Magnetic Resonance Imaging (MRI) And Functional Computerized Tomography (4DCT) In Radiotherapy

Magnetic resonance imaging (MRI) is traditionally used in the diagnostic and staging parts of a patient's treatment pathway. More recently however, it has also been used to help determine the planning target volume (PTV). In many CNS patients their diagnostic MRI scan is fused with their radiotherapy planning CT scan. (REF) The oncologist is then able to outline the tumour mass on the MRI as a GTV, which can then be superimposed on the planning CT scan, to check the area for treatment and the margins to be used. This fusion of images, is advantages for two reasons. Firstly the MRI image shows far greater resolution in soft tissue, than the CT, and secondly because the CT scan is carried out post surgery following de-bulking, whilst the MRI is carried out prior to any clinical intervention. Therefore the oncologist is in effect, able to treat where the gross tumour was actually initially located.

Functional MRI (fMRI) is a relatively new technology, which is used with the aim to try to determine precisely which part of the brain is handling which critical functions. This is called brain mapping and is used primarily during surgery before the patient comes for any radiotherapy. The use of fMRI has been extended more recently, as it has now also been used as tool, in order to monitor the growth and function of any remaining brain tumour following treatment.

Functional computerised tomography (4DCT) is a normal CT scanner with software incorporated, or hardware adaptations, which allow it to look at organ motion in relation to bony anatomy. These fall into three main categories; 1 Breath Hold, 2 Gating and 3 Tracking the movements of the tumour.

Another method, which can also be used to monitor and control organ motion linked to breathing, is a type of active breathing control device. An active breathing device allows imaging in only one specific part of the patient's breathing cycle. The benefit of this is that it allows for the scan to be constructed with the tumour in one position, and therefore hopefully not as effected by organ motion. The same active breathing device is then used each day while the patient is having their radiotherapy treatment, so the tumour is localised while the treatment machine is delivering dose. This technique does require much cooperation from the patient, and would only be suitable for patients whose thoracic tumours were not too severe to have caused severely laboured, or erratic breathing patterns.

Another type of tumour tracking device uses reference points that are actually attached to the patient's skin surface. These reference points are then tracked while the patient is being CT scanned to determine at which point in their breathing cycle they were at any given point during the scan. When the data from the CT scan is then being consolidated, this additional motion information is added allowing the actual specific motion of any tumour to be linked to each patient's own specific breathing cycle.


The 3rd 4DCT method effectively builds a margin around the GTV as it continually moves within the body. This technique aims to determine the true extent of each patient's actual tumour motion, so a personalised plan can be produced which gives a treatment dose to the GTV, even though it is moving throughout the treatment. The main problem with this technique is a large volume normally needs to be treated, if the tumour motion is of significance and is therefore not generally useful when using radical radiotherapy treatment techniques.