Phase encoding scheme handling¶

From version 3.0RC1 onwards, MRtrix3 is capable of importing information from DICOM relating to the phase encoding of the acquired images, and encoding this information within key-value fields inside an image header. This information can then later be used by the dwifslpreproc script, specifically using its -rpe_header command-line option, to automatically set up and execute FSL’s topup and eddy commands without requiring explicit input from the user regarding the phase encoding design of the imaging experiment. This page explains how this information is encoded and manipulated by MRtrix3.

Note

Due to variations in sequences, acquisition protocols, DICOM encoding, and image manipulation operations, MRtrix3 cannot be guaranteed to obtain or retain the phase encoding information correctly in all circumstances. Manual inspection of these data and/or the outcomes of image processing when making use of these data is advocated. If circumstances leading to incorrect import or encoding of this information in MRtrix3 is reproducible, please report this to the MRtrix3 developers so that the software can be updated to support such data.

Phase encoding information storage¶

The phase encoding information for a particular image file can be stored in one of two ways.

• The most convenient of these is storage of (one or more) key-value field(s) encapsulated within the image header, just as can be used for the diffusion gradient scheme. This ensures that the information is retained through image processing, as each MRtrix3 command passes the header entries of the input image through to the output image.
• Alternatively, this information can be stored within a JSON file that accompanies the relevant image file(s). This information would then typically be imported / exported using the -json_import and -json_export options in mrconvert.

Precisely how this phase encoding information is encoded however depends on the nature of the phase encoding information for that image; specifically, whether the phase encoding information is identical for all volumes within an image file (or if it contains just one volume), or whether the phase encoding information varies between volumes within the image series.

Fixed phase encoding¶

In the case where both the phase encoding direction and the EPI total readout time are equivalent for all volumes within an image, this information is encoded within two fields: “PhaseEncodingDirection” and “TotalReadoutTime”. These fields are consistent with BIDS (the Brain Imaging Data Structure).

PhaseEncodingDirection can take one of six values: “i”, “i-“, “j”, “j-“, “k”, “k-“. These correspond to the first, second and third axes of the corresponding image. Exactly how these axes map to “real” / “scanner” space axes (i.e. left-right, posterior-enterior, inferior-superior) depends on the The image transfom that is stored in the respective image header, which may potentially have been modified internally by MRtrix3 on image load (discussed further in the Non-axial acquisitions section). Here, we begin with the most simple use case:

Take an image that conforms to the RAS (Right-Anterior-Superior) convention common to both NIfTI and MRtrix3. If the phase encoding is applied A>>P (anterior-posterior), this is the second spatial axis, or axis “j”. However, the phase encoding is also reversed along that axis (“RAS” indicates that voxel positions along the second axis increase when moving toward the anterior of the brain, whereas “A>>P” indicates the opposite). Hence, in this example, PhaseEncodingDirection would have the value “j-“.

Note

The phase encoding direction is defined specifically with respect to image axes. It is therefore not affected by the image strides, which only affect how the data for these axes are arranged when they are stored as a one-dimensional list of values within a file.

Note

The phase encoding direction does not relate to “x”, “y” and “z” axis directions in “real” / “scanner” space, as do other representations of orientation information in MRtrix3. This is because phase encoding specifically affects the appearance of the image along the image axis in which phase encoding was applied. The mapping from image axes to real-space axes is determined by the image header transform.

To demonstrate this: Imagine if an EPI image were to undergo a rigid-body rotation. The EPI field inhomogeneity distortions would still align with the relevant image axis after this rotation; but the effective x-y-z direction of phase encoding in real-space would change.

TotalReadoutTime provides the total time required for the EPI readout train. Specifically, this is the time between the centre of the first echo, and the centre of the last echo, in the train; this is consistent with BIDS, and is sometimes referred to as the “FSL definition”, since it is consistent with relevant calculations performed within FSL tools. It should be defined in seconds.

Variable phase encoding¶

If the phase encoding direction and/or the total readout time varies between different volumes within a single image series, then the two key-value fields described above are not sufficient to fully encode this information. In this situation, MRtrix3 will instead use a key-value entry “pe_scheme” (similar to the “dw_scheme” entry used for the diffusion gradient scheme).

This information is stored as a table, where each row contains the phase encoding direction and the readout time for the corresponding volume; the number of rows in this table must therefore be equal to the number of volumes in the image. In each row, the first three numbers encode the phase encoding direction, and the fourth number is the total readout time. The direction is specified as a unit direction in the image coordinate system; for instance, a phase encoding direction of A>>P would be encoded as [ 0 -1 0 ].

Non-axial acquisitions¶

When images are acquired in such a manner that the axes do not approximately correspond to RAS convention (i.e. first axis increases from left to right, second axis increases from posterior to anterior, third axis increases from inferior to superior), MRtrix3 will automatically alter the axis Strides & transform in order to make the image appear as close to an axial acquisition as possible. This is briefly mentioned in The image transfom section. The behaviour may also be observed by running mrinfo with and without the -config RealignTransform false option, which temporarily disables this behaviour.

Because phase encoding is defined with respect to the image axes, any transformation of image axes must correspondingly be applied to the phase encoding data. When the phase encoding information is stored within the image data, MRtrix3 should automatically manipulate such phase encoding information in order to maintain correspondence with the image data.

