Medical Imaging Technology: What are the chief benefits of MRI in terms of planning radiotherapy for patients?
Louise Jordan: Computed tomography (CT) imaging is common practice in many oncology centres due to its quick scanning times and the geometric accuracy it provides due to the lack of inherent geometric distortion. It also supplies electron densities of tissues, which are used to calculate targeted radiation doses. However, the limited soft tissue contrast on CT images can make it difficult to accurately define tumour volumes against surrounding structures. It has been well documented that magnetic resonance imaging (MRI) offers superior anatomical soft tissue contrast in comparison to CT, and for this reason many cancer imaging departments are now turning their interest to MRI in the radiotherapy planning process.
Current radiation therapies, such as 3D conformal radiotherapy, IMRT (intensity-modulated radiation therapy) and SRS (stereotactic radiosurgery), allow the delivery of very high radiation doses to small volumes of tissue within a single fraction. It is therefore imperative that tumour boundaries are accurately defined to avoid radiation damage to surrounding healthy structures and organs at risk such as salivary glands and facial nerves. In the field of oncology, MRI is used in the radiotherapy planning of head and neck tumours, as it offers outstanding anatomical detail with excellent characterisation of soft tissue structures.
CT images can only be obtained in the axial plane, whereas MRI has multiplanar capabilities, due to the gradients used in image acquisition. The spatial resolution, tissue contrast achieved, and multiplanar capabilities of MRI, permits 3D reconstructions in every plane, allowing a more accurate volumetric delineation and dose calculation to be made. The benefit of MRI also applies to retreatments, whereby contrast-enhanced imaging can be used to identify changes due to recurrent cancer, as opposed to post-treatment fibrosis.
Why is MRI more likely to trigger feelings of anxiety than other procedures?
Claustrophobia is defined as a fear of confined, or enclosed, spaces. Publications vary greatly, reporting between 1% and 30% incidence of the condition in MRI, including many patients who did not previously consider themselves to be particularly, or at all, claustrophobic.
The patient is accurately immobilised to plan and execute radiotherapy of head and neck tumours, due to the sensitivity of adjacent anatomy such as optical structures, facial nerves, salivary glands and major blood vessels. Any discrepancies between planning scans and radiotherapy treatment could result in geographical miss and local failure causing catastrophic consequences. An immobilisation ‘mask’ called a beam directional shell (BDS), is custom-made for each patient to restrict movement of the head and neck during radiotherapy planning scans and throughout treatment in order to ensure conformity of position. The restriction of the BDS along with the confined environment in the bore of the scanner can provoke claustrophobic reactions and lead to refusal of the examination.
How is the mask made?
At the initial appointment, the patient attends for a mould to be made of the back of the head that will act as a cradle for the head in the finalised mask. The patient wears a swimming cap to create a smooth surface. The back of the head is then encased in plaster. As the latter covers the ears it becomes difficult to hear anything in the room. The sound of the technician shaping and smoothing the plaster is amplified and reverberates in the ears (I liken this to the sound of a cheese grater being scraped across the plaster). The patient must wait until the plaster has hardened and set.
The second session involves making a plaster cast of the face and neck, which is then used to shape a clear, plastic mask. On their third visit, the patient attends to ensure their plastic mask fits exactly to their facial contours and a shoulder-restrictive mesh is applied to prevent movement. The patient is encased in the mask at each appointment to replicate positioning during radiotherapy planning and throughout subsequent treatment.
The mask-making procedure itself has anxiety-inducing elements to it, but, in fact, this may be beneficial, as the patient is introduced to the BDS gradually and is given a sense of the restriction they will endure in the following appointments.
The back of the mask is attached to the tabletop insert. The patient lays back into the cradle and the top of the mask is fitted and clipped onto the tabletop insert to fasten it securely into position. Once the mask is fitted, the head, neck and shoulders cannot be moved. A plastic rest curves over the mask, two body array coils are placed on top of the rest and over the neck to cover the required field of view.
How many MRI procedures can’t be completed due to anxiety?
Estimations of the number of patients who refuse MRI examinations due to claustrophobia vary, as it is very difficult to establish without accurate record-keeping. Patients may refuse the examination before their appointments, with various reasons given for non-attendance. In these scenarios, the cost to the department is difficult to calculate, as the appointed time-slot is generally rescheduled. When the patient does not attend an appointment without prior warning, additional costs are incurred, as the scanner sits idle.
What can medical imaging professionals do to help patients overcome anxiety?
Patient preparation is essential and it is imperative that an explanation of what the treatment entails is given succinctly by medical referrers. This site adopts a multidisciplinary approach in which all preparation, planning and treatment is carried out in the same unit.
This undoubtedly improves clinicians’ understanding of procedural techniques, allowing them to explain the process effectively to patients and their carers. This also guarantees that all those employed in the department understand the patient process, guaranteeing that staff members are as knowledgeable and confident as possible.
One of the most common strategies used in UK hospitals to alleviate fear of MRI examinations is the use of patient information leaflets. All patients referred for treatment of head and neck tumours in this department receive written information detailing the procedure and what to expect.
The MRI information leaflet invites the patient to visit the department, allowing them to become familiar with the environment, while giving staff the opportunity to build a rapport with them and to impart any extra information that might help alleviate any fears.
In this digital age, many people now also turn to the internet for guidance. Hospitals should exploit this opportunity to provide patients with another means of accessing pretreatment advice.
It is imperative that the radiographer establishes good communication with the patient prior to scanning, during setup and throughout the procedure. The patient is informed of the setup and procedure and is told to communicate via prearranged hand signals.
As soon as they lay on the table, the patient is given an alarm ball that they can squeeze if they need to alert the radiographer at any time. This is because once the mask is fitted, the jaw is immobilised and it is very difficult, if at all possible to talk.
Sedation is available in this department as a short-term management of severe anxiety, and is offered as a last resort to patients whose anxiety levels remain high. Staff members in department are experienced in detecting individuals who are having difficulties coping and are able to request a prescription of lorazepam at any point during mask-making, therapy planning and treatment. While this department has not carried out any audit or accurate research to formerly establish the effectiveness of sedation, the refusal rates it experiences are very low. In fact, in the past year, only one patient has refused treatment planning, and was therefore unable to proceed to treatment.
It is vital to minimise the time a patient spends inside the mask and that the duration of the examination is less than 12 minutes in total. As gadolinium is required, a cannula is inserted before the examination begins.
Will MRI scanners in the future be better for anxious patients?
It is important to manipulate scan parameters to achieve images with the necessary signal-to-noise ratio, spatial resolution and contrast in the shortest time feasible. Imaging techniques of the future could be developed to allow reduction of scan time, while also providing high-quality images.
The benefit of modern scanners with open designs and short, wide bores are highlighted in current literature. However, a randomised control trial by Enders et al (2011) disputed this, concluding that even recent MR scanner design could not prevent claustrophobia, and suggested that developments to create a more patient-centred environment were needed. This department incorporates both, with a dedicated wide-bore scanner used in tandem with anxiety-reduction techniques that are delivered by skilled staff in order to make the entire patient experience more tolerable.
In my experience as a radiographer, many patients have attended appointments with anxiety, fear, and claustrophobic tendencies, but have been able to overcome these to complete the mask-fitting, planning scans and radiotherapy treatment, having recognised them as vital components to their cancer management.
This indicates that further research may be beneficial to evaluate the effect of cancer diagnosis on patient anxiety and refusal rates in MRI.
