Jochen Keupp^{1}, Doneva Mariya^{1}, Jakob Meineke^{1}, and Peter Forthmann^{1}

^{1}Philips Research, Hamburg, Germany

T2w-MRI plays an important role in prostate cancer providing information on the location/grade in diagnosis or surveillance. T2-mapping may provide objective characterization but is hampered by long acquisition time, which has been addressed by dedicated acceleration techniques (e.g. kt-T2 mapping). We investigate further acceleration of T2-mapping by prospective variable sub-sampling in the echo time domain, comparing regular or irregular patterns in combination with compressed sensing using low rank and sparsity constraints, towards a routine clinical T2 mapping protocol with increased volume coverage. Prostate and phantom T2-maps with 24 slices (1×1×3mm3 voxel) were acquired in 5½ minutes with promising map quality.

Reconstruction used a fast low rank and sparsity regularization algorithm

$$\underset{M}{\operatorname{argmin}} \left( \frac{1}{2} \| EM-P \|^2_2 + \lambda_1 \| W(M) \|_1 + \lambda_2 \| M \|_{*} \right)$$

Here, M is a matrix with N columns of vectorized echo images, P a matrix of N measured k-spaces, E the encoding matrix, including sub-sampling and coil sensitivity maps, $$$\lambda_1$$$ and $$$\lambda_2$$$ are regularization parameters (values chosen here 0.025 and 0.0015), W is the wavelet transform, $$$\| M \|_{*}$$$ is the nuclear norm of matrix M. T2 maps were computed from the multi-echo series using the standard MR system reconstruction (maximum likelihood estimation).

Experiments were performed on a 3T MRI system (Ingenia, Philips, NL) using a reference phantom (Eurospin-TO5, Diagnostic Sonar, GB, 12 samples T2=50…370ms, T1=220…1620ms, see Fig2.a) and volunteer examinations (n=3), with informed consent obtained. The following imaging parameters were used: FOV 160×160×72 mm

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Figure 1: Sampling patterns used for
prospective 2D sub-sampling of multi spin echo T2 mapping varied in the echo
time dimension (top down) at an acceleration factor of R=11. (a) Irregular
pattern defined using a sampling density compensated Poisson disk approach in
the phase encoding ky and echo number (echo#) dimensions. (b)
Regular pattern comparable to sampling patterns in previous kt-T2 mapping
approaches^{2}. For both pattern types, 3 calibration lines were used
in the k-space center.

Figure 2: Phantom testing for fast T2 mapping
with compressed sensing (CS) in the echo time dimension. (a) shows the phantom
setup and tube numbers (Eurospin-TO5). A single slice reference (b), scan time
25 minutes, is quantitatively comparable to the multi-slice fast acquisition
with irregular sampling (c) and regular sampling pattern (d). CS acquisitions
(b/c) were completed in 5½ minutes for 24 slices (acceleration R=11). A rainbow
color scale is used with T2=0…480ms.

Figure 3: Phantom results showing selected individual
echo images for fast T2 mapping, based on irregular sampling (a) or regular
sampling patterns (b). The CS reconstruction of irregular sampling allows
consistent image quality across the echo time dimension, while the regular
pattern shows some residual artifacts with varying strength and patterns across
different echoes.

Figure
4: Multi-slice T2 mapping of a volunteer prostate, completed in 5½ minutes
(acceleration R=11) with 1 mm in-plane spatial resolution (showing 6 out of 24
slices). Prospective irregular (Poisson disk), a) and regular (b) sub-sampling
with different patterns for each echo time was used in combination with a CS
reconstruction with low rank and sparsity constraints. (greyscale T2=0…250ms). In
(c) the T2W anatomical reference is shown.