Count Them In
By Keith Loria
Vol. 23 No. 4 P. 12
How Photon Counting Is Changing CT for the Better
Photon-counting CT is a new way of capturing X-ray data from CT. The first clinical photon-counting detector (PCD) CT system available for patient care, Siemens Healthineers North America’s NAEOTOM Alpha, was FDA approved in September 2021. That was followed by NeuroLogica Corp, a subsidiary of Samsung Electronics Co, receiving FDA clearance for its OmniTom Elite PCD in March 2022.
A recent study performed by Cynthia McCollough, PhD, and her colleagues from the department of radiology at the Mayo Clinic in Rochester, Minnesota, examined Siemens Heathineers’ system and concluded that it outperforms and has demonstrated several advantages over standard CT, including suppression of electronic noise, improved spatial resolution, and reduced radiation dose.
“Until the approval of the Siemens NAEOTOM Alpha, all clinical scanners used a type of detector that is known as a scintillation detector,” McCollough says. “This means that the X-rays were first converted into visible light, and then the visible light was converted into electrical signal.”
Scintillation detectors are also referred to as energy-integrating detectors because the signal represents a summation of the light energy that is created when X-rays hit the detector. Information about individual photons is lost in the two-step scintillation process.
“In contrast, photon-counting detectors use a one-step process, whereby the X-ray energy is converted directly into electrical signal,” McCollough says. “This allows information about individual photons to be retained, such that the numbers of X-rays in different energy ranges can be counted. This is a game changer because a number of advantages come from using photon-counting detectors, which are also referred to as energy-resolving quantum detectors.”
One advantage is noise reduction. Because the energy of a photon must exceed an energy threshold that is set by the manufacturer, around 20 kiloelectronvolts (keV) to 25 keV, the electrical noise in the detector circuitry can be rejected, as it will fall below that energy level, McCollough explains. This eliminates electronic noise streaking artifacts through dense anatomy, where the number of photons is simply so low that the “signal” is primarily from electrical noise.
“Photon-counting detectors give equal weight to every photon that is registered, meaning that low-energy photons, for example, around 40 keV to 50 keV, that carry a lot of information about iodinated structures in the patient, are treated equally as the higher-energy photons, for example, 100 keV,” McCollough says. “This is a major improvement compared with energy-integrating scintillating detectors, whose signal is proportionately weighted by the brightness deposited by a photon.”
In energy-integrating detectors, the higher-energy 100 keV photon creates much more visible light than the lower-energy 40 keV photon. Thus, the high-energy photon contributes much more signal, even though it contains much less iodine information.
“We refer to the situation with energy-integrating detectors as ‘energy weighting,’ while in photon counting it is referred to as ‘count weighting,’” McCollough says. “The count weighting scenario really boosts the iodine signal in the image for the same amount of injected contrast material compared with non–photon-counting detector scanners.”
To improve the spatial resolution of energy-integrating detectors, the detector material must be sliced and diced into smaller and smaller blocks and light, and reflecting walls, referred to as septa, need to be placed between each individual detector cell. As the detector elements get continually smaller, a larger percentage of the X-ray detector is occupied by these septa. “This means that any X-rays hitting the septa are not used in creating the image, since the walls do not act as scintillators, and this wastes radiation dose delivered to the patient,” McCollough says. “There is also a practical limit to how small the detectors can be diced and the walls made.” In PCDs, the detector material is not sliced and diced. It is a single, large piece of semiconductor material, she explains. The X-rays are converted to electrical charge and the electrical charge is drawn to discrete electrical connectors—positively charged anodes—on the back side of the detector. These electrical signal–collecting anode pixels can be made extremely small without sacrificing radiation dose efficiency.
“The game changer here is that the spatial resolution characteristics of the system are fundamentally much better with photon-counting detectors,” McCollough says. “If a physician needs to create an image of medium-smooth sharpness, the photon-counting system has such high sharpness in it that it can smooth out the collected data and still end up at a medium sharpness level. The smoothing, however, dramatically reduces the noise.”
Thus, PCDs allow users to achieve the same noise level using much less dose or, if they want to use the same dose level, much lower noise levels.
The systems can also achieve much higher spatial resolution than is possible with any energy-integrating scintillating detector system on the market. The cutoff frequency on the photon-counting system is 40 line pairs/cm, which corresponds to 125 microns.
“Work that we presented at the 2021 RSNA meeting showed that we could resolve breast microcalcifications using the photon-counting system after applying some additional noise reduction techniques developed at the Mayo Clinic,” McCollough says. “That is, spatial resolution good enough to see breast microcalcifications is in the system; the increased noise that occurs at really sharp reconstruction kernels just has to be controlled.”
Additionally, because the X-ray detector in photon-counting CT can sort the detected photons into two or more energy bins, both dual-energy CT and K-edge CT imaging is possible. The multienergy scan mode is on all the time because every acquisition sorts the detected photons into multiple energy bins.
“Currently, dual-energy CT requires special hardware [or] acquisition modes, is limited to certain tube potentials, or is not compatible with certain types of dose reduction techniques,” McCollough says. “That is not the case with the photon-counting CT system manufactured by Siemens.”
