Innovations conference 2014 dr shalini vinod dedicated magnetic resonance imaging for radiotherapy
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Dr Shalini Vinod - Dedicated Magnetic Resonance Imaging for Radiotherapy
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Semelhante a Innovations conference 2014 dr shalini vinod dedicated magnetic resonance imaging for radiotherapy
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Innovations conference 2014 dr shalini vinod dedicated magnetic resonance imaging for radiotherapy
- 1. Liverpool & Macarthur Cancer Therapy Centres
- 2. Dedicated Magnetic Resonance Imaging for Radiotherapy Planning SK Vinod, LC Holloway, E Juresic, R Rai, L Cassapi, A Xing, G Goozee, D Moses, GP Liney
- 3. Liverpool & Macarthur Cancer Therapy Centres Background • MRI is increasingly being used for volume definition in radiotherapy – Superior soft tissue contrast compared to CT – Can also provide functional imaging • Diagnostic MRI not ideal – Different patient positioning – Time interval between diagnostic imaging and radiotherapy planning – Cost & access issues
- 4. Liverpool & Macarthur Cancer Therapy Centres Aims • To describe the implementation of MRI simulation for common tumour sites treated with radiotherapy: – Head & Neck cancer – Cervix cancer – Prostate cancer – Rectal cancer – Brain cancer
- 5. Liverpool & Macarthur Cancer Therapy Centres MRI Simulator • MRI simulator (3T) installed June 2013 at Liverpool Hospital • Dedicated MRI physicist • Dedicated MRI radiographer • Trained radiation therapists • 0.2 FTE radiologist
- 6. Liverpool & Macarthur Cancer Therapy Centres MRI Simulator • Flat bed • Laser lights • Wide bore • RT specific coils/supports
- 7. Liverpool & Macarthur Cancer Therapy Centres Methods • Basic requirements for MRI simulation developed by the MRI Sim Group • Diagnostic protocols for MRI acquisition used as starting point for scans • Site-specific tumour groups consulted about requirements from MRI scans – Tumour vs Lymph node vs Normal tissue visualisation • Protocols refined with site-specific input
- 8. Liverpool & Macarthur Cancer Therapy Centres Methods • Healthy volunteers scanned to optimise images before clinical scan acquisition • Volunteers scanned in supine and prone position with and without bellyboard to optimise imaging for rectal cancer • Standard bladder and bowel protocols for pelvic imaging (as per CT sim)
- 9. Potential problems with MRI • Patient set-up • Distortion – Geometric – Patient • Longer scan times – Patient motion – Organ motion • Claustrophobia • Contraindications – pacemakers, implants etc Liverpool & Macarthur Cancer Therapy Centres
- 10. Results – Basic requirements • Patient set-up – Identical patient positioning to CT simulation and treatment – Flat bed couch with 32 channel integrated spine coil – Use of MRI compatible immobilisation devices – MRI bore size may preclude use of some immobilisation devices – Placement of RF coils must not alter patient anatomy Liverpool & Macarthur Cancer Therapy Centres
- 11. Head & Neck Cancer – Patient position Liverpool & Macarthur Cancer Therapy Centres
- 12. Rectal Cancer – Patient position Liverpool & Macarthur Cancer Therapy Centres PRONE SUPINE
- 13. Results – Basic requirements • Field of View < 30cm – To minimise geometric distortion – To adequately image required anatomy for fusion • Slice thickness 2mm – To enable detailed visualisation of anatomy – To optimise fusion to simulation CT and better interpolation in RTP Liverpool & Macarthur Cancer Therapy Centres
- 14. Prostate Cancer – CT vs MRI Liverpool & Macarthur Cancer Therapy Centres
- 15. Results – Basic requirements • Bandwidth ≥ 440 Hz – To minimise patient distortion from susceptibility artefact and chemical shift • Scan acquisition in one concatenation & use of parallel imaging – To minimise scan time • Patient motion • Patient tolerability Liverpool & Macarthur Cancer Therapy Centres
- 16. Head & Neck Cancer MRI simulation Liverpool & Macarthur Cancer Therapy Centres
- 17. Results – Functional imaging • Functional sequences – Diffusion weighted imaging – Dynamic contrast-enhanced imaging – Perfusion imaging Liverpool & Macarthur Cancer Therapy Centres
- 18. Cervix Cancer – Functional Images T2 TSE RESOLVE DWI Liverpool & Macarthur Cancer Therapy Centres T2 HASTE ADC MAP
