Fazed by Phase

The word “phase” has a few uses in MR physics.

Let’s review them.
It can get confusing.


Phase Encoding

Generically, phase refers to the difference between two points in the time of a cyclical motion or process. Perhaps the most common use of phase in MRI is in phase encoding. Phase encoding is the introduction of a phase variation between the precession of the net magnetisation of spin isochromats across the field-of-view, the degree of which is changed over a series of signal acquisitions, to simulate frequency encoding (rate-of-change-of-phase is frequency). This encoding allows spatial localisation of MR signals in the phase-encoding direction, using the Fourier transform.

A brief (and dense) snippet of explanation such as this does not do justice to phase encoding, which can be difficult to grasp, but it is covered elsewhere on this site and in every MRI textbook.

Phased array

The term “phased array” originates from development of radio wave transmission. In wave theory, a phased array is a group of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions.

In MRI signal reception, we can use a local receiver coil to get higher SNR, because a smaller coil is sensitive to noise signals from a smaller area of the patient. To get the desired anatomical coverage of a larger area, we use an array of the smaller coils (sometimes called coil elements). To get optimal SNR from a phased array coil, it is necessary to make sure that the noise from coil to coil is largely uncorrelated. Part of achieving this involves ensuring minimal electromagnetic interaction between the coils. So in a similar manner to radio wave transmission, adjusting the receive sensitivity of an array of smaller coils is about the shape and arrangement of those coils. There are several competing factors to be considered in the design of a phased array coil, and determining the optimal arrangement of the coil elements is an area of ongoing development.

Phased array coils are sometimes called multiple-element coils. Sometimes the number of independent receive circuits (or channels) is referred to (which also indicates the number of elements), e.g. a 32 channel array. The number of independent RF receiver channels must match (or be greater) than the number of coil elements used in the receiver coil, unless the scanner’s RF system is independent of the number of coil elements in the receiver coil (pdf) in which case any number of receiver coil elements is compatible, with all coil elements used independently.

Heart Phases

A single heart beat can be divided into multiple equal parts in time, called heart phases. “Phases” in this sense is used in the same sense as the phases of the moon as it waxes and wanes: the division of a cyclical process into equal small parts. In cardiac MRI, a functional “cine” imaging scan may be acquired over a few heartbeats, from which we create a single heart-beat movie. The number of frames in that movie of one beat is the number of heart phases; it is the temporal resolution. More heart phases means more data acquisition and a longer breath hold for the patient, but makes for a smoother movie of the beat. Fewer heart phases means a jerkier movie of the heart beat, and possibly blurred myocardial wall boundaries.

Preparation Phases

When you press Start Scan, the scanner does not immediately begin acquiring data. Instead it makes a few clicks, pops, and buzzes, before the scan starts. These are preparation phases, with which the scanner gathers essential data, and optimisation information for the coming scan. It will: check that the right coil is attached (and that all the channels are working); check for signal correction levels; make sure the receiver coil is receiving at the right frequency; determine the right amount of RF power to be used; make sure the various RF frequencies are levelled; check what the optimum resonant frequency (f0) is now that a patient is in the bore; check patient width; check the difference in the delays of the x, y and z gradient channels; perform B0 shimming (and re-check f0); check what signal will cause clipping and adjust the receiver gain; correct for intensity distortions in echo readout; make a noise measurement; gather data for phase correction…

Not all preparation phases are required for every scan. Additionally, some preparation steps can be skipped if a recently acquired scan is sufficiently similar (and the scanner can automatically re-use some of the results from the preparation phases in the previous scan). In one modern scanner, coil sensitivity data for parallel imaging, and B1 calibration scans (to remove so-called dielectric shading) are made part of the preparation phases and are automatically acquired when they are needed.