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Echo Spacing and Total Readout Time Philips Ingenia 3T
Everything is made for Siemens, so Philips people must dig for regular variables needed for Susceptibility Distorsion Correction.
Most of this was copied directly from Brain Voyager.
The echo spacing can be calculated from a formula using the water-fat shift per pixel, which is mentioned in the *.PAR header and the DICOM in position 2001,1022. For 3T, the water-fat shift per pixel is for example 12.0. The formula becomes:
echo spacing in msec = 1000 * (water-fat shift (per pixel)/(water-fat shift (in Hz) * echo train length))
where
- echo train length (etl) = EPI factor + 1
- water-fat-shift (Hz) = fieldstrength (T) * water-fat difference (ppm) * resonance frequency (MHz/T)
- water-fat difference (ppm) = 3.35 [2]
- resonance frequency (MHz/T) = 42.576 (for 1H; see Bernstein pg. 960)
- Here is a Matlab script to calculate the echo spacing (in milliseconds): calc_echo_spacing_philips_mod.m (zipped).
In case acceleration like SENSE or GRAPPA is used, the echo spacing needs to be divided by the acceleration factor, for example if echo spacing is 0.5ms with factor 2, then effective echo spacing is 0.5/2 = 0.25ms. Partial Fourier for phase and slice as well as multiband (MB-EPI) can apparently be ignored in calculations for echo spacing.
Philips already takes acceleration into account for the Echo Train Length. In fMRI prep is also considered.
total readout time = echo spacing * no. echoes
Total readout time = (reconstructed number of phase-encoding lines - 1) * effective echo spacing
Total readout time (FSL) = (MatrixSizePhase - 1) * EffectiveEchoSpacing
Total Readout Time (SPM) = 1/(BandwidthPerPixelPhaseEncode)
References
[1] For Siemens scanners, the fieldmap sequence is gre_field_mapping. This sequence acquires two similar T2 images with different echo times. Magnitude and phase images are reconstructed (usually one only receives the magnitude data). Since the two phase images are subtracted from each other, the result is two magnitude and one so-called 'differential phase map'. After unwrapping (removing jumps of 2 pi) and rescaling to Hertz, this differential phase map is used to calculate the shift for each pixel.
[2] Haacke et al: 3.35ppm. Bernstein et al (pg. 960): Chemical shifts (ppm, using protons in tetramethyl silane Si(CH3)4 as a reference). Protons in lipids ~1.3, protons in water 4.7, difference: 4.7 - 1.3 = 3.4.