Teaching Case

A 4-year-old boy, whose findings of intracranial hypertension were investigated, was referred for cranial MRI examination. During the MR Safety screening, it was learned from the parents that there were no contraindications to the examination. However, the examination was stopped after a scout scan because an MRI artefact of the nasopharynx area was seen, which could indicate the presence of potentially unsafe material. When the parents were interviewed, no history could be obtained explaining the cause of the artefact. It was investigated with digital radiography (DX) and Computed Tomography (CT).

Artefacts were detected in the retropharyngeal area in cranial MRI scout scans (Figure 1). Suspicious millimetric density was seen in the tonsillar region on the lateral cranial DX (Figure 2). CT was performed because the headache increased and the artefact could not be explained. In the left tonsillar area, a high-density material (1050 HU) was detected to be located in the submucosal area (Figure 3).

Background

Artefacts develop when imaging near metal material arise due to a heterogeneous static magnetic field that creates large changes in the rate of movement or resonance frequency through the object [1].

These artefacts are especially caused by metal implants. The external magnetic field B0, which is normally homogeneous, is made heterogeneous by the metal. Materials that create magnetic sensitivity are classified as diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic and ferrimagnetic (Figure 5) [2].

A blooming artefact is a susceptibility artefact encountered in some MRI sequences in the presence of paramagnetic substances that affect the local magnetic environment. It is a work that helps the diagnosis of pathologies such as bleeding lesions and siderosis [3].

Imaging Perspective

Artefacts that occur in imaging near metal are caused by an inhomogeneous static magnetic field that causes large changes in precession or resonance frequency across the object. The main artefacts that occur in imaging are signal loss due to phase loss, geometric distortion, displacement artefacts in slice selection and reading directions, and failure of fat suppression (FS) artefacts [1]. Frequency shifts near metallic implants can cause large changes in fat resonance. As a result, FS cannot be performed [4].

Near metal objects, magnetic field changes can be very large [1]. This leads to phase reduction or loss of coherence and loss of signal. In images, this manifests as a black area where the signal would normally be found (Figure 1). Some metal implants may cause heating due to interaction with radiofrequency fields. The most common example is guidewires [1,6].

Careful parameter and pulse sequence selections can avoid or reduce artefacts. By using spin-echo imaging, phase-degrading effects can be almost completely avoided. Two general approaches are proposed to reduce metal artefacts. The first is parameter optimization in standard sequences. Most of the metal artefacts can be reduced by using a mid-range matrix, increasing the gradient receiver bandwidth, increasing the number of exits, increasing the slice selection bandwidth, using spin echo sequences, not preferring gradient echo sequences, and imaging in a lower-power magnetic field. The other approach is special series such as view angle tilting (VAT), multi-acquisition variable resonance image combination (MAVRIC), and slice encoding metal artefact correction (SEMAC). These sequences were developed to eliminate artefacts by making the specified optimizations [7].

Clinical Perspective

Following the detection of a metal artefact in the boy's nasopharynx, the removal of the foreign body prevented the infective-inflammatory processes that could be caused by the foreign body and its possible intra-tissue migration.

Outcome

The corpus alienum in the left tonsillar area, which could not be detected during the endoscopic examination, was surgically removed (Figure 4). When the parents were interviewed again, it was understood that their house was being renovated and that this foreign object was due to the swallowing of the material used in construction. After recovery, an MRI examination was performed successfully

Take Home Messages / Teaching Point

MR Safety screening and patient preparation should be done carefully and in detail, especially in pediatric patients. Inspection of scout/localizer/survey scans for implant-related artefacts is important for MR Safety. Recognition of MRI artefacts directs the investigation of the cause.

Consent was obtained from the parents. All patient data have been completely anonymised throughout the entire manuscript and related files.