Dr. Shaihan Malik, PhD is a Reader – Associate Professor of medical imaging/MRI physics at King’s College London currently involved in researching heating effects from radiofrequency waves at 7T, as well as Quality Assurance (QA) for clinical research and interventional MRI at a variety of field strengths.
A key reason for purchasing a Multisample120E phantom from Gold Standard Phantoms was to help with our research on heating effects at 7T and to be able to validate new coils used on this powerful scanner. With this phantom we can validate predictive models of heating effects via MR thermometry combined with temperature probes inserted directly into the phantom. By measuring temperature changes and relating it to the specific absorption rate (SAR), we can validate that a coil is behaving as expected, as part of our quality assurance procedures.
The process of purchasing the phantom was smooth, we communicated with the team about our requirements for MR thermometry including the placement of probes and custom reference points and worked collaboratively to find a solution. GSP also sent us a prototype that we were able to trial and send back for alterations before the phantom was finalised and purchased. Preliminary measurements show the phantom is working well and allows us to simultaneously take readings from the embedded temperature probes and perform MR thermometry. Moving forward we are finalising the MR thermometry protocol, and we plan to use this technique to validate safety models both commercially purchased coils (for off label use for example) or for those developed in-house by our RF Engineering lab, led by Özlem Ipek.
Our second research focus involves quantitative MRI and the Multisample120E allows us to validate the T1 and T2 mapping protocols as we have a known reference value in the vials for comparison. The Multisample120 is filled with a susceptibility matching fluid to avoid image distortions, and this is usually just water with some doping to change the relaxation times. However we have found that at 7T (i.e. 300 MHz) the high dielectric constant of water creates strong B1 homogeneity which then leads to poor performance of quantitative MRI methods. We have found the phantom to be very useful to investigate alternative replacements for a flood-fill liquid – we haven’t yet found a solution but hope to soon.
Dr. Vasia Papoutsaki PhD is an MRI Physicist, senior research fellow at the Center for Medical Imaging, University College London. Her research primarily involves quality assurance for quantitative imaging biomarkers. She has previously developed a variety of test objects (phantoms) for T1/T2 weighted imaging and diffusion weighted imaging, and also been involved in the preparation for imaging with Hyperpolarized 13C imaging.
“One of my main research areas is developing and validating novel imaging biomarkers, and I am working with Professor Shonit Punwani on this topic. Our current focus is on the validation of the apparent diffusion coefficient (ADC) in diffusion weighted MRI (DW-MRI) for use in the detection of prostate cancer. To do this we have started a clinical trial, where we are attempting to mimic the anatomy and ADC values of prostate cancer using test objects, or phantoms. Since the phantoms that we are using show temperature-dependent values of their diffusion characteristics, we absolutely need to control for the environmental parameters, and in particular we need to maintain the temperature of the liquid surrounding the test objects stable or controlled over the time of the examination. This trial is expected to last three years in the UK and US across multiple MRI scanner vendors.
“Diffusion weighted MRI is based on the Brownian motion of water and is the only MRI technique able to reflect tissue cellularity. Currently, the Prostate-Specific Antigen test (PSA) is the first step in prostate cancer screening. However, this is inaccurate, resulting in high rates of false positives and false negatives, which leads to excessive numbers of follow up biopsies to establish diagnosis, which are painful and time-consuming. Recently, the National Institute for Healthcare and Excellence (NICE) in the UK included the use of multi-parametric MRI as an intermediate step in an attempt to reduce the number of biopsies recommended by changing the diagnostic pathway, and we are now working to further increase the accuracy and specificity of MRI.”