The biggest challenge in bringing PCD CT to the marketplace has been in the manufacturing of the detectors. Completely new fabrication techniques needed to be developed and then implemented on a large scale.
“But now that it’s been shown that it can be done, I anticipate that more and more manufacturers will follow suit and add photon-counting detectors to their scanner portfolio,” McCollough says. “Also, when someone gets a photon-counting detector scanner, they shouldn’t just operate it the way they have always operated other high-end scanners, or they will get images that look like those from other high-end systems. The routine image series should be virtual monoenergetic images, slice thickness values should bereduced, and much sharper reconstruction kernels should be used if one wants to see the advantages of the technology.”
Comparing and Contrasting
Matthew Fuld, PhD, product manager for photon-counting CT at Siemens Healthineers North America, notes the company’s offering is the world’s first photon-counting CT scanner and is an endeavor that Siemens Healthineers has been working toward for almost 20 years.
“It’s a big achievement that this actually works and we can scan head to toe—all the body regions. And we chose not to release this new type of technology that is complicated from a technical perspective in terms of a vendor implementing it in a simple product; we went to the pinnacle of what is computed tomography in the industry today, which is dual source,” he says. “And so, by offering it in this package of dual-source CT (DSCT), it really pushes the boundaries clinically of what someone can do with CT that they really were not able to do before.”
For example, people who have a very high calcium burden—eg, those with coronary artery disease or atherosclerosis—have calcium buildup in their coronary arteries that makes it difficult for a radiologist or cardiologist to visualize the actual lumen of the vessel because the heart is moving fast.
“Because of the combination of DSCT temporal resolution, higher spatial resolution, and spectral information at the same time, it allows us to eliminate the calcium from visualization,” Fuld says. “So then, a clinician can see the pathway of a coronary artery without the bias of the calcium, allowing them to make a more confident diagnosis.”
NeuroLogica’s OmniTom Elite is the first single-source photon-counting CT scanner with a single detector on a mobile system. It can generate spectral CT images at multiple energy levels, meaning it provides the ability to capture CT data in multiple energy bands, leading to potentially more accurate visualization and segmentation of bone, blood clots, plaque, hemorrhage, and intracranial tumors.
Jason Koshnitsky, senior director of global sales and marketing mCT for NeuroLogica Corp, notes the system also has the ability to provide real-time mobile imaging to administer point-of-care CT to critical patients without the need to transport them to a separate imaging department.
“The whole industry feels PCD is really the next generation, and we’re looking to expand the diagnostic possibilities of CT at the patient’s bedside,” he says. “We’re really at the beginning of all this, and we have a lot of goals and potential.”
With additional research, Koshnitsky feels PCD can possibly improve image quality, reduce dose, reduce artifacts, and offer other potential applications.
The main difference between NeuroLogica’s system and Siemens Healthineers’, Koshnitsky says, is the utilization of a single source; NeuroLogica’s system uses one X-ray tube, the same as its current scanners, as opposed to Siemens Healthineers’ dual X-ray tubes and two sets of detectors.
“The other difference is their machine is a full-body machine,” he says. “Our product will be for head only—it’s a brain scanner.”
Changing Clinical Practice
Some of the most exciting applications of PCD CT that McCollough is seeing with Siemens Healthineers’ NAEOTOM Alpha scanner involve imaging of anatomy where high spatial resolution adds value. These include applications in the inner ear as well as imaging of blood vessels, lungs, bones, and the heart.
“Every image looks better compared with the energy-integrating detector system, with other specifications being comparable,” she says.
Koshnitsky says clinically, PCD is going to have a major impact, especially when it comes to dose reduction.
Considering what it may do in the future, he cites colonoscopy as an example. Today, it requires two days to prepare—one day drinking fluids and one day getting scoped. With PCD technology, clinicians may one day be able to perform a CT virtual scan that removes the bowels from the colon and allows radiologists to see whether any polyps exist. It’s definitely a possibility, he says.
Lung diseases, such as interstitial lung disease, offer another. In patients with interstitial lung disease, their lung parenchyma can be altered to a variety of different texture patterns. Understanding what those alterations are, which patterns they form, and what is the distribution is critical to understanding their progression of the disease.
“Now that there are new treatments to help reduce symptoms and reduce progression of interstitial lung disease, it’s very important to understand whether a drug is working and whether it’s going to be valuable for a particular patient because they’re quite expensive,” Fuld says. “We can use a tool that gives a much better depiction of the texture patterns of the lung to help inform the physician whether or not a treatment path is working. That will be very helpful guiding long-term decisions.”
Fuld notes the uniqueness of photon counting for clinical practice.
“We have as an industry developed many technologies over the last decades that have changed fundamentally the behavior of what we do with a CT scanner,” he says. “This is not a technology that just provides spectral information or high resolution or just goes fast. This is a technology that provides all of those things at the same time, while doing it at low dose.”
Koshnitsky feels the next step in PCD will be applying AI to harness the power of photon counting to a greater degree.
McCollough notes there are additional benefits that will come into play as the technology evolves, including the ability to distinguish between two different contrast materials, such as iodine and gadolinium, but the photon-counting CT systems already outperform current state-of-the-art CT systems and are game changers in the industry.
— Keith Loria is a freelance writer based in Oakton, Virginia. He is a frequent contributor to Radiology Today.