Sie befinden sich hier

Inhalt

T2*- and Diffusion Measurements with 3He

M. Bock

Introduction

MR imaging of human lungs and airways is possible with hyperpolarized gases such as ³He [1]. Apart from the longitudinal relaxation time T₁ two additional parameters determine the contrast of ³He images: the diffusion coefficient D and T₂^*. As D of ³He in the gas phase is 5 to 6 orders of magnitude larger than that of water, already the relatively weak imaging gradients of several mT/m create a significant diffusion weighting. In the human lung T₂^* is very short, because air-tissue boundaries induce rapid susceptibility changes.

To measure these two parameters simultaneously in an acquisition time, that does not exceed the life-time of hyperpolarization in the human lung of about 30 s, a sequence was developed, that employs two gradient-echo trains with different inter-echo spacings and readout gradient strengths.

Materials and methods

The sequence for the simultaneous measurement of D and T₂^* was implemented on a clinical MR scanner (SIEMENS Magnetom 63/84 SP, Erlangen, Germany) using a home-built, double resonant ³He/¹H Helmholtz coil (H.J. Zabel, DKFZ Heidelberg). Two gradient-echo trains with 8 alternating readout gradients each were sampled after a slice-selective rf excitation (Fig. 1). Readout gradient strength and bandwidth of the second echo train were half of that of the first to maintain spatial resolution (echo spacings: DTE₁ = 4.56 ms, DTE₂ = 7.12 ms, bandwidths: BW₁ = 391 Hz/pixel, BW₂ = 195 Hz/pixel).

Fig. 1: Timing of the 3He sequence. 2 series of 8 gradient echoes are acquired. Because echo spacing and readout gradient strength are different in the 2 series, the individual contributions of D and T2* to the signal decay can be separated.

The decay of the MR signal S during an echo train (normalized to the signal of the first echo S(0)) is described by

with

where n = 0..7 is the echo number and i = 1 or 2 is the series index. As the b-value bi(DTEi) is dependent on the details of the gradient switching, it is different for series 1 and 2. Values of D and T2* are calculated from the decay constants ai, which are determined by a fitting procedure.
Measurements with both hyperpolarized and non-prepolarized gas samples of 3He in glass cells were performed. Hyperpolarization of about 60% was created at the University of Mainz (Group: E. W. Otten) by laseroptical pumping of metastable 3He with subsequent collisional polarization transfer to the ground state [2]. MR imaging was performed using very low flip angles (a < 1°) to preserve hyperpolarization (Matrix / FOV / SL / NEX = 96-128 / 500 mm / 40 mm / 1). The non-prepolarized sample contained a 3He/O2 gas mixture in a sealed cell (T1 = 2.5 s). At TR = 100 ms flip angles around the Ernst angle aE = 16° were chosen. The other imaging parameters were: Matrix / FOV / SL / NEX = 642 / 300 mm / 40 mm / 1. For comparison, 3He diffusion coefficients were also measured with a spectroscopy sequence.

Results and discussion

Fig. 2 shows the result of a measurement with hyperpolarized 3He. The measured value of D is in good agreement with the spectroscopy measurement (1.87× 10-4 m2/s) and the literature value (1.83×10-4 m2/s, T = 20°C, p = 1 bar). In general it was observed, that T2*-values showed larger uncertainties than values of D.

From the 2 image series maps of D and T2* can be calculated. Changes in the structure of lung tissue modify local diffusion coefficients and T2*. Parameter maps derived from 3He imaging data might help to identify regions in the human lung, that are affected by pathologic processes.

Fig. 2: Normalized 3He MR signal of the hyperpolarized sample as a function of echo spacings n TE1 (squares) and n TE2 (circles). From the fitted constants a1 = 7.5 ms and a2 = 10.1 ms and b-values b1(TE1) = 2.62 103 s/m2 and b2(TE2) = 2.76 103 s/m2 D and T2* were calculated.

References

[1] Bachert P., Schad L.R., Bock M., et.al.: Magn. Reson. Med. 36, 192-196 (1996)
[2] Eckert G., Heil W., Meyerhoff M., et. al.: Nucl. Instrum. Meth. A 346, 45-51 (1994)