- 19. MRI simulation by tumour site 20 18 16 14 12 10 8 6 4 2 0 Aug-13 Sep-13 Oct-13 Nov-13 Dec-13 Jan-14 Feb-14 Mar-14 Apr-14 May-14 Jun-14 No of patients Liverpool & Macarthur Cancer Therapy Centres H&N MRI Gynae MRI Prostate MRI Rectum MRI Brain MRI
- 20. Liverpool & Macarthur Cancer Therapy Centres Conclusions • Presence of MRI simulator allows improved patient access for radiotherapy planning • Specific requirements for MRI for radiotherapy planning which are different to diagnostic requirements • Balance between acquiring adequate images vs length of scan (patient tolerability & motion) • Multidisciplinary input required between RO, ROMP, RT, MRI radiographers and radiologist for successful implementation of MRI simulation
Notas do Editor
- Magnetic resonance imaging is being increasingly used for definition both tumour volumes and normal tissues for radiotherapy planning. It has superior soft tissue contrast compared to CT and different sequences can also be used to indicate function. To date radiotherapy departments have relied on fusion of MRI scans taken in diagnostic radiology departments. However these are not ideal for radiotherapy planning. The patient positioning is different, the external body contour required for planning may not be present, the scans are usually taken on a curved bed whilst all radiotherapy treatment occurs on a flat bed. In addition there is often an interval between when the patient was scanned and radiotherapy planning during which the tumour may have changed ie grown. Cost and access issues also impact on widespread uptake of MRI information for radiotherapy planning.
- The aim of this study is to describe specific MRI simulation for radiotherapy planning for common tumour sites. These tumour sites were chosen MRI has been shown superior to CT for demonstrating the extent of cancer for all these tumour types. In addition these cancers are usually treated with radiotherapy in the definitive setting (rather than post-op setting) meaning that there is a gross tumour to volume.
- A MRI simulator was installed in the radiation oncology department at Liverpool Hospital in June 2013 for clinical and research use. It was positioned adjacent to the CT simulator. It is not adequate to just have new technology, staff are crucial to successful implementation. There was a MRI simulation steering group charged with implementing MRI for simulation. We employed a MRI physicist, MRI radiographer and MRI radiologist who all supplied a skill set which was not present in our department.
- This is a picture of our MRI simulator. Differences between this and a diagnostic MRI included a flat bed which is necessary for patient stability and reproducibility between MRI, CT and treatment. The wide bore allows for radiotherapy immobilisation devices such as masks etc to go through the bore. Some of the RT specific coils and supports are seen on the couchtop. There are also laser lights for patient positioning as we have in our CT simulator and on the linacs to ensure reproducibility of patient position from planning through to treatment.
- The MRI sim group developed a set of basic requirements for images taken on the MRI simulator using diagnostic protocols as a starting point. Site-specific tumour groups were consulted in order to refine the protocols in order to best demonstrate tumour and normal tissue anatomy relating to particular anatomical sites.
- There are several potential problems with MRI sim which need to be overcome. First patient set-up needs to be identical to CT simulation and that on treatment. This includes standard filling of the bladder and having an empty rectum for pelvic tumours. MRI scans are subject to distortions which can result from both the magnetic field in general and also patient specific distortion related to individual anatomy. Scan times may be long compared with CT during which time patient and organ motion can occur. In addition some patients may not be able to tolerate MRI due to claustrophobia or other contraindications such as pacemakers etc.
- At Liverpool the MRI sim is performed after the CT simulation with identical patient positioning and immobilisation devices. However MRI bore size, which is smaller than CT may preclude use of some immobilisation devices. The imaging coil is integrated into the flat bed. It is important that the use of any other coils on the anterior surface of the patient be placed in such a way that they don’t alter patient anatomy.
