matter: intermediate signal intensity (grey)
  • white matter: hyperintense compared to grey matter (white-ish)

T1 Gadolinium enhanced

Pathological tissues (tumours, areas of inflammation/infection) will demonstrate accumulation of contrast (mostly due to leaky blood vessels) and therefore appear as brighter than surrounding tissue. Often post contrast T1 sequences are also fat suppressed (see below) to make this easier to appreciate.

T1 fat suppressed

Fat suppression (or attenuation or saturation) is a tweak performed on many T1 weighted sequences, to suppress the bright signal from fat.

T2

T2 weighted (T2W) sequences are part of almost all MRI protocols. Without modification the dominant signal intensities of different tissues are:

• fluid (e.g. urine, CSF): high signal intensity (white)
• muscle: intermediate signal intensity (grey)
• fat: high signal intensity (white)
• brain
  • grey matter: intermediate signal intensity (grey)
  • white matter: hypointense compared to grey matter (dark-ish)

T2 fat suppressed

In many instances one wants to detect oedema in soft tissues which often have significant components of fat. As such suppressing the signal from fat allows fluid, which is of high signal, to stand out. This can be achieved in a number of ways

MRI

• MRCP and MRI rectum - non con
  • Pineapple juice given for MRCP - negative contrast agent which suppresses the fluid in the stomach
• Contrast - always T1
  • Extra-cellar
    • Gadolinium
    • Distribute to oedema and inflammation just like CT
  • Primovist
    • Hepatobillary agent
    • Taken by hepatocytes then excrete it into the bile
    • Useful to look for bile leaks