Quantitative magnetic resonance methods for in vivo investigation of the human liver and spleen. Technical aspects and preliminary clinical results.
Thomsen C.: Acta Radiol Suppl 1996;401:1-34
University Hospital, Copenhagen, Denmark.
This project was initiated
with the introduction of magnetic resonance (MR) in Denmark in
order to evaluate the possibilities of this technique as a diagnostic
aid in non-focal liver and splenic diseases. The signal intensities
in the MR image are sensitive to the longitudinal relaxation (T1),
the transverse relaxation (T2), flow and chemical shift. All these
parameters may be quantified by developing specific pulse sequences
sensitive to the parameter in question. Previous studies had indicated
that relaxation time measurements might be of value in the diagnosis
of liver cirrhosis and haemochromatosis. Measuring relaxation
times in these 2 groups of patients posed different challenges.
In patients with liver cirrhosis a method had to be developed
for simultaneous T1 and T2 relaxation time measurements, which
was robust to the respiratory motion of the liver. A combination
of multi-echo pulse sequences with different repetition times
was chosen, because motion effects were partly refocused. Multi-acquisition
was used to improve the signal-to-noise ratio in the heavily saturated
experiments with short repetition times, to further reduce the
sensitivity to motion. To test the quality of this pulse sequence,
phantom experiments were performed, and sensitivity to motion
was tested by measuring with and without respiratory synchronization.
Respiratory synchronization gave a marked improvement in focal
liver diseases, whereas no difference was found in non-focal diseases.
Standard imaging sequences with a minimum echo time of 30 ms could
not be used to measure the short T2 relaxation times found in
patients with increased liver iron. A volume-selective multi-echo
spectroscopic pulse was developed with a minimum echo time of
4 ms. Biexponential signal decay could be shown in patients with
increased liver iron by using this sequence. Patients with liver
cirrhosis, as a group, had increased T1 relaxation times compared
to normal volunteers, but an overlap in T1 values was found. No
correlation between the degree of fibrosis and the T1 relaxation
time was found. Liver iron concentration could be quantified either
by using the fast component of the T2 signal decay or by using
the decreased signal in spin-echo and gradient echo images. Patients
with leukemias and myeloproliferative disorders had prolonged
T1 relaxation times in the spleen, but a considerable overlap
was found between this group and a group of patients with benign
hyperplasia and patients with splenomegaly secondary to portal
hypertension. Volume-selective proton spectroscopy was developed
and used to quantify the liver fat concentration. The accuracy
of the method was about 3 g/100 g. With the implementation of
a second generation scanner system it became possible to develop
a pulse sequence, using the phase information in the MR signal,
to measure portal vein flow during breath-holding. This method
made it possible to estimate the portal vein flow during fasting,
and the flow increase after eating. Quantitative MR methods may
contribute to the diagnosis of non-focal liver diseases by estimation
of liver fat and liver iron and by assessment of portal vein blood
flow. Increased T1 relaxation time is a sign of a disease process
in the liver rather than specific for any liver disease.
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