Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

A higher frequency encode gradient strength has a larger bandwidth per pixel,and a weaker gradient strength has a smaller bandwidth per pixel. Therefore, witha larger gradient strength, water and fat are contained within the pixel boundaries,while for a lower gradient strength, water and fat are separated by one ormore pixels.RF bandwidth/gradient strength considerations and pulse sequences can mitigatechemical shift problems. Large gradient strength confines the chemical shiftwithin the pixel boundaries, but a significant SNR penalty accrues with the broadRF bandwidth required for a given slice thickness. Instead, lower gradient strengthsand narrow bandwidths can be used in combination with off-resonance "chemicalpresaturation" RF pulses to minimize the fat (or the silicone) signal in the image(right image of Fig. 15-36C). STIR (see Chapter 14) parameters can be selected toeliminate the signals due to fat at me "bounce point." Additionally, swapping thephase and frequency gradient directions or changing the polarity of the frequencyencode gradient can displace chemical shift artifacts from a specific image region.Identification of fat in a specific anatomic region is easily discerned from the chemicalshift artifact displacement caused by changes in FEG/PEG gradient directions.Ringing ArtifactsRinging artifact (also known as the Gibbs phenomenon) occurs near sharp boundariesand high-contrast transitions in the image, and appears as multiple, regularlyspaced parallel bands of alternating bright and dark signal that slowly fades with distance.The cause is the insufficient sampling of high frequencies inherent at sharpdiscontinuities in the signal. Images of objects can be reconstructed from a summationof sinusoidal waveforms of specific amplitudes and frequencies, as shown inFig. 15-37 A for a simple rectangular object. In the figure, the summation of frequencyharmonics, each with a particular amplitude and phase, approximates thedistribution of the object, but initially does very poorly, particularly at the sharpedges. As the number of higher frequency harmonics increase, a better estimate isachieved, although an infinite number of frequencies are theoretically necessary toreconstruct the sharp edge perfectly. In the MR acquisition, the number of frequencysamples is determined by the number of pixels (frequency, k" or phase, k y ,increments) across the k-space matrix. For 256 pixels, 128 discrete frequencies aredepicted, and for 128 pixels, 64 discrete frequencies are specified (the k-spacematrix is symmetric in quadrants and duplicated about its center). A lack of highfrequencysignals causes the "ringing" at sharp transitions described as a diminishinghyper- and hypo intense signal oscillation from the transition. Ringing artifactsare thus more likely for smaller digital matrix sizes (Fig. 15-37C, 256 versus 128matrix). Ringing artifact commonly occurs at skull/brain interfaces, where there isa large transition in signal amplitude.The wraparound artifact is a result of the mismapping of anatomy that lies outsideof the FOV but within the slice volume. The anatomy is usually displaced to theopposite side of the image. It is caused by nonlinear gradients or by undersamplingof the frequencies contained within the returned signal envelope. For the latter, thesampling rate must be twice the maximal frequency that occurs in the object (the