Information Technology Reference
In-Depth Information
Many clinical MRI protocols exist that have trade-offs in diagnosis of various
backbone abnormalities. The technician has four main parameters to tune before
MRI acquisition that control the appearance (intensity) of the resulting image: (1)
proton density, (2) longitudinal relaxation time (T1), (3) transverse relaxation time
(T2), and (4) the
flow. The proton density refers to the concentration of protons in
the tissue in the form of water and macromolecules (proteins, fat, etc.). Both T1 and
T2 relaxation times de
fl
ne the way that the protons revert back to their resting states
after the initial RF pulse. The most common effect of the
fl
flow is the loss of signal
from rapidly
flowing arterial blood.
Two common pulse sequences for MR imaging are widely used: T1- and T2-
weighted spin-echo sequences. The T1-weighted sequence uses a short Repetition
Time (TR) and a short TE (Echo Time) (TR
fl
≤
1,000 ms, TE
≤
30 ms). The T2-
weighted sequence uses a long TR and long TE (TR
80 ms).
Moreover, two major techniques are used for suppression of fat signals in MRI:
Short Tau Inversion Recovery (STIR) and selective partial inversion recovery
(SPIR). The STIR sequences suppress fat signal by using an initial 180
≥
2,000 ms, TE
≥
radiofre-
quency pulse to invert the longitudinal magnetization. Image acquisitions are then
performed with the inversion time equivalent
°
to the known null point for fat
×
(approximately 0.69
T1) [
92
]. SPIR is a more recent fat-suppression technique
that is based on the use of frequency-speci
c pulse sequences [
93
]. Only the fat
magnetization pulse is inverted leaving water resonances as is. This technique is
useful for suppressing any tissue-speci
c pulse given the known-frequency of that
tissue. However, the SPIR technique is extremely sensitive to the magnetic
eld
inhomogeneity. SPIR is used with both T1- and T2-weighted MRI [
44
].
Furthermore, another important MRI sequence generator that is related to
common current clinical MRI is called MR Myelography [
57
] (Myelo is a new
Latin word, from Greek muelos, which means spinal cord). In this method, the
background signal is suppressed by using heavily T2-weighted fast spin-echo pulse
sequences and obliterating fat signal by pre-saturation. The resulting slices are then
projected into a composite image using a standard maximum intensity projection
(MIP) algorithm [
57
].
It is worth mentioning that inter-observer variability exist in lumbar diagnosis
similar to many diagnosis tasks from various imaging modalities including X-ray
radiographs, MRI, CT, Single-Photon Emission Computed Tomography (SPECT),
and High Resolution (HR). However, MRI shows high inter-observer reliability
compared to plain radiographs in lumbar area diagnosis (e.g., [
62
]). Mulconrey
et al. [
70
] showed that abnormality detection for degenerative disc and Spondyl-
olisthesis with MRI has
= 0.728, respectively, which is considered
high in showing inter-observer reliability where this reliability is considered perfect
when 0.8
κ
= 0.773 and
κ
≤
κ
≤
1.
