Preparation for magnetic resonance imaging

MRI Characteristics and Sequence Selection

Magnetic resonance imaging is the modality of choice for cross-sectional imaging in children, with the exception of lung parenchyma imaging, the bone cortex, acute trauma, and certain congenital heart diseases. The key advantages of the method are the absence of ionizing radiation and rich tissue contrast capabilities. However, pediatric MRI requires meticulous attention to patient preparation, motion artifact compensation, and signal-to-noise ratio (SNR) optimization.

The success of the examination is largely determined by the choice of pulse sequences:

• Ultrafast sequences (e.g., steady-state free-precession). Ideal for visualizing the heart and bowel loops. Their drawback is the absence of differential T1 or T2 weighting, providing only a general structural assessment without tissue detail.
• Anatomic sequences (e.g., volumetric T2-weighted spin-echo). Provide the best visualization of anatomy, but scanning takes over 10 minutes. Any macroscopic patient movement during this period critically degrades image quality.

Age-Based Approach and Sedation in Children

Children represent the most challenging patient group for MRI. The preparation approach strictly depends on age:

1. Infants. The feed-and-wrap method (feed-and-wrap) supplemented with acoustic protection is most effective. The optimal time is postprandial sleep, provided feeding occurred after a 4-hour fasting period.
2. Children under 6–8 years. Usually require medicinal sleep. General anesthesia is considered safe; rare fatal outcomes (approximately 10 per 101,885 cases, or 0.001%) are exclusively associated with severe underlying pathology. For children weighing under 15 kg, sedation is an option. However, it is less reliable (success rate of about 75%), does not secure the airway, and does not allow for breath-holding. Sedation is suboptimal for abdominal and thoracic MRI, but can be used for musculoskeletal and nervous system evaluations.
3. Older children. The main focus is on alleviating anxiety. Preliminary familiarization with the environment (child life specialists, MRI mock-ups) is used. Inside the scanner, integrated entertainment systems are utilized, and parents or guardians are permitted to stay nearby.

Radiofrequency Coils and Artifact Mitigation

The primary factor determining the signal-to-noise ratio is the proper selection of the radiofrequency coil. The use of multichannel coils provides a higher signal level.

Coil coverage should be approximately 1.5 times the size of the region of interest (ROI). Using too small a coil results in insufficient signal, while using too large a coil registers excess noise. In practice, due to a shortage of specialized pediatric devices, children are often scanned using traditional adult coils (e.g., an adult knee coil is used for a newborn's chest MRI).

To minimize motion artifacts, the following approaches are combined:

• Peristalsis suppression: Critical for high-quality T2-weighted spin-echo imaging. Achieved by intravenous administration of glucagon or hyoscine butylbromide.
• Physiological gating (Gating): Allows «freezing» of periodic motion by acquiring data only during strictly defined time windows. Gating can be physical (respiratory bellows, ECG leads) or image-based (liver dome navigation, phase scout techniques). For thoracic MRI, a combination of ECG and respiratory gating is mandatory.
• Special techniques: If synchronization is not feasible, targeted saturation (targeted saturation) or advanced k-space trajectories (k-space) are applied.
• K-space manipulations: The physician can alter the phase-encoding direction to shift the motion artifact away from the zone of interest. For instance, when scanning the thymus in the axial plane, image quality is dramatically improved by setting the encoding direction from right to left.

Class and exam questions

What else to study on this topic

• MR Myelography: Indications and Comparison with X-Ray Methods
• Spinal Angiography and the Application of MRA
• Current Status of CT: Iterative Reconstruction and Dual-Energy CT
• Features and Limitations of CBCT in Pediatric Practice

Other topics in the "Additional Topics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.