Steady-state free-precession SSFP cine images can be used for function analysis. Four-chamber view with signal averaging of the aorta. The region outside the field of view FOV wraps around at the other side of the image white arrows a.
When using parallel imaging and an acceleration factor of two, the region outside the FOV wraps around in the middle of the image b. This artefact is often resolved by increasing the FOV or the number of phase encoding steps oversampling. Regarding the asymmetry of the torso, the number of phase encoding steps may be adequate for acquisition in the opposite direction.
If the aliasing persists, the body part may be projected outside the area of interest. Saturation bands involve the application of an inversion pulse on a specified area. When a saturation band is targeted on the body part outside the FOV, the signal from this particular area is inverted and can no longer cause aliasing. Parallel imaging is a technique that reduces the acquisition time by undersampling in the phase encoding direction. Parallel imaging uses the information about the local sensitivity of each coil element.
The FOV is intentionally made too small in the phase encoding direction, and the resulting aliasing is unwrapped using this information [ 12 , 13 ]. The selected acceleration factor defines the extent of undersampling performed. For example, when the acceleration factor is two, only half of the available K-space is used. However, the signal to noise ratio is often reduced at higher acceleration factors. When aliasing occurs, the region outside the FOV will be projected in the region of interest.
By reducing the parallel imaging acceleration factor, the artefact will be forced to the borders of the image Fig. Alternatively, the FOV can be increased to enclose all body parts. In flow sequences, the phase shift of moving hydrogen protons is observed. This phase shift is proportional to the velocity of flowing protons. The encoded velocity VENC is a parameter on the scanner that represents the maximum velocity present in the imaging volume. Any velocity greater or smaller than the preselected VENC causes aliasing.
Aliasing appears as black holes Fig. It is important to detect this artefact, since it will lead to underestimation or overestimation of the true velocity. The VENC should be manually adjusted, until the velocity encoded on the scanner slightly exceeds the velocity in the body of the patient. Correct adjustment of the VENC will eliminate the artefact. Aliasing in flow sequences in a patient with hypertrophic cardiomyopathy and turbulence in the left ventricular outflow tract. The aliasing artefact decreased and later vanished upon elevation of the velocity-encoding VENC gradient.
Ghosting refers to the appearance of parallel lines or double contours in the image Fig. These are often repeated projections of the abdominal or chest wall. This artefact is most often caused by respiratory motion during the acquisition [ 15 ]. Measures to eliminate the ghosting artefact primarily concern controlled breathing during the acquisition and coaching of the patient.
When the patient is unable to perform breath-holds in expiration, breath-hold in inspiration can be used. Single shot imaging or reducing the spatial resolution can be used to speed up the acquisition and decrease the breath-hold duration. A navigator sequence can be used to monitor the movement of the diaphragm during free breathing. Image acquisition is synchronised to the diaphragmatic excursions. Navigator-gated imaging can be time-consuming, because data acquired outside a pre-set acceptance window are rejected [ 16 ]. Alternatively, real-time imaging does not require breath-holds and is not ECG triggered.
It is suitable for patients with difficulty in performing breath-holds or in case of arrhythmia. However, the acquisition time will increase and the image quality decreases severely. The signal from the abdominal wall may be inverted by the application of a saturation band. Normally, data are collected during the complete heart cycle and retrospectively assigned to specific phases of the cardiac cycle retrospective triggering. In the presence of a poor ECG signal or arrhythmia, data acquisition may become challenging.
When a trigger artefact is present, myocardial borders become less well defined or blurry. The image quality may decline and render the examination non-diagnostic and the measurements or calculations performed unreliable. Arrhythmia rejection is a software option that can be used in patients suffering from an irregular heartbeat.
Images obtained during irregular RR intervals are rejected. Perioperative Transesophageal Echocardiography E-Book. Vascular Ultrasound. Neurovascular Imaging. Shoki Takahashi. Roland Droh.
MR Angiography of the Body. Emanuele Neri. The Aortic Valve. Oral Radiology - E-Book. Stuart C. Clinical Cardiac MRI. Jan Bogaert. Ultrasonography in the ICU. Paula Ferrada. Jeffrey J Goldberger. Transesophageal Echocardiography for Congenital Heart Disease. Pierre C. Keratoconus and Keratoectasia. Ming Wang. Carlo Catalano. Quick Guide to the Management of Keratoconus. Mazen M. Thorsten Johnson. Imaging Coronary Arteries. David A. Ultrasound of the Male Genitalia. Bruce R. Clinical Echocardiography. Michael Y.
Cryoablation of Cardiac Arrhythmias E-Book. Audrius Bredikis. Systemic Vasculitis. Lotfi Hendaoui. Essentials of Electrodiagnostic Medicine. William Campbell. Strategy in Bedside Hemodynamic Monitoring. Jean-Francois Dhainaut.
Cardiac CT Imaging. Matthew J. Cataract Surgery Complications.
Lucio Buratto. MRI of the Gastrointestinal Tract. Jaap Stoker. Cyprian Mendonca. Vascular CT Angiography Manual. Robert Pelberg. Radiology Illustrated: Hepatobiliary and Pancreatic Radiology. Byung Ihn Choi. Walter Sekundo. Jeanine J. Essential Equations for Anaesthesia. Edward T.