I have previous experience in producing phantoms, however we lacked specialist equipment, which is why we decided to work with a manufacturer. The Gold Standard Phantoms team (led by Dr. Aaron Oliver-Taylor, GSP CTO) understood all the requirements we needed for this trial and was able to suggest ideas I was not aware of. In total, the collaboration process lasted six months, which also involved double validation of our diffusion solution in the US. This was something Gold Standard Phantoms were willing to accommodate, while other companies weren’t, and this, along with their excellent knowledge of MRI and phantoms, is why we decided to work with them. The end product became the MultiSample190, I have been using it for six months and the results have been very good.”
“The outcome of this trial, undertaken in collaboration with the Quantitative Imaging Biomarkers Alliance (QIBA®), is to validate the ADC values in prostate cancer by establishing the influence changing the MRI sequence parameters has on the apparent diffusion coefficient and how this impacts the translatability of the value across protocols and scanners in terms of diagnosis and cut off values for patients. Its results will be used as part of the guidance provided by QIBA® for the use of ADC in prostate cancer.“
“Overall, we have been very happy working with Gold Standard Phantoms and I would definitely recommend them to other clinicians and researchers that need a test object.”
Professor Mark “Marty” Pagel, leads the Contrast Agent Molecular Engineering Laboratory Imaging in the Department of Cancer Systems Imaging and the Department of Imaging Physics at the MD Cancer Centre, Houston, USA. His particular interests are in molecular imaging for the tumour microenvironment where he uses MRI, PET/MRI and MRI fingerprinting. The aim of the molecular imaging performed is to provide validation for the tumour acidosis, hypoxia and vascular perfusion, and enzyme activity in the tumour microenvironment between blood vessels and cells. In addition to this, his team have developed a method of CEST MRI to quantify the extracellular pH of tumours and the oxygenation with a variety of other techniques. Their research is primarily preclinical (75%) with 25% in clinic, most of these studies are at the translational stage consisting of approximately 20-60 participants.
“In order to translate our small animal imaging methods to the clinic it is important to verify that our instruments are rigorous and related to each other. To facilitate this, a “standard” is required. In CEST MRI this is not established, whereas in more commonly used methods it is. Therefore, we have taken the initiative to create a standard for CEST MRI to be used by the research and radiology communities for quality assurance/quality control for routine CEST MRI. Currently, we are focusing on establishing a standard for APT CEST, metabolite-weighted CEST and acidoCEST that measures pH.”
“For these projects we have relied on the MultiSample120E as it accommodates our requirement of having numerous tubes (24 in total). This is necessary as we are using multiple materials that are commonly used by others for CEST MRI. The large number of tubes makes this phantom more versatile, meaning we can create a standard that more researchers will be able to use for various types of CEST. This feature is what cemented our decision to use the MultiSample120E over other companies who do not offer this level of versatility. Another advantage of the GSP MultiSample120E is that it is extremely easy to change the tubes and content, whereas phantoms from other manufacturers have fixed configurations or more intricate designs, which makes them harder to take apart.”
“A reason we require several tubes is that some are used for MR thermometry. As temperature impacts all CEST methods in order to perform a standardised protocol, you must maintain the phantom at a universal and constant temperature. However, magnet rooms are not well controlled in terms of temperature and to compensate for this, some researchers use a water ice slurry. However, this only lasts 30-45 minutes, which isn’t always sufficient. Therefore, we use a material from a third party suggested by GSP that maintains 37°C for 6-10h which can be reheated multiple times in order to monitor the temperature of the phantom. This further highlights the advantages of the screw top design, which allows us to quickly and easily fill the tubes before this material cools and changes to a solid state.”
“Although we’ve made many phantoms ourselves, these are only sufficient as initial prototypes for preclinical imaging and are not appropriate for clinical use. We’ve also used phantoms from other companies, however we found these didn’t accommodate enough tubes for this application and we have had to modify them. Another issue we had with their diffusion phantom was that it is too big to fit within our coil for the 7T Siemens system we use, so far only the MulitSample120E has been able to fit.”
“I appreciate working with GSP as you are a mobile and entrepreneurial company, which is important when producing innovative phantoms and MRI methods.”