Data acquired during the development of a R2′ mapping technique 
with prospective correction for macroscopic magnetic field gradients

The data in this archive was acquired during the development of the GASE (Gradient-Echo
Slice Excitation Imaging Asymmetric Spin Echo; GESEPI ASE) technique. GASE can be used to 
map the reversible transverse relaxation rate R2′ (a contrast used in Magnetic Resonance 
Imaging - MRI) without the need to separately acquire a magnetic field map to correct for 
residual magnetic field gradients not compensated by magnet shimming. This technique has 
application in measuring blood oxygenation using the quantitative BOLD (1,2) 
(baseline levels) and calibrated BOLD (3) (dynamic changes) techniques as 
well as iron deposition (4).

This dataset consists of three experiments;
- phantom.zip containing an initial validation of the GASE technique.
- fmapping.zip used to investigate the distribution of magnetic field gradients in the head.
- gesepi.zip where a range of GASE variants were tested against uncorrected ASE.

Images are encoded as compressed NIFTI files and contain basic information about voxel
size, repetition time and orientation. Further information is contained in a YAML
formatted file, which is both human and machine readable. These files are inherited by 
image files at lower levels of the directory structure unless they are overridden by a 
file at the lower level.

Phantom data - phantom.zip  

This dataset consists of ASE and GASE4 images acquired with different applied magnetic 
field gradients in the z-direction: 0, 100, 150 microTesla/meter. A standard phantom based 
on the FBIRN agar doped construction was used.

Field mapping data - fmapping.zip

This dataset consists of high resolution magnetic field maps in order to investigate the
distribution of magnetic field gradients in healthy volunteers. High resolution
Magnetisation Prepared RApid Gradient Echo (MPRAGE) images for each subject 
were acquired for coregistration and segmentation purposes. MPRAGE images are brain 
extracted (5) in order to preserve anonymity of the subjects. 

GESEPI data - gesepi.zip

This dataset consists of 4 different ASE variants for comparison with standard ASE: GASE4,
GASE8 and GASE128 (see glossary below). High resolution MPRAGE images for each subject 
were acquired for coregistration and segmentation purposes. MPRAGE images are brain 
extracted (5) in order to preserve anonymity of the subjects. 

See each individual directory for file naming conventions.

Glossary

EPI - Echo Planar Imaging
ASE - Standard Asymmetric Spin Echo data acquired with EPI
GASE4 - GESEPI ASE data acquired with 4 subslices (partitions) at 1.24mm each with 3D EPI
GASE8 - GESEPI ASE data acquired with 8 subslices (partitions) at 0.63mm each with 3D EPI
GASE128 - GASE4 data with doubled in-plane resolution from 64 to 128 matrix

References

1. An H, Lin W. Quantitative measurements of cerebral blood oxygen saturation using 
   magnetic resonance imaging. J. Cereb. Blood Flow Metab. 2000;20:1225–1236. 
   doi: 10.1097/00004647-200008000-00008.
2. He X, Yablonskiy DA. Quantitative BOLD: Mapping of human cerebral deoxygenated blood 
   volume and oxygen extraction fraction: Default state. Magn. Reson. Med. 
   2007;57:115–126. doi: 10.1002/mrm.21108.
3. Blockley NP, Griffeth VEM, Simon AB, Dubowitz DJ, Buxton RB. Calibrating the BOLD 
   response without administering gases: Comparison of hypercapnia calibration with 
   calibration using an asymmetric spin echo. Neuroimage 2015;104:423–429. 
   doi: 10.1016/j.neuroimage.2014.09.061.
4. Ordidge RJ, Gorell JM, Deniau JC, Knight RA, Helpern JA. Assessment of relative brain 
   iron concentrations using T2-weighted and T2*-weighted MRI at 3 Tesla. 
   Magn. Reson. Med. 1994;32:335–341.
5. Smith SM. Fast robust automated brain extraction. Hum. Brain Mapp. 2002;17:143–155. 
   doi: 10.1002/hbm.10062.