Revision History


For the multiphase, T1-weighted series, ACR reviewers will evaluate the following parameters obtained from the DICOM header:

  • Slice thickness

  • Interslice gap

  • In-plane pixel size (frequency-encoding direction)

  • In-plane pixel size (phase-encoding direction)

  • Temporal resolution


Spatial Resolution

There are 5 determinants of voxel dimensions in an MRI examination:

  1. Slice thickness (ST)

  2. Field of view along the phase-encoding direction (FOVp)

  3. Field of view along the frequency-encoding direction (FOVf)

  4. Number of phase encoding steps (Np)

  5. Number of frequency encoding steps (Nf)


The voxel volume is the pixel area multiplied by the slice thickness.


Pixel Size and Voxel Volume Calculations

In-plane pixel size (phase) = (FOVp/Np)

In-plane pixel size (frequency) = (FOVf /Nf)

Pixel area = (FOVp/Np) x (FOV/Nf)

Voxel volume = (Pixel area) x (ST)



Alterations in any of these 5 parameters will change the voxel volume, the SNR of the image, and the amount of partial volume averaging exhibited in each image. Alterations in the number of phase-encoding steps (Np) affects scan time, while alterations in the number of frequency-encoding samples (Nf) may affect the maximum number of slices as well as the minimum possible TE for the imaging sequence. Voxels that are too large for the anatomic structures being examined cause the viewer to observe what is perceived as blurriness in the image. While decreasing the size of the voxels has the potential to increase the sharpness of the image, there is a concomitant decrease in SNR. When the SNR is too low for the anatomic structures being examined, the viewer observes what is perceived as image "graininess." Increasing the SNR of the image by increasing the voxel volume, increases the smoothness of the image, but at the expense of increased image blur, increased partial volume averaging, or both.


The Breast MRI Accreditation Program’s requirements for spatial resolution for the T1-weighted multi-phase series are as follows:

  • The acquired (not interpolated) slice thickness must be ≤ 3.0 mm. Cases with slice thicknesses > 4.0 mm will fail; cases with slice thicknesses > 3.0 but ≤ 4.0 mm may fail if the reviewers note deficiencies in other categories.

  • The in-plane pixel resolution must be ≤ 1.0 mm (for both phase and frequency). Cases with in-plane pixel resolution > 1.2 mm will fail; cases with in-plane pixel resolution > 1.0 but ≤ 1.2 mm may fail if the reviewers note deficiencies in other categories (e.g. poor positioning, significant artifacts, etc.).

  • The interslice gap must be ≤ 0 mm. Cases with gaps > 0 mm will fail.


Facilities should check the DICOM header information and use the formulas listed above to determine if their exams meet the ACR’s spatial resolution requirements.


Temporal Resolution

The total time between the end of the contrast injection and the end of the early phase post-contrast T1 series is evaluated to assess temporal resolution. Do not include the time for the saline flush in this value.



For example:

  • The difference between the time at the end of contrast injection and the time that the early phase post-contrast T1 series begins = 40 seconds = Time Delay

  • The difference in time between the time that the early phase post-contrast T1 series begins and the time that the early phase post-contrast T1 series ends = 60 seconds = Acquisition Time

  • Total Time = Time Delay + Acquisition Time = 40 seconds + 60 seconds = 100 seconds (1.67 minutes)


The first post-contrast sequence must have been completed in ≤ 4.0 minutes of completion of contrast injection. Cases with a total time > 5.0 min will fail; cases with a total time > 1.0 but ≤ 5.0 min may fail if the reviewers note deficiencies in other categories.


Requested acquisition times are specific to the indicated individual series, i.e., time for the T2-Weighted Bright Fluid series, time for the Pre-Contrast T1 series, time for the Early Phase (1st) Post-Contrast T1 series, or time for the Delayed Phase (last) Post-Contrast T1 series. When completing the Clinical Test Image Data section, do not provide the time it takes to complete the entire multi-series set.


You must provide the requested information as defined here. If, for example, your facility calculates time delay using the difference between the end of injection and the end of the first dynamic contrast pass, you may not provide this information as an alternative.



Revision History for this Article

Date

Section

Description of Revision(s)

12-12-19

All

Article created; FAQs incorporated; No criteria changes



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