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Photon-counting CT to revolutionize imaging technology

Photon-counting detectors are a potential new medical CT technique. Experts believe this is the start of a new CT scanners revolution, and it’s the most significant technological advancement for this workhorse imaging modality in years.

Few predicted the tremendous growth of X-ray computed tomography (CT) technology, and even fewer foresaw the rapid development of clinical applications of CT back in the late 1980s. As a matter of fact, there was so much enthusiasm around the newly introduced magnetic resonance (MR) imaging that predictions of MR taking over CT imaging were accepted by many. More than 30 years later, CT has not only survived the challenges from other imaging modalities but has also moved to the frontline of hospital’s diagnostic imaging. For the first time, in 2021, the Indian market too saw CT as a modality overtake MRI. The trend no doubt is expected to reverse itself in 2022, with MRI back to be the larger segment in 2022.

There are many factors that contributed to the success of CT, and there are multiple ways to summarize the technological advancements over the past 30 years. These advances can be examined based on their clinical impact, performance improvements, or the underline technologies themselves. From a clinical impact point of view, coronary CT angiography (CCTA) is no doubt one of the major driving forces for many technological developments. It demands fast data acquisition to freeze the heart’s motion, superior spatial resolving power to characterize small pathologies, and sufficient coverage to enable imaging of the entire heart over one or a few cardiac cycles. Nearly all technological advancements over the years have contributed in one way or another to the success of CCTA today. Of course, stringent requirements for other clinical applications, such as trauma, oncology, and stroke, also played key roles in the technology development.

Needless to say, the future technology is both exciting and diverse. Nowadays, artificial intelligence (AI) and deep learning technologies have become, and will continue to be, a powerful tool and a disruptive technology that pushes the frontier of CT. AI-supported improvement and automation of the CT scan workflow and approaches to enhance the clinical information of CT images have changed the way technicians and radiologists work. Modern CT scanners provide anatomy-aligned reconstructions and advanced visualizations as part of standard image reconstruction tasks, and even automated identification, and quantification of pathological processes are on their way to routine integration into the CT workflow. On the information presentation front, additive manufacturing [or three dimensional (3D) printing] will continue to impact the way radiologists interact with other medical professions and patients. The rapid development of virtual reality and augmented reality has and will continue to impact many radiology departments, ranging from training to operation, and new workflows. Interestingly, despite all technological advances and improvements, all modern CT scanners are still based on the third-generation rotate–rotate geometry. There are new developments on the X-ray tube technologies that may allow multiple X-ray sources to be placed on the same CT gantry, and potentially lead to a new generation of CT scanners with less or no mechanical motion.

Photon-counting detectors are a new technology with the potential to provide CT data at very-high spatial resolution, without electronic noise and with inherent spectral information. Photon-counting detectors were evaluated in prototype CT benchtop systems more than 10 years ago. The performance of the detectors used in these early systems, however, was not adequate for clinical CT imaging, mainly because they did not tolerate the high X-ray flux rates of medical CT. Significant recent progress in detector material synthesis has meanwhile enabled the installation of preclinical whole-body photon-counting CT prototypes for human use.

Photon-counting detectors have several advantages, compared to solid-state scintillation detectors. The detector cells are defined by the strong electric field between common cathode and pixelated anodes, there are no additional separation layers. The geometrical dose efficiency is only reduced by unavoidable antiscatter collimator blades or grids. Different from scintillator-based detectors, each macropixel confined by collimator blades can be divided into smaller subpixels, which are read-out separately to increase spatial resolution.

All current pulses induced by absorbed X-rays are counted during the measurement time of one projection as soon as they exceed a threshold energy. Low-amplitude baseline noise is well below this level and does not trigger counts – even at low-X-ray flux, only the statistical Poisson noise of the X-ray quanta is present in the signal. CT scans at very-low radiation dose or CT scans of obese patients show, therefore, less image noise, less streak artifacts, and more stable CT numbers than the corresponding scans with scintillation detectors. Radiation dose reduction beyond today’s limits seems possible.

There is no down-weighting of lower energy X-ray photons as in solid-state scintillation detectors. Photon-counting detectors can, therefore, provide CT images with potentially improved CNR, in particular in CT scans with iodinated contrast agent. Silicon detectors have been realized with eight energy bins, which may allow additional flexibility in terms of energy-weighting of signals to optimize specific task-based performance, as well as accommodating a larger number of possible simultaneously administered contrast materials.

Compared to established dual-energy acquisition techniques, photon-counting detectors are often assumed to provide better energy separation and less spectral overlap. However, unavoidable physical effects reduce the energy separation. The current pulses induced by X-rays, absorbed close to pixel borders, are split between adjacent detector cells (charge sharing). This leads to erroneous counting of one high-energy X-ray photon as several lower-energy hits.