Where management of such information becomes more complex and prone to errors is when it is included in the sidecar information of a JSON file, e.g. as is commonly now utilised alongside NIfTI images such as in the BIDS format. This becomes more complex at both read and write stages, each in their own complex way.

1. When reading a JSON file, MRtrix3 will take the transformation that was applied to the corresponding input image, and apply that to the phase encoding information.

This has two curious consequences:

1. Running:

mrconvert image.nii -json_import image.json - | mrinfo - | grep PhaseEncodingDirection


and:

cat image.json | grep PhaseEncodingDirection


may produce different results. This is because once imported via the -json_import option, the phase encoding direction is altered to reflect how MRtrix3 interprets the image data, rather than how they are actually stored on file.

2. Running:

mrconvert image.nii - | mrconvert - -json_import image.json image.mif


may produce the incorrect result. This is because information regarding the transformation that is applied to the NIfTI image in the first mrconvert call in order to approximate an axial acquisition is no longer available in the second mrconvert call. When the -json_import command-line option is used, it is interpreted with respect to the input image for that command - which, in the above case, is an MRtrix piped image to which the axial transformation has already been applied - and so must always be used immediately in conjunction with loading the image with which that JSON file is associated.

2. When writing a JSON file, MRtrix3 will attempt to modify the phase encoding information in order to conform to the limitations of the output image format alongside which the JSON file is intended to reside.

Unlike the MRtrix image formats (.mih / .mif), NIfTI & NIfTI-2 (.nii) do not support arbitrary image strides. When writing image data with non-trivial strides to a NIfTI image, MRtrix3 will reorder the three spatial axes in order to approximate an axial alignment, performing the corresponding modifications to the image transform. Any exported JSON file must therefore also have the same transformation applied.

This has the curious consequence that:

mrconvert input.mif output.mif -json_export output.json
cat output.json | grep PhaseEncodingDirection


and:

mrconvert input.mif output.nii -json_export output.json
cat output.json | grep PhaseEncodingDirection


may produce different results. A JSON file must only be interpreted in conjunction with the singular image file alongside which it was generated. In the case of the MRtrix image format, we suggest relying instead on the storage of sidecar information within the Header key-value pairs rather than in such an external file, as it allows MRtrix3 to apply any requisite modifications to such data to echo modifications to the image without user intervention.

Note

This concept also has consequences for the dwifslpreproc script when manually providing the phase encoding direction. The axis and sign of phase encoding provided to the script must reflect the direction of phase encoding after MRtrix3 has performed this transformation, i.e. as it is read by any MRtrix3 command or as it appears in mrview, not the actual encoding of axes within the file.

Manipulation of phase encoding data¶

The primary purpose of storing this phase encoding information is to automate the correction of EPI susceptibility distortions. However this can only occur if the information stored is not invalidated through the manipulation of the corresponding image data. Therefore, any MRtrix3 command that is capable of manipulating the image data in such a way as to invalidate the phase encoding information will automatically modify this phase encoding information appropriately. This includes modifying the representation of this information between the fixed and variable phase encoding cases.

Consider, for instance, a pair of b=0 images, where the first was acquired with phase encoding direction A>>P, and the second was acquired using phase encoding direction P>>A:

$mrinfo AP.mif ****************************** Image: AP.mif ****************************** ... PhaseEncodingDirection: j- TotalReadoutTime: 0.0575 ...$ mrinfo PA.mif
******************************
Image:            PA.mif
******************************
...
PhaseEncodingDirection: j
...


Now watch what happens when we concatenate these two images together:

\$ mrcat AP.mif PA.mif AP_PA_pair.mif -axis 3
mrcat: [100%] concatenating "AP.mif"
mrcat: [100%] concatenating "PA.mif"
# mrinfo AP_PA_pair.mif
******************************
Image:            AP_PA_pair.mif
******************************
...
pe_scheme:     0,-1,0,0.0575
0,1,0,0.0575
...


When the two input images are concatenated, MRtrix3 additionally concatenates the phase encoding information of the input volumes; since it detects that these are not consistent between volumes, it stores this information using the pe_scheme header entry, rather than PhaseEncodingDirection and TotalReadoutTime.

The mrconvert command has a number of additional functionalities that can be used to manipulate this information:

• The -import_pe_table and -export_pe_table options can be used to import/export the phase encoding information from / to file as a table, i.e. in the format used for the pe_scheme header entry described above. Note that even if all volumes in the image have the same phase encoding direction and total readout time, these options will still import / export these data in table format.
• The -import_pe_eddy and -export_pe_eddy options can be used to import/export the phase encoding information in the format required by FSL’s eddy tool. The FSL documentation page describes this format in more detail.
• The -json_import and -json_export options can be used to import/export all header key-value entries from/to an external JSON file. This may be useful in particular for operating within the BIDS specification. There is a caveat here: If you use the -json_export option on an image with fixed phase encoding, the PhaseEncodingDirection and TotalReadoutTime fields will be written as expected by BIDS; however if the image contains variable phase encoding, then the pe_scheme header entry will be written to the JSON file, and this will not be appropriately interpreted by other BIDS tools.
• The -set_property option may be useful to override these header entries if they are deemed incorrect by some other source of information.