Magnetic resonance imaging in pediatrics

Resolution and Signal-to-Noise Ratio

An MRI scanner measures signal intensity on a voxel-by-voxel basis. For a high-quality image, the voxel signal must exceed background noise. Pediatrics has specific challenges: due to the small size of anatomical structures, children require a significantly smaller voxel size than adult patients.

The problem is that a smaller voxel contains fewer protons (hydrogen nuclei) and therefore yields a lower MR signal. Decreasing the field of view (FOV) while maintaining the data acquisition matrix inevitably leads to a drop in the signal-to-noise ratio (SNR).

To compensate for the loss of image quality, specialists use several approaches:

• Increasing the number of signal acquisitions (number of acquisitions).
• Decreasing the receiver bandwidth.
• Lowering the parallel imaging factor.
• Using k-space oversampling.
• Widespread implementation of pediatric coils that physically improve signal reception.

Tissue Contrast and Enhancement

Even a low signal-to-noise ratio can be considered acceptable if tissue contrast is optimized in the image. This is achieved through a combination of various weightings and intravenous contrast administration.

Features of tissue visualization in children:

• Tumors: Pediatric mass lesions are characterized by restricted water diffusion. Therefore, diffusion-weighted imaging (DWI) is an obligatory component of the protocol.
• Adipose tissue: T1-weighted images (T1WIs) with fat suppression nicely visualize the pancreas and enhance the effect of gadolinium. T1WIs without fat suppression are needed to detect fat-containing lesions (e.g., teratomas or lipoblastomas) and assess anatomical planes.
• Hyaline cartilage: Makes up a vast portion of a child's skeleton (epiphyses, apophyses). Cartilage provides an intermediate signal on fat-suppressed T1WIs, proton-density spin-echo (SE), and fat-suppressed SSFP.

Intravenous administration of gadolinium increases diagnostic efficacy. 3D SPGR sequences in the arterial or early portal venous phase perfectly delineate lesion margins. It is important to remember that in infants under six months of age, renal function is physiologically reduced, making routine assessment of the glomerular filtration rate (GFR) mandatory prior to injection.

Scan Planes and Basic Abdominal Protocol

In 2D scanning, slices are positioned strictly perpendicular to important anatomical planes in at least two projections. The axial plane is most informative for the abdominal cavity (perpendicular to the aorta and inferior vena cava), while the sagittal plane is optimal for the pelvis and midline structures such as the thymus. With modern 3D scanning, the acquisition plane is less critical, as the physician performs post-processing multiplanar reconstruction (MPR).

Standard abdominal MRI protocol for a mass lesion: Hyoscine is administered prior to the scan to reduce peristalsis.

1. Localizers.
2. STIR (coronal, respiratory-gated, wide field of view).
3. T2-weighted 3D spin-echo with variable flip angle.
4. DWI (axial, b-values 0 and 1000).
5. T1-weighted spin-echo with fat suppression (axial).
6. 3D gradient echo (GRE) with fat suppression during a breath-hold (before and after power-injector gadolinium administration).

Managing Motion Artifacts and Lung MRI

Patient motion and physiological respiratory excursions are the main enemies of pediatric MRI. The problem is addressed by using respiratory (respiratory gating) and cardiac synchronization, as well as introducing non-Cartesian reconstruction algorithms and specialized sequences (PROPELLER, BLADE, MultiVane, RADAR).

These technologies distribute motion artifacts in radial directions. Thanks to them, MRI has been successfully applied to evaluate the lung parenchyma and airways in children (especially those with cystic fibrosis and immunodeficiencies). For research purposes, hyperpolarized gas MRI, oxygen-enhanced MRI, and Fourier decomposition are used, allowing precise correlation of reduced perfusion zones with areas of bronchial changes.

Nuclear Medicine in Pediatrics

Radioisotope diagnostics play a crucial role in pediatric urology, oncology, and the investigation of fever of unknown origin. The modern trend is a shift from planar scintigraphy to SPECT and PET-MRI, which dramatically reduces the radiation dose to the child compared to PET-CT.

Specific radiopharmaceuticals:

• Tc-99m-DMSA and Tc-99m-MAG3: basic tracers for the urinary tract (diagnosis of renal parenchymal scarring, vesicoureteral reflux).
• I-123 MIBG: critical for neuroblastoma staging.
• Tc-99m-pertechnetate: search for gastrointestinal bleeding (Meckel's diverticulum).
• F18-FDG: the gold standard for lymphoma staging.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Fourier decomposition technology in the assessment of pulmonary perfusion
• Physics of 3D variable flip angle spin-echo sequences (SPACE, VISTA, CUBE)
• Specifics of radiopharmaceuticals in pediatric nephrology: choosing between Tc-99m-DMSA and Tc-99m-MAG3
• Physical principles of radial motion artifact suppression methods (PROPELLER, BLADE)
• Criteria for choosing between fat-suppressed and non-fat-suppressed sequences in the evaluation of mass lesions

Other Topics in "Pediatric Diagnostic Radiology"

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