Breast MRI Clinical Image Review Category A: Pulse Sequences and Image Contrast (Revised 12-12-19)
Modified on: Fri, 13 Dec, 2019 at 11:21 AM
The selection of appropriate pulse sequences is critical. They are the major determinant of image contrast and thus determine the appearance of both normal tissues and breast pathology. In order to depict and characterize abnormalities, a comprehensive breast MRI exam should include pulse sequences that provide more than one type of image contrast. The specific type of pulse sequences (e.g., conventional SE, fast SE, turbo SE, gradient-echo) and the precise imaging parameters (e.g., TR, TE, flip angle, echo train length) are not specified and are left to the discretion of the imaging facility.
The major categories of image contrast assessed in the evaluation process are:
T2-Weighted/Bright Fluid Series
Term used to indicate an image where most of the contrast between tissues or disease states is due to differences in the tissue T2 (or T2*), with fluid such as blood or cystic fluid appearing bright. The term “T2-weighted” may be misleading in that spin density differences and T1 differences also may contribute to image contrast. The T2-weighted/bright fluid sequence should demonstrate fluid as sufficiently bright to be considered a true bright fluid sequence. The case will fail if bright fluid is absent or indistinguishable from background tissues, such that the deficit could lead to misdiagnosis or is inadequate to reliably distinguish cysts from solid masses. If the bright fluid contrast is present, but is faint or highly variable, the case could fail if other image problems exist.
An Ax T2 fat-sat sequence is desirable. With a T2 fat-sat (or STIR) the fat is suppressed (i.e., becomes dark) and the bright fluid is much easier to see.
A Short TI Inversion Recovery (STIR) image with TI set to suppress fat signal may be considered a T2-weighted/bright fluid series if it successfully shows fluid to be bright. A T2*-weighted or a non-spoiled (steady state) T1-weighted gradient echo pulse sequence also may be satisfactory as a bright fluid sequence if it shows fluid as being adequately bright (i.e., brighter than all other tissues in the breast).
The Committee on Breast MRI Accreditation has determined that contrast should generally not appear in a good quality T2-weighted bright fluid series. Also, the vast majority of exams performed at US facilities do T2W imaging before, rather than after, administration of the contrast agent. The reason that you would not want to do T2W imaging after contrast administration is that the basis for bright fluid in T2W imaging is the longer T2 of fluid, cystic or vascular, compared to cellular tissue. Administering contrast agent shortens the T2 values of perfused fluids and tissues, particularly vessels, so contrast agent decreases the brighter T2W appearance of the vascular bed. It would not necessarily affect cysts. If there are no cysts in the breasts, the only way ACR reviewers can evaluate the presence of bright fluid contrast in T2W images is to see bright vessels. The addition of contrast agent during T2W imaging is likely to decrease their visibility and could compromise bright fluid evaluation.
Multi-Phase T1-Weighted Series
Pre-contrast T1-weighted without or with fat suppression: Term used to indicate an image where most of the contrast between tissues and disease states is due to differences in T1. A T1-weighted image is achieved by imaging with a short TR relative to the longest tissue T1 of interest and a short TE relative to the tissue T2 (to reduce T2 contributions to image contrast). Short TR/short TE sequences are a necessary component of the required examination. This sequence must have sufficient dark fluid contrast to be considered a true T1-weighted sequence. Fat suppression may be used for this sequence, along with the post-contrast series that follow. If fat suppression is not used, subtraction images with these pre-contrast images subtracted from post-contrast images must be reconstructed and submitted.
Post-contrast T1-weighted with fat suppression or subtraction (early and delayed phases): The intent of early phase and delayed phase post-contrast imaging is to capture information on lesion enhancement in the early and late phases of postcontrast enhancement. These pulse sequences must be identical to the pre-contrast T1-weighted series described above in terms of spatial and temporal parameters. There are 2 acceptable exceptions to this requirement:
Small deviations between pre- and post-contrast series may appear in the DICOM header files of some series, even though identical acquisition parameters were selected
Aurora EDGE software uses a distinctly different acquisition time for the pre- contrast series compared with that of the post-contrast series.
If fat suppression is used for the pre-contrast T1, it should also be used for this sequence. If fat suppression is not used, subtraction images with pre-contrast images subtracted from these post-contrast images must be reconstructed and submitted, along with the un-subtracted source post-contrast T1-weighted images. At least 2 phases of post-contrast images must be submitted: the earliest and the latest phase post-contrast series.
All sequences must demonstrate sufficient signal to noise (SNR) and not appear too grainy. The details of the pulse sequence, along with other selected parameters such as imaging matrix and number of signal acquisitions (per phase-encoding step) acquired, will determine the image acquisition time and SNR. In general, short acquisition times are desirable to limit patient motion and discomfort and provide flexibility such that additional sequences may be obtained if desired; however, short acquisition times should not be employed if they can only be obtained at the expense of overall image quality and diagnostic value.
IV contrast must be evident in the 2 post-contrast T1-weighted series. Please see the documents on MR Contrast Agents and Contrast Media at the ACR’s Radiology Safety webpage for contrast safety information.
Your facility does not need to submit post-processed data from CAD systems. The only reason you would need to submit any post-processed data is if you do not perform fat-saturation on your multiphase T1-weighted series and need to submit subtracted images corresponding to the early-phase and late-phase post-contrast series, along with the originally acquired pre-contrast, early-phase post-contrast, and late-phase post-contrast images. If you do fat-sat, you need only send the requested acquired images. If you do not do fat-sat, you can still do subtractions on the MR system itself for the first and last post-contrast series and send those. You still need not send any CAD-processed images. Most sites with MRI CAD systems just send the acquired images to CAD and PACS and depend on the CAD system to do subtractions, MIPS, etc. In that case, if you didn’t do fat-sat, the subtractions might come from the CAD system or you might do the subtractions on the MR system (which you normally wouldn’t do) and send those. The latter option is simpler for most sites.
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