- This is a photograph showing the immobilisation for H&N treatment. Two small flexible coils are placed laterally around the fixation shell using two coil supports. The 18 channel body array is connected to one of the available ports at the bottom of the table using a long cable
- These are examples of T2 TSE images acquired in the prone position (superior images) and supine position (inferior images) in volunteers. The image quality of the scans taken in the supine position is better than those in the prone position. The position of rectal cancer patients and planning technique was subsequently changed to incorporate the better visualisation of tumour in the supine position.
- The field of view not be too large due to geometric distortions which occur with MRI which are worse at the edge of the field. With modern scanners distortion errors are 1-2mm within a radius of 10-15cm. The slice thickness has to be quite small for a more detailed visualisation of tumour and organs at risk. Diagnostic protocols often use slice thickness of 3-5mm where as 2-3mm is what is required for radiotherapy planning. Patients still have a simulation CT to enable radiotherapy dose calculation and having an identical slice thickness with CT optimises registration of the two imaging modalites. Note: In MRI spatial encoding is achieved through combination of static magnetic field and gradient coils which superimpose linear changes in the field. Geometric distortions is due to inhomogeneity of static magnetic field or non-linearity of gradients.
- These images acquired in a patient with prostate cancer show the superior visualisation of the prostate on MRI. The prostate apex is particularly difficult to see on CT (Top left) but much better visualised on MRI (T2 TSE, Top right). The overall anatomy of the prostate is also better delineated on MRI (bottom right) compared to the CT (bottom left).
- Patient related artefacts such as susceptibility artefact and chemical shift artefact can be minimised by having a bandwidth >=440Hz. Adjacent images in MRI acquisition may have different image contrasts. This is due to cross excitation of adjacent slices and is usually overcome by leaving a gap between slice acquisition. However for radiotherapy planning we require all the anatomical information without any gaps. Concatenation involves imaging alternate slices over two acquisitions so that there is no gap in imaging ie image slices 1,3,5etc and then return to image slices 2,4,6 etc. Parallel imaging is a method of acquiring the scan data more efficiently by using multiple coils arranged around the area to be imaged. Note: Chemical shift artefact is caused by the magnetic shielding of the nucleus by electron clouds that surround it. The relevant chemical shift is between protons in fat and water. Protons on fat experience a weaker magnetic field than protons in water meaning that fat containing tissues will be shifted in the direction of the frequency encoding gradient. The susceptibility artefact is due to the differences in the degree of magnetization of human tissues in response to an applied magnetic field. This is greatest at tissue:air interfaces. Both these patient-related artefacts can be minimised by setting the pixel bandwidth at twice the water-fat shift ie >440Hz.
- These are images acquired in a head & neck tumour patient. The top two images serve to illustrate the image quality and coverage obtained using dedicated RF coils which extends from midbrain down to sternal notch. The bottom images show a slice taken through the tumour using (left to right) DIXON T2-w in-phase, water-only and DIXON T1-W water-only post-contrast
- The MRI image sequences can also be used to capture functional information using techniques such as Diffusion Weighted Imaging, Dynamic Contrast enhanced imaging and perfusion imaging. These techniques can potentially identify the active tumour within a whole organ eg prostate or cervix.
- Example images acquired in a cervical tumour patient; (from top left going clockwise) transverse T2 TSE, sagittal T2-w haste, ADC map and RESOLVE DWI with b= 800 s/mm2 both of which demonstrate an area of reduced diffusion.
- Since the implementation of MRI simulation the its use across the 5 tumour sites has increased. The greatest use currently is in prostate and gynae cancers. MRI sim is being offered to all patients with a clinical indication. It is mainly being used for tumour delineation but also organs at risk esp neural tissues. We have access to a radiologist to help with interpretation of MRI images as there is a learning curve for all tumour sites. At present we still need to perform simulation CT scans to allow electron density data for radiotherapy planning but work is currently underway to overcome this limitation of MRI.
- Future plans Use of functional imaging (DWI, DCE) to identify active tumour regions MRI during treatment to assess - Tumour changes, Normal tissue changes Evaluate prognostic value of imaging parameters in radiotherapy MRI simulation to replace CT simulation