An in depth summary of the "Biomedical Imaging (BMs23 course)". This summary contains all material discussed in the course and you should easily pass the exam using this. In addition, this summary can be used outside this course for a clear and complete description of MRI, ultrasound, PET, SPECT, X...
- Echo time (TE) & Repetition time (TR)
- Interpreting TR & TE times and their effects on contrast
- Tissue contrasts in MRI
- Additional remarks regarding TE, TR, T1 time, and T2 time
- Paramagnetic substances and magnetic field disturbances
- MRI in prostate cancer
2. ULTRASOUND
- Sound and sound waves
- Transducers
- Creating an image
Magnet
Superconductive coiled alloy in a copper matrix, charged with
electrical current (1 to 7 Tesla). Liquid cooling with helium (4.2
Kelvin) for effortless electricity flow. The main magnetic field is
called B0.
As a result of the magnetic field, protons will spin perpendicular to the field orientation
(‘rechterhandregel’) at a specific frequency called the ‘Larmor’ frequency.
This is given by: f0 = gamma * B0
- f0 is the precession (larmor) frequency (Mhz)
- gamma is the gyromagnetic ratio (Mhz/T)
- B0 is the magnetic field strength (T)
The protons can exist into two energy states, a low energy state (spin-up) and a high energy state
(spin down). The difference of energy between the two is given by: deltaE = gamma * h * B0
- gamma is the gyromagnetic ratio (Mhz/T)
- h is the Planck’s constant (6.626 x 10^-34)
- B0 is the magnetic field strength (T)
There are more than one protons in a system. Protons in high/low states cancel each other out. The
ratio of proton high vs low is given by: N+/N- = exp[-deltaE/ (k *T)]
- deltaE = gamma * h * B0
, - k is Boltzmann constant (1.381 x 10^23 J/K)
- T is temperature in Kelvin (K)
In MRI you only observe the global (netto) magnetization. The net magnetization (M) is the averaged
sum of many individual quantum spins as a vector.
Radiofrequency coils (RF)
RFs are either transmitter or receiver, or both. They
transmit/receive radio frequencies perpendicular to the
main magnetic field. Using a RF coil, a perpendicular field
(B1) can be applied to create torque on the original M
vector. With continued application of the B1 field, M
precesses at continually larger angles away from B0 (flip
angle). Important in this is that the B1 field frequency
should be the same as the Larmor frequency (Swing
Analogy).
A coil perpendicular to the main magnetic field (B0) can also record fluctuations in magnetization.
This is because a change in magnetic field, due to the precessing magnetization vector, induces a
current in the receiving RF coil. Because the coil is perpendicular to the B0 field, we record changes in
transverse magnetization.
When we terminate B1, the magnetization relaxes to its original state M0. This relaxation is
characterized into 3 plans (x,y,z). With z being parallel to the main magnetic field (so perpendicular to
RF pulse).
Transverse relaxation can be quantified with
T2, while longitudinal relaxation is quantified
with T1. T2 is at 37% point of transverse
relaxation and T1 is at 63% of longitudinal
relaxation. T1 is always longer than T2, since it
represent the timescale for the relaxation of
the longitudinal component, which involves
the relaxation of spins in the transverse plane.
T1 and T2 differ for tissues since they depend
on local macromolecules (this is why MRI is
excellent in soft-tissue). The differences in relaxations determines the contrasts in MRI.
Measuring T2 (transverse)
Transverse relaxation is due to dephasing of spins.
As seen in the picture, spins diphase, which
results in loss of Mx,y components of the
magnetization. Dephasing is caused by interaction
with the local environment. Since no magnet is
perfect, inhomogeneities occur, which further
decrease T2 into its observed T2* value. So T2 is
the original true T2 of the tissue, while T2* is less
than T2, because of inhomogeneities in the
magnetic field.
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