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K-space, reconstruction, T1 and T2 pulse sequences

ND320 C3 L1 14 K-Space, Reconstruction, T1 And T2 Sequences

What happens during reconstruction of a captured MR signal?

SOLUTION:

Because this is how we can vary what types of tissue exhibit the highest signal
So that we can leverage our ability to control magnetic fields inside the MR scanner in order to focus on relevant aspects of the anatomy or pathology

Summary

K-space and Reconstruction

The currents measured by RF coils get turned into a digital format, and represented as vectors in “K-space”. The concept of K-space goes back to the wave theory in physics and basically defines a space of vectors that describe characteristics of electromagnetic waves.

In our case, these wave vectors carry information about the characteristics of the matter in the space that has been measured. Essentially, these vectors record the spatial frequency of signal intensity, and thus, through the process that involves an Inverse Fourier Transform and a lot of de-noising and other optimizations, get turned into a familiar 2D image that represents a slice through a human body with different anatomy having different pixel intensity. This process is referred to as image reconstruction in MR physics. Typically, image reconstruction is performed on a computer that is directly embedded into an MR scanner, and the problem of optimizing or scaling image reconstruction alone is a very interesting one.

If you are familiar with Fourier Transforms, but the concept of “spatial frequency” still seems way too mind-boggling - don’t worry, it could be. However, image analysis through FTs is a common technique in image processing. I have a link to some good materials on the subject later which will hopefully help you understand this concept a bit better if you are interested.

Similar to a CT scan, multiple slices imaged with a pre-set spatial interval through a human body are combined to obtain the 3D image. Note, however, that due to greater control over the electromagnetic fields, MR scanners can obtain data directly for a 3D volume in a single “sweep”, without having to go slice-by-slice.

Pulse Sequences

We can vary the combination of gradient fields, RF pulses, and aspects of the signal that is getting measured. Together, these are called a pulse sequence.

Two very common sequences are called “T1-weighted” and “T2-weighted” sequences (technically - these two are looking at different aspects of the same combination of electromagnetic fields). T1 produces greater contrast resolution for fat, and T2 produces greater detail in fluids. Quite often, a contrast medium is used along with a T1 sequence to make certain structures stand out. Thus, the gadolinium agent is often used in neuroradiology to improve the visibility of things like tumors and hemorrhages.

Many more sequences exist, including many which are proprietary to scanner manufacturers, and there is a field of medical physics that deals exclusively with pulse sequence design.

New Vocabulary

K-space data: “raw” data generated by an MRI scanner. Images need to be reconstructed from it
Pulse sequence: a combination of magnetic fields and sequence in which they are applied that results in a particular type of MR image