When diagnosing coronary disease, for instance, seven million invasive catheterization procedures are performed each year, half of which do not lead to therapy. CT angiography provides a non-invasive diagnostic alternative. But conventional CT scans suffer from image artefacts due to calcifications or implanted stents or pacemakers, precluding a large patient population from this approach. The spectral imaging offered by photon-counting technology enables differentiation between calcification, stents, vessel walls, and contrast media. This allows the removal of unwanted data from the image, increasing diagnostic accuracy. With photon-counting technology, one can make non-invasive coronary imaging available to all patients that could benefit from it.

Dr TBS Buxi
Chairman, CT & MRI Departments,
Sir Ganga Ram Hospital

Latest advancements in CT combine improvements in the hardware coupled with smart workflow and AI-driven decision-making support to improve high-quality diagnostic outcome at the lower cost of care. Modern CT scanners bring lot of new AI-enabled software and features, which enable consistency from scan to scan for high quality and fast result. New patient-side gantry controls provide simplified and intuitive workflow for the technologist. Enhance patient care by letting the technologist do more directly from the scanner. Functions like patient selection from HIS/RIS, protocol selection, patient orientation, ECG guidance, interventional controls, and initialization of gantry are possible directly at the scanner. AI-enabled positioning is a new development to improve the accuracy of vertical positioning, improve consistency from user to user, and reduce positioning time to speed up workflow. Advanced interventional technology is designed to improve accuracy to provide confidence and patient safety. It Improves efficiency by automatic needle tracking to ensure the throughput is maintained. This AI-enabled technique automatically calculates depth, angle, and tip to target and deviation for smooth procedures from plan to completion. AI-based motion-compensatory reconstruction in cardiac is a new technique to automatically generate motion-corrected images for both retrospectively and prospectively gated exams. This new reconstruction technology improves image quality at high heart rates. AI-based image reconstruction is designed to lower the radiation dose, and lower noise and improve low-contrast detectability and thus provide an image appearance that closely resembles filtered back projection. Fast reconstruction speed fits seamlessly in your day-to-day operations. Overall, new CT scanners are designed with intelligent innovations that help you enhance patient care, improve image quality, and maximize system availability. While the availability of technique and high-end equipment are extremely important, the crux remains on the attending radiologist’s knowledge, and the clinical need in the patient for requirement of a particular diagnosis. The key to success is a multifactorial scenario, requiring perfect understanding of the suspected clinical diagnosis and the methodology required for conducting the CT scan.

Cancer patients undergo diagnostic scanning and many follow-up exams to assess treatment response and disease progression. With conventional CT, however, the image-acquisition parameters can influence the image itself, preventing accurate comparison of scans over time. Photon-counting CT, on the other hand, provides consistent signal quality, with stable Hounsfield unit values for each patient across every scan.

Photon-counting detectors are a promising new technology for future medical CT. Photon-counting detectors have already been used for several years in high-energy physics and nuclear imaging. However, these previous-generation photon-counting detectors could not be used with a clinical CT scanners because they could not keep up with the high rate of photons reaching the detector. Currently, prototypes are used to evaluate the potential and limitations of photon-counting CT in clinical practice.

Crystal breakthrough. Recently, the US Food and Drug Administration (FDA) cleared the world’s first photon-counting CT scanner, the Siemens Naeotom Alpha, on September 30, 2021. Both FDA and CT experts say this is the start of a revolution in new CT scanners technology, and is the biggest shift in technology for this workhorse radiology modality in years. The centerpiece of the Naeotom Alpha innovation is the new photon-counting detector, which uses an active detection layer that offers advantages over conventional CT detectors. The FDA sent out a rare press release on the approval, noting its importance to medical imaging.

All major imaging companies are developing photon-counting CT detectors. While this technology has been researched academically for several years, it appears several vendors are close to commercializing these detectors, with Siemens only being the first to market.

At RSNA 2020, GE Healthcare said photon-counting CT detector technology is the way of the future and highlighted its purchase of the Swedish company Prismatic Sensors AB in November 2020. The company is developing photon-counting, deep silicon detectors that GE will use for its prototype photon counting CT system. GE began operation of this prototype system in 2021.

Samsung is working with a large US university to develop its photon-counting detector technology and discussed the new detector at RSNA 2020.

Philips Healthcare also has prototype scanners in operation.

Photon-counting detector CT is only in its infancy, with the first human scanner only recently appearing. The next several years are sure to reveal a wealth of information on the potential applications of photon-counting detector CT.

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