University College London and ISDI Collaboration
Interview with Rimcy Palakkappilly Alikunju and Kirstie Mogilner
KM: Hi can you tell me a bit about your journey which led to your collaboration with ISDI?
RPA: My background is in Electronics and Communication Engineering with Masters in VLSI (Very Large-Scale Integration) and Embedded Systems. I was working as a Lecturer in one of the Further Education College in London when I saw the post for a PhD at University College London (UCL) looking at ‘Innovative CMOS Sensor Solutions for X-ray Detection and Imaging’. This really appealed to me as I have done my Master’s project in ASIC design based on MOS transistors and also has worked with microprocessor/microcontroller-based projects in academia where sensors play a key role in detecting various signals. So, to have a real-life experience starting from sensor design to fabrication is more than like a dream coming true. Moreover, the wide range of diverse skills that the research topic can offer in terms of industry and academia motivated me to apply for the research role.
In my current role, I am a graduate researcher doing my PhD under Prof. Alessandro Olivo’s guidance in the Medical Physics and Biomedical Engineering Department at UCL. My collaboration with ISDI began as UCL already had a relationship with ISDI and my project is jointly funded by ISDI and Engineering and Physical Sciences Research Council (EPSRC). The project requires the development of a new X-ray detector at ISDI. Also, so far, I have been doing my testing and characterisation studies using the ISDI detectors and equipment and that is where ISDI comes into play.
KM: What has your experience been like working with the ISDI team? Can you name anyone in particular who you collaborated with closely who contributed to your learning/experience?
RPA: It’s a pleasure to work with ISDI and to be honest it is a very professional organisation. All the team members are very friendly and pro-active, knowledgeable, and moreover, they are experts in their field. I have been able to meet and collaborate with all the team members and I have worked especially closely with James Brodrick, Asmar Khan, Thalis Anaxagoras, Edward Bullard, Yiannis Stamatis, Nicolas Iacovou and Jason Morehen. But I don’t want to single anyone out as everyone has been very helpful. It is a very good working environment, and I can’t thank enough for the valuable support ISDI has given me for the accomplishment of my work and for the paper which has just recently published.
KM: So from your deep dive into scintillators & FOPs have you found out anything that surprised you?
RPA: It was in fact a study of how scintillators of different thicknesses contribute to the performance characteristics of the X-ray detectors. For example, using a thin scintillator gives higher resolution but using a thicker scintillator, higher quantum efficiency can be obtained due to higher X-ray absorption power. In addition to scintillators, I have also studied different X-ray detector configurations by varying specific parameters like FOPs, substrate coating and using different pixel pitch. In terms of my PhD project, this study will help me with which scintillators to choose and which detector configurations to use.
Thinking application-wise, CT and dental for example, which requires a low dose, a higher DQE detector might be a good choice. But in the case of industrial applications, where dose is not a primary concern, higher resolution detectors will be a better choice. All these results are presented in the paper and will help the researchers or X-ray detector companies or any prospective users. They will be able to look at the results and use this information to choose the most suitable detector configuration for their imaging needs.
KM: That is exactly what I thought when I looked at the paper – this will be really useful for the end user or the customer, enabling them to think about what they actually need for their application. What will you be doing next in the world of imaging?
RPA: ISDI has recently manufactured a double layer detector (which I am calling a ‘sandwich’ detector) which I plan to use for my PhD Dual Energy project. This will really help in applications like Cone Beam Computed Tomography (CBCT) for radiation therapy or Breast CT or Dental. By Dual Energy, images of the phantom material(s) at two different energies are taken and due to the difference in attenuation at these energies, the sandwich detector will provide relatively higher contrast images than a single or conventional CT aiding in material discrimination such as microcalcifications in Breast CT for example.
KM: Can you tell me more about the dual layer detector?
RPA: Yes, this means there are two sensors on top of each other. One sensor will detect the low energy photons and the other will detect the higher energy photons. Hence two images at two different kV’s are obtained simultaneously – single shot dual energy imaging. The sandwich detector is thus expected to lead to high-quality images at lower doses, allowing better diagnostics in medical applications without increasing the radiation burden (this is still in the experimental stage and needs to be proven).
Dual Energy ‘Sandwich’ Detector
KM: I have seen that used in security imaging
RPA: Yes, the concept is not new, and it has been used in security applications, but we are trying to extend its use into medical applications like CBCT, Breast CT, dental etc., though there are already dual layer detectors being used in chest applications.
KM: For your PhD how are you utilising this ‘sandwich’ detector?
RPA: In the initial phase, experiments will be conducted on how it can be used for material discrimination by identifying the effective atomic number and electron density of the item being imaged by employing dual energy algorithms on the dual layer detector images. For example, in airport security applications, where the content inside a luggage is unknown, X-ray images can be fed into the dual energy algorithm which will give information about the effective atomic number (Zeff) and electron density of the materials contained in the luggage. Thus, any potentially dangerous material(s), if any, can be identified.
The overall aim of my project is to obtain a sufficiently high DQE at both sensor layers coupled with a spectral separation, leading to a high performance of the dual-energy algorithms. The detector is mainly aimed at material discrimination at this point, but we will have to wait to see how effectively it can be used after the research.
KM: It sounds exciting – hopefully we can share the results when your dual energy project is finished. Finally, what did you think of ISDI detectors?
RPA: ISDI detectors proved to be very good based on the characterisation studies conducted. From the results so far, they have given good quantum efficiency and MTF values which is suited for a wide range of applications.
Hand phantom X-ray Image (taken at 70kV)
You can download Rimcy’s paper here:
Effect of different scintillator choices on the X-ray imaging performance of CMOS sensors
This paper compares the performance characteristics of CMOS X-ray detectors in various configurations by varying certain parameters of a typical X-ray detector such as fibre optic face plate (FOP), scintillator substrate coating, sensor pixel pitch and scintillator thickness
In particular, we studied the effects that certain changes in configuration have on the overall X-ray performance of the detector, namely (i) using a fibre optic plate (FOP) versus not using it, (ii) having different substrate coatings for the scintillators, (iii) using sensors with different pixel pitches, and finally (iv) using scintillators of different thicknesses.
X-ray characterisation metrics such as sensitivity, pre sampling modulation transfer function (pMTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) as per the IEC 62220-1 standard specifications in each case.
Based on these characterisation studies, specific X-ray applications can be matched with different detector combinations. For instance, computed tomography (CT) and dental applications demand low Air Kerma, hence detector configurations with high DQE values are preferred. High-resolution detectors with greater pMTFs are preferred for industrial X-ray applications. Thus, this study can help researchers and practitioners choose a detector configuration based on the specific demands of their application.
1. UCL Department of Medical Physics and Biomedical Engineering, Malet Place, London WC1E 6BT, UK
2. ISDI Ltd, Highgate Business Centre, 33 Greenwood Place, London NW5 1LB, UK