MRI Physic Radiology

Regarding nuclear magnetic resonance imaging (MRI):

• In an external magnetic field, more hydrogen nuclei align with their magnetic

  moments parallel to the external field than antiparallel

• Protons subject to a strong static external magnetic field start to precess in phase

• The frequency of precession (Larmor frequency) of protons in a static magnetic field

  of 1.5 T equals 42.6 MHz

• At 1.5 T, the precessional frequency of hydrogen nuclei in fat is 220 Hz lower than

  that of hydrogen nuclei in water

• Apart from hydrogen, other nuclei that can be polarized in an external magnetic

  field include carbon 12C and oxygen 16O

Which of the following are true about MRI signal formation in biological tissues?

• A free induction decay (FID) signal can only occur after a 90 degree pulse

• After a 90 degree pulse, the longitudinal magnetization recovers at the same rate as the

  transverse magnetization decays

• The rate of the longitudinal magnetization recovery does not depend on the external

  magnetic field strength

• Dephasing of the transverse magnetization is mainly due to spin–spin relaxation

• The FID is not usually considered in clinical MRI

3. In magnetic resonance imaging

• Signal is generated from 12C nuclei

• Paramagnetic contrast agent is used to enhance tissue contrast

• The SI unit for main magnetic field strength is Gauss

• Scanning time is reduced by using shorter time to repeat (TR)

• Vascular pulsation artefacts occur in the phase encoding direction

Regarding magnetic resonance imaging (MRI)

• Larmor frequency is proportional to the proton density

• the flip angle depends on the applied radiofrequency strength

• the free induction decay signals are detected with gradient coils

• nuclei with odd number of nucleons are used

• the longitudinal magnetization lies perpendicular to the X-Y plane

In spin–echo (SE) imaging:

• The first radiofrequency pulse in a spin–echo pulse sequence is a 180 degree pulse

• Rephasing of the transverse magnetization is caused by the 180 degree pulse

• Effects of local field inhomogeneities on the MRI signal are eliminated

• For practical purposes, the delay between excitation and the refocusing pulse is kept constant, but signal readout can be performed at any chosen echo time (TE)

• Rephasing can only be performed once, and the whole sequence must be repeated to obtain another echo

Regarding the image quality in MRI

• increasing TR will decrease the signal intensity on T2-weighted

• decreasing pixel size will decrease signal-to-noise ratio

• image resolution is improved by increasing the matrix size

• shimming improves field homogeneity

• image quality is unaffected by TE (echo time)

Regarding MRI - Spin Echo

• TR is the time between two 90 degree radiofrequency pulses

• Signal read-out during the 180 radiofrequency pulse

• Field inhomogeneity is due to introduction of patient onto the magnetic field.

• Spin-Spin relaxation is T1 relaxation.

• T2 is the time for the MR signal to fall to 63% of its maximum value.

Concerning image contrast in spin–echo imaging:

• The best contrast between tissues with different T1s is obtained with short repetition times (TRs) and short echo times (TEs)

• T2 weighting is obtained with short TEs and long TRs

• An image obtained with a TE of 15 ms and a TR of 2000 ms would be proton density (PD) weighted

• Fat appears with a high signal on T1-, T2- and PD-weighted sequences

• The fat signal is partially suppressed on fast (turbo) spin–echo imaging

Regarding spatial encoding in standard (i.e. non-fast/turbo) spin–echo imaging:

• The slice-selection gradient is usually applied just before the 90 degree and 180 degree radiofrequency (RF) pulses

• A phase-encoding gradient is usually applied along the larger dimension of the imaged object

• The entire phase-encoded information is obtained during a single readout

• A frequency-encoding gradient is applied together with the phase-encoding gradient

• A spin–echo sequence requires a rephasing gradient applied after the slice-selection gradient

 Regarding MRI

• 1. Tissue with higher proportion of free water has longer T2 relaxation time

• 1. Lamour frequency is inversely proportional to strength of external magnetic field.

• 1. The central area of K space represent contrast of image

• 1. Chemical shift artefact can be reduced by increasing receiver bandwidth

• 1. Higher gradient amplitude increase the slice thickness