Aliasing
Aliasing occurs when the FOV in the phase-encoding direction is smaller than the anatomy being imaged. The data are under-sampled and the image appears to be wrapped from one side to the other along this direction. This can occur in two directions when 3D data are acquired, but it does not occur along the frequency-encoding direction.
Artifacts are unavoidable in MRI. Even when the MR scanner is functioning correctly, specific imaging parameter choices will produce artifacts. This section describes the artifacts seen most commonly in cardiac MRI and proposes solutions for their reduction or removal.
Gradient Decay or Fall-Off
This artifact occurs as a result of excitation and reception of signal from tissue in the falloff zone of the spatial encoding gradients, particularly in the superior-inferior direction. Spatial encoding gradient fields decay from their maximum amplitude to zero outside the physical dimensions of the gradient coil. RF excitation and reception of signal from tissue
within the fall-off zone of these gradient fields results in tissue at an incorrect physical location being mapped onto the reconstructed image. This can also occur when the RF coil used for excitation is larger than the physical extent of the gradient encoding fields.
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The triboelectric effect is the build-up of static charge due to the rubbing of two different materials against each other (for example, an ungrounded conductor and the insulator of the ECG cable).
Effect on ECG Waveform
Introduces a slowly varying change in the baseline voltage (baseline drift) of the ECG signal
Blood is composed of formed elements (45% by volume) and plasma. Plasma is composed of approximately 90% solvent and 10% solute. The solvent is water and the solute is composed of salts (sodium, potassium, calcium, magnesium, chloride, and bicarbonate), plasma proteins (albumin, fibrinogen, and globulins), and other substances (nutrients,
waste products, respiratory gases, and hormones). Blood is a conductive medium—positive and negative charges exist and allow current flow. When moving blood is exposed to a magnetic field, charge separation occurs, inducing a time-varying electrical dipole signal that is a function of the magnitude and direction of flow within the field of the MR scanner.
This dipole is detected as a time-varying electrical signal superimposed onto the ECG waveform. This second electrical signal precedes ventricular contraction and consequently is detected after the QRS complex of the ECG waveform. Ventricular re-polarization, as described by the ST segment, is detected at approximately the same time as the flow of
blood through the aorta and, hence, is often superimposed onto the T wave of the ECG.
This is most commonly known as “T-wave swelling” (or “T-wave elevation”) and can cause the T wave to be of even greater amplitude than the R wave. T-wave swelling often results in unreliable triggering and poor image quality.
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