Article
Magnetic resonance imaging (MRI) scans are a routinely used imaging modality in modern medical practice and can produce very detailed images of the hard and soft tissues. While they can be taken on any region of the body, they are often used for investigations in the head and neck region.
Although comprising a relatively small proportion of orthodontic patients overall, there are an increasing number presenting for treatment with a need to undergo routine MRI scans as part of their ongoing medical care.
MRI scans work by creating a magnetic field around the body and the protons contained within the body's water molecules align with the magnetic field before moving back to resting position.
The extent to which a material gets magnetized in an external magnetic field is known as susceptibility. Diamagnetic substances disperse the magnetic field with no significant field distortion and almost all the body tissues belong to this group. Ferromagnetic materials (such as stainless steel) are strongly susceptible giving rise to inhomogeneity of the magnetic field This can create significant artefacts on the MRI images, appearing as large dark areas around the metal with distortion of the surrounding structures. Paramagnetic substances such as titanium are weakly susceptible to magnetic fields and cause relatively lesser distortion of the magnetic field, thus dramatically reducing susceptibility artefact. The degree of susceptibility artefacts depends on the types of MRI sequence used and field strength of the scanner, being worse on a 3 Tesla (T) MR scanner compared to a 1.5 T.
While not important for more distant sites, where orthodontic patients are having scans in the head and neck region (for example, for investigation of neurological disorders such as epilepsy or monitoring following neuro-oncology treatment) the appliance material choice needs to be considered.
Where a patient presents needing a one-off emergency MRI scan of the head and neck region, appliances are usually removed and can be replaced later. However, for those individuals who are anticipated to need one or more MRI scans throughout the duration of treatment, this isn't a practical option owing to the repeated bonding and debonding of the appliance.
In this journal, a recently published protocol on the management of patients expected to undergo regular MRI scans of the head and neck region proposed the use of ceramic brackets only in order to avoid artefact generation1.
In most cases, a viable alternative to this can be the use of titanium brackets and tubes. Titanium brackets have multiple benefits when compared with ceramic brackets. From an aesthetic perspective, to the untrained eye they resemble standard metal brackets, which will normally be comparative to appliances worn by patients’ peers. The bonding and debonding techniques are identical to standard metal brackets, without the need for any additional instruments in the inventory. When using a ceramic appliance, it may be necessary to place stainless steel bands or tubes on the molar teeth, which will require removal prior to scanning to prevent artifact generation. Titanium brackets also avoid the risk of enamel abrasion or fracture which can be associated with ceramic brackets.
At Alder Hey Children's Hospital, following liaison with medical and surgical colleagues, the authors routinely use the titanium Orthos (Ormco) appliances (Figure 1), which are over 99% titanium, for children who require regular MRIs of neurocranial structures. These appliances can be of a comparable cost to ceramic appliances (with brackets costing around £4 and tubes costing around £8). The authors have found this appliance to hold up well during orthodontic treatment, effectively expressing the inbuilt prescription (which can be customized to vary minimally from our usual MBT prescription). The brackets can be left in place and the wire removed and replaced around the scan. For this cohort (which encompasses most patients requiring frequent MRI scans), the literature agrees that titanium brackets do not need to be removed to image at 1.5 T or 3 T2,3,4,5.
The authors would recommend liaison with the medical consultants involved in the patient's care to make them aware of the planned arrangements for orthodontic treatment. Artefacts created by any orthodontic appliance on the MRI image may result in incorrect diagnosis or difficulties in diagnosis and it is important that all relevant clinicians are advised of any proposed treatment and the subsequent logistical implications for undertaking future scans.