Quantitative T2 Magnetic Resonance Imaging

The exceptional soft tissue contrast of which
MRI benefits, makes it an important tool for
medical diagnosis. Not only the contrast itself,
but also the possible quantification of relaxation
times T1 and T2 causing this contrast is
of interest. This quantification has proven to be
clinically useful in the context of neurological
diseases such as schizophrenia, autism, Parkinson’s
disease and many others.
The principle method used to quantify
transverse relaxation time T2 is the spin echo
(SE) sequence which takes rather long. T2 quantification
for medical diagnosis is not often used
because of this. A recently developed T2 quantification
method, driven equilibrium single pulse
observation of T2 (DESPOT2) offers the possibility
of volumetric T2 quantification within a
clinical acceptable time with a resolution of less
than 1 mm isotropic. The DESPOT2 method
uses two balanced steady state free precession
(bSSFP) acquisitions and prior knowledge of T1
to determine T2.
The bSSFP acquisition on brain tissue
is known to be magnetization transfer (MT)
sensitive. Within this thesis’ chapter 2, the effect
of MT on the observed T2 by DESPOT2 is
investigated, and the outcome compared to the
SE observation of T2. Within this chapter it is
presented that MT reduces the observed T2 and
that this reduction can be avoided by the use of
elongated excitation pulses.
The introduction of elongated RF excitation
pulses introduces finite pulse effects: magnetization
decay during part of the RF excitation
pulse. Since the DESPOT2 method is based
on a theory assuming instantaneous excitation,
the observed T2 calculation in this case contains
a flaw which error size depends on the RF pulse
duration. In chapter 3, the finite pulse effect
on the DESPOT2 T2 calculation is investigated
and a correction for this effect is presented.
The DESPOT2 theory with incorporated finite
pulse effect allows the observation of T2 to be
independent of the RF pulse duration.
Although it is now possible to acquire
MT free bSSFP images and calculate the T2
with the DESPOT2 method without the finite
pulse effect manipulating the observed T2 value,
the DESPOT2 method still underestimates the
T2 compared to the T2 observed by SE. In chapter
4 it is shown that this underestimation is
caused by the microscopic complexity of brain
tissue which is overlooked by the observation of
a single T2. Within the limit of a single pool the
two methods observe approximately identical
T2 values since the single pool model on which
both methods are based is restored. In brain tissue,
the pool fractions are not approaching this
limit and therefore the T2 observed by the two
methods is different. Within the SE observation,
T2 does not depend on the echo spacing as is
commonly thought; however, the time span over
which the T2 decay is sampled should be longer
than the T2 observed. The DESPOT2 observation
depends strongly on the flip angles used;
however, as long as both flip angles remain <<90°
the T2 observed is always lower than that observed
by SE. Further, the difference between
the two methods has shown to be depending
stronger on the fractional pool sizes than on the
exchange rates.
Although the MT effect within the bSSFP
acquisitions can be avoided by elongated RF
excitation pulses and the thereby introduced
finite pulse effects corrected within the DESPOT2
T2 calculation, the DESPOT2 method
still overlooks the microscopic complexity of
brain tissue. Because of this, an underestimation
of T2 compared to SE T2 observations occurs,
of which the amount depends on the fractional
pool sizes and the exchange rates.