Technical progress in MR imaging has enabled significant reductions of acquisition times to the point that acquisition of real-time images is now possible. Nevertheless, motion artefacts remain critical since patient movement can severely re

duce image quality and subsequent diagnostic accuracy. To avoid motion artefacts in pediatric patients, sedation or even anesthesia with intubation and controlled ventilation is often required. MR imaging may be performed in spontaneous breathing patients if respiratory-gated sequences are utilized. A pressure sensor attached to a belt encircling the patient's upper abdomen can be used to record respiratory motion. Synchronization between the detected abdominal wall excursion and the MR acquisition has been shown to decrease motion artefacts considerably. However, the technique is limited by the indirectness of the mechanic coupling between the abdominal wall motion and the structures to be imaged in the chest. Nevertheless, the technique is suitable for the evaluation of intraabdominal pathologies.

With the introduction of navigator sequences a new method of respiratory monitoring has become available. For these navigator sequences a single slab measurement perpendicular to the diaphragm detects the motional excursion of the diaphragm and can be used as a trigger signal for the MR acquisition. This allows for compensation of respiratory motion. However, acquisition of high resolution cross-sectional images in the thorax has to be combined with ECG gating and thus measurement times may drastically increase.

As a consequence, controlled ventilation combined with breath-holding is often preferred for examinations of intrathoracic lesions in order to reduce motion artefacts. State of the art MR imaging equipment now permits many acquisitions to be completed in just a few seconds making anes-thesiologically controlled breath-hold imaging possible in children.

The strong magnetic field inherent to MRI requires special equipment for patient monitoring such as special pulse oximetry devices or ECG paddles. In addition, a ventilation unit and medical perfusor devices have to be MR compatible or should be placed outside the magnetic field and connected by long infusion lines.

Patient positioning within the magnet may be head-first or feet-first since this has no influence on the quality of imaging. However, patient positioning should be consistent with the requirements for careful monitoring and possible ventilation.

For the functional evaluation of flow phenomena, phase related imaging with monitoring of the cardiac cycle is mandatory. To better detect the R-R interval, MR-compatible ECG electrodes should preferably be placed on the left lateral chest wall.

Depending on the patient's size or the size of the malformation to be examined, different kinds of coils may be used.

In small children the use of phased array body surface coils covering almost the whole body permits examination of the entire intracorporal vas-

Fig. 2. MIP reconstruction of the arterial phase of a thoracic MRA in a premature baby (30th week of pregnancy) (Gd-BOPTA, 0.2 mmol/kg, total contrast agent volume 0.15 ml). Limitations of resolution are apparent due to the extremely small size of the anatomic structures (diameter of aorta approx. 1.5 - 2 mm). Nevertheless, diagnosis of a typical aortic coarctation (arroW with a stenosis distal to the left subclavian artery was possible cular architecture in one imaging study. In small babies, newborns or for more localized malformations, wrap around surface coils may be used, however even with these coils the borders of resolution may be reached in premature babies (Fig. 2). Alternatively extremity coils can be used for imaging of children. Head coils are generally not advisable since the geometry of the thorax of a baby does not fit the volume for which head coils are optimized.

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Essentials of Human Physiology

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