COST B11 Working Group 3 - Texture formation

Chairman: Prof Dr Lothar R. Schad, Heidelberg
Protocoller: Anne M. Fenstad and Arvid Lundervold, Bergen

The WG 3 meeting was held on Friday 5th November from 9:00-10:30 AM. There were three talks:

Standardized MR Pulse Sequences for Test Object Imaging: What has to be Considered?

The purpose is to agree on a standardized MR pulse sequence that one can perform on the same test object using different MR scanners and obtain similar texture measure results.

Five different groups of MR parameters must be considered. These are related to: contrast, slice, coil, resolution/SNR, and reconstruction, respectively. There will have to be an agreement on the setting of these parameters. A conventional spin-echo (SE) sequence is suggested as a first choice. This because of reduced artifact sensitivity, availability and the possibility to use T1, T2 and proton contrasts in SE acquisitions. The disadvantages are that the image contrast depends on quality of rf-pulses and a slice cross-talk. Regarding slice parameters, one has to decide on thickness and position of the slices, and the excitation order - linear (ascending/descending) or interleaved slice order. For the resolution parameters one must consider field-of-view (FOV), the reconstructed matrix, measured k-lines, rectangular FOV ratio, read/phase oversampling and number of acquisitions, aiming at a good signal-to-noise ratio. On the Siemens Vision there are two possibilities for filter settings in reconstruction of the image. One can use raw data filtering (Hamming/Hanning, oversampling) or image filtering (mean, median). Some of this could be "hidden" in the reconstruction process and might vary between scanners from different MR manufacturers. Basically, there are two different coils: transmit and receive. Coil parameters will influence RF-excitation profile and sensitivity. For receive only coils (surface coils and phased array coils), one must consider mechanisms for inhomogeneity correction and image combination.

After considering all these parameters the proposal for a MR-protocol will be:

Stability, Transferability and Normalization of Texture Parameters.

The talk gave a description of texture formation in medical ultrasound images. An overview of texture parameters and texture analysis from previous work on ultrasound was summerized: One important issue was the stability of the parameter estimates. The stability were very dependent on ROI-size. If the ROIs were greater than 800 points there seems to be no problem. If the ROI was between 400 and 800 points there was an instability effect for some parameters, and for ROI less than 400 points there were no valid evaluatin possible. An other important message was that there should be an equal distribution of ROI-sizes over classes. These experiences (over many years) with texture in medical ultrasound could also be applicable to MRI and should be considered in following work.

How does the signal-to-noise ratio influence texture measures?

The measured signal in MRI (complex-valued k-space) can be identified as the true signal added a Gaussian noise term. The noise variance of this thermal noise is a sum of independent stochastic processes related to the body, the coil, and the electronics. Definitions of signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were given together with methods for parameter estimation from observed data.

In the noise/texture experiments three groups of textural features were used: histogram-based, gradient-based and co-occurrence matrix-derived parameters (i.e. mean, variance, mean absolute gradient, variance of absolute gradient, angular second moment, contrast and correlation). All parameters were calculated within user-defined regions of interest (ROIs) using the MaZda program (v.2.11) developed within the COST B11 by the Lodz group. In addition, Matlab was used for analysis and graphics, XV was used for color-table editing, and XITE (i.e. xregion) was used for detailed drawing of ROIs.

Three datasets were used in the analysis: the "straws data" from Heidelberg (Imaging of Test Objects), raw (k-space) data from a SE head acquisition, and data from a breast study. Details about the acquisition parameters and noise characteristics are given in the slide presentation.

A few significant findings and observations from this study are:

Results from this study are planned to be presented in a separate paper. A short abstract is also submitted to ISMRM 2000 (pdf).

After each talk there were given time for discussion and comments.

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