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QUESTIONS
» Image Artefacts

An image artefact is a feature which misrepresents the object in the field-of-view. This section explores some artefacts which occur in magnetic resonance imaging.

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RF Noise Spikes

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What image artefact do RF noise spikes produce?

A ringing effect at high-contrast boundaries.

A noisy band across the middle of the image.

Light and dark stripes across the image.

INCORRECT. A ringing effect which fades from a high contrast boundary is truncation artefact. Truncation artefact arises from non-infinite k-space coverage, not from errors in k-space data (which an RF spike will produce).

Hint: the effect of a spike of RF energy will depend on the particular k-space data point which is being measured at the time of the spike. At the moment an RF spike occurs, a much larger data value will be recorded for that k-space data point. Remember that every single point in k-space affects the entire image, and try again.

INCORRECT. You may be thinking of RF zipper artefact. Zippers can arise from external RF noise (or FID artefact, or excitation pulse feedthrough), but do not result from instantaneous RF spikes.

Hint: the effect of a spike of RF energy will depend on the particular k-space data point which is being measured at the time of the spike. At the moment an RF spike occurs, a much larger data value will be recorded for that k-space data point. Remember that every single point in k-space affects the entire image, and try again.

rfspikes
Multiple RF spikes

CORRECT. Prominent light and dark stripes or waves may be seen in the image, sometimes referred to as a "corduroy" artefact. (See the K-Space Tool for illustration of RF noise spikes in k-space.)

A spike in the signal is essentially a delta function as far as the Fourier transform (FT) is concerned. The FT of a delta function is simply a sine wave; this is where the regular fringes across the image come from. The location of a spike in the raw data (i.e. at the data point in k-space which were we acquiring when the spike occurred) determines the periodicity of the fringes: the fine (high spatial frequency) or broad (low spatial frequency) nature of the waves. The location in k-space also affects the angle of the waves. Sometimes criss-cross patterns are formed by the effect of multiple spikes. Electrostatic discharge between cables can be a source of RF spikes, and since dry air encourages build up of static electricity, a humidity level is specified for the scanner room. RF noise spikes can even arise from static electricity from nylon bed-clothing.

Further reading on this topic:
Books: Q&A in MRI p147
Online: Chickscope, K-Space Tool

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