Quantitative magnetic resonance methods for in vivo investigation of the human liver and spleen. Technical aspects and preliminary clinical results.

Abstract

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.