GE Round factor for Total Readout Time

rordenlab · Feb 3, 2021 · 6445c1c · 6445c1c
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diff --git a/GE/README.md b/GE/README.md
@@ -34,7 +34,7 @@ Some sequences allow the user to interpolate images in plane (e.g. saving a 2D 6
 
 ## Total Readout Time
 
-One often wants to determine [echo spacing, bandwidth](https://support.brainvoyager.com/brainvoyager/functional-analysis-preparation/29-pre-processing/78-epi-distortion-correction-echo-spacing-and-bandwidth) and total read-out time for EPI data so they can be undistorted. Speifically, we are interested in FSL's definition of total read-out time, which may differ from the actual read-out time. FSL expects “the time from the middle of the first each to the middle of the last echo, as it would have been had partial k-space not been used”. So total read-out time is influenced by parallel acceleration factor, bandwidth, number of EPI lines, but not partial Fourier. For GE data we can use the Acquisition Matrix (0018,1310) in the phase-encoding direction, the in-plane acceleration ASSET R factor (the reciprocal of this is stored as the first element of 0043,1083) and the Effective Echo Spacing (0043,102C). While GE does not tell us the [partial Fourier fraction](https://bids-specification.readthedocs.io/en/stable/04-modality-specific-files/01-magnetic-resonance-imaging-data.html), is does reveal if it is present with the ScanOptions (0018,1022) reporting [PFF](http://dicomlookup.com/lookup.asp?sw=Ttable&q=C.8-4) (in my experience, GE does not populate [(0018,9081)](http://dicomlookup.com/lookup.asp?sw=Tnumber&q=(0018,9081))). While partial Fourier does not impact FSL's totalReadoutTime directly, it can interact with the number of lines acquired when combined with parallel imaging (the `Round_factor`).
+One often wants to determine [echo spacing, bandwidth](https://support.brainvoyager.com/brainvoyager/functional-analysis-preparation/29-pre-processing/78-epi-distortion-correction-echo-spacing-and-bandwidth) and total read-out time for EPI data so they can be undistorted. Speifically, we are interested in FSL's definition of total read-out time, which may differ from the actual read-out time. FSL expects “the time from the middle of the first each to the middle of the last echo, as it would have been had partial k-space not been used”. So total read-out time is influenced by parallel acceleration factor, bandwidth, number of EPI lines, but not partial Fourier. For GE data we can use the Acquisition Matrix (0018,1310) in the phase-encoding direction, the in-plane acceleration ASSET R factor (the reciprocal of this is stored as the first element of 0043,1083) and the Effective Echo Spacing (0043,102C). While GE does not tell us the [partial Fourier fraction](https://bids-specification.readthedocs.io/en/stable/04-modality-specific-files/01-magnetic-resonance-imaging-data.html), is does reveal if it is present with the ScanOptions (0018,1022) reporting [PFF](http://dicomlookup.com/lookup.asp?sw=Ttable&q=C.8-4) (in my experience, GE does not populate [(0018,9081)](http://dicomlookup.com/lookup.asp?sw=Tnumber&q=(0018,9081))). While partial Fourier does not impact FSL's totalReadoutTime directly, it can interact with the number of lines acquired when combined with parallel imaging (the `Round_factor` 2 (Full Fourier) or 4 (Partial Fourier)).
 
 The formula for FSL's definition of TotalReadoutTime (in seconds) is:
 

diff --git a/console/nii_dicom_batch.cpp b/console/nii_dicom_batch.cpp
@@ -1569,7 +1569,9 @@ tse3d: T2*/
     }
 	if (d.effectiveEchoSpacingGE > 0.0) {
 		//TotalReadoutTime = [ ceil (PE_AcquisitionMatrix / Asset_R_factor) - 1] * ESP
-		float totalReadoutTime = ((ceil (0.5 * d.phaseEncodingLines / d.accelFactPE) * 2.0) - 1.0) * d.effectiveEchoSpacingGE * 0.000001;
+        float roundFactor = 2.0;
+        if (d.isPartialFourier) roundFactor = 4.0;
+        float totalReadoutTime = ((ceil (1/roundFactor * d.phaseEncodingLines / d.accelFactPE) * roundFactor) - 1.0) * d.effectiveEchoSpacingGE * 0.000001;
 		printf("ASSET= %g PE_AcquisitionMatrix= %d ESP= %d TotalReadoutTime= %g\n", d.accelFactPE, d.phaseEncodingLines, d.effectiveEchoSpacingGE, totalReadoutTime);
 		json_Float(fp, "\t\"TotalReadoutTime\": %g,\n", totalReadoutTime);
 		float effectiveEchoSpacingGE = totalReadoutTime / (reconMatrixPE - 1);