Controlling chemical-shift at 7 Tesla

Chemical-shift displacement in the slice-selection direction can cause artefacts in magnetic-resonance (MR) images acquired at higher field strengths. This is especially challenging for 7-Tesla (7T) MRI.

The artefacts present with a surprising variety. To learn how to keep them under control, check-out our publication and the corresponding section in our 7T MRI review.

 It required a truly interdisciplinary effort and essential contributions from the Siemens on-site scientists at SCMI to recognize, understand, and mitigate the image-quality problems caused by chemical shift at 7 Tesla.

Chemical-shift artefacts
In magnetic-resonance imaging (MRI), hydrogen atoms in both fat and water molecules give rise to a bright signal. However, since they are bound to different atoms (carbon and oxygen, respectively), they generate signals with different frequencies.

This causes fat (such as in bone marrow) and water (such as in skeletal muscle) to show up at slightly different positions in a MR-image – even if they were located at the same position in the body.

Radiology professionals are familiar with the fat displacement along the readout (or frequency-encoding) direction and radiologists, who read MR-images, routinely account for it. When doing so, many among them likely take for granted, however, that the fat- and water-signal sources depicted in a single image are located in the same slice within the examined body.

Through-slice chemical-shift artefacts
At lower field strengths, this assumption holds well enough. But at a field strength of 7 Tesla, the fat and water components shown in an image may physically be located in two quite different slices. Their spatial separation can easily reach the 2-fold of the prescribed slice thickness – when using similar radiofrequency pulses as at lower fields.

Thus, in the above images of a human knee, the bone-marrow ("fat-like") signal from a slice further in the back of the knee is imaged onto the "hole" in the water-image part, where we expect to see the fibular-head bone marrow.

In the image shown on the left, this ends up looking almost as it should, since the fibular head actually extends far enough beyond the imaged slice to the back. In contrast, since the slice shown on the right is close to the rear end of the fibular head, there is no more corresponding bone marrow located even further back. Thus, no bone-marrow signal shows up where we expect it.

Interpreted in isolation, the right image above could suggest a bone-marrow pathology, while, in fact, it is only the artefactual result of the imaging process.

Minimizing chemical-shift artefacts
MRI technology allows us to mitigate such undesired effects. However, there is no free lunch:

At all field strengths, protocols are optimized for smaller fat shifts in the readout direction by an increased receive bandwidth. This comes at the cost of a reduced signal-to-noise ratio of the images and a careful tradeoff has to be made.

In contrast, through-slice chemical-shift artefacts can be minimized by increasing the spectral width of the applied radio-frequency pulses. Luckily, this may not have a strong effect on the signal-to-noise ratio. Here, it is an increased amount of radio-wave energy that is deposited in the examined body parts (increased specific absorption rate, SAR), which needs to be balanced versus the improved image quality.

Once the concepts are understood, acquisitions without any chemical-shift displacement in either slice-selection direction or frequency-encoding direction become possible, as shown below in images of a veal steak.