Diagnostic radiology tests utilising various imaging equipment like CT/X-ray are essential to arrive at an accurate diagnosis. iDose4 is an iterative-based reconstruction technology introduced by Philips in its CT scanners through which the scanner is enabled to reduce radiation dose up to 80 per cent without compromising on the quality of images. iDose4 is also capable of improving the image quality at low radiation dose. “This technology is amongst the most successful low-dose imaging technologies in the CT industry, which got widely appreciated by the entire radiology community,” says Raveendran Gandhi, senior director-Radiology, Philips Electronics, who shared more details on this technology.

Fig. 1: (a) Full dose images (FBP - filtered back projection), (b) iDose4 images (80% less dose)
Fig. 1: (a) Full dose images (FBP – filtered back projection), (b) iDose4 images (80% less dose)

Radiation—A concern and the reason behind iDose4
Talking about the effects of radiation exposure, Raveendran Gandhi said, “Exposure to ionising radiation leads to several biological side effects. Multiple clinical studies on radiation effects on foetus have consistently found an association between exposure to medical radiation during pregnancy and risk of childhood cancer in the offspring.”

Fig. 2: iDose4 images of the heart showing calcification and narrowing of coronary arteries
Fig. 2: iDose4 images of the heart showing calcification and narrowing of coronary arteries
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According to Raveendran Gandhi, several clinical studies basing radiation effects on pediatric CT risks suggest that pediatric CT will result in significantly increased lifetime radiation risk over adult CT, both because of the increased dose per milliampere-second, and the increased lifetime risk per unit dose. In the UK, it has been estimated that approximately four per cent of diagnostic radiology procedures are CT examinations, but, their contribution to the collective radiation dose is approximately 40 per cent.

This emphasises the need for low-dose CT imaging technology.

iDose4—A different approach to image reconstruction
“For decades, filtered back projection (FBP) has been the industry standard for CT image reconstruction. While it is a very fast and fairly robust method, FBP is a sub-optimal algorithm choice for poorly sampled data or for cases where noise overwhelms the image signal. Such situations may occur in low-dose or tube-power-limited acquisitions, for instance in the scans of morbidly obese individuals,” says Raveendran Gandhi. Talking about the noise, he further adds, “Noise in CT projection data is dominated by photon count statistics. As the dose is lowered, the variance in the photon count statistics increases disproportionately. When this very high level of noise is propagated through the reconstruction algorithm, the result is an image with significant artefacts and high-quantum mottle noise.”

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iDose4 is based on a completely different approach to image reconstruction (IR) techniques. Raveendran Gandhi says, “IR techniques attempt to formulate image reconstruction as an optimisation problem, i.e., IR attempts to find the image that is the ‘best fit’ to the acquired data. The noisiest measurements are given low weight in the iterative process; therefore they contribute very little to the final images.” He further adds, “Hence, IR techniques treat noise properly at very low signal levels, and consequently, reduce the noise and artefacts present in the reconstructed image. This results in an overall improvement of image quality at any given dose.” With IR techniques, the noise can be controlled for high spatial resolution reconstructions, hence, providing high-quality, low-contrast and spatial resolution within the same image. These are possible due to the recent innovations in hardware design and algorithm optimisations.

Fig. 3: iDose4 images of a patient who had undergone coronary angiography and stenting. iDose4 improved spatial resolution enabled clear visualisation of the patency of the stent
Fig. 3: iDose4 images of a patient who had undergone coronary angiography and stenting. iDose4 improved spatial resolution enabled clear visualisation of the patency of the stent

Working of iDose4. Whilst explaining the working of iDose4, a noteworthy point mentioned by Raveendran Gandhi is that in iDose4 algorithm, iterative processing is performed in both the projection and image domains. The reconstruction algorithm starts with projection data, where it identifies and corrects the noisiest CT measurements—those with very poor signal-to-noise (S/N) ratio, or very low photon counts. Each projection is examined for points that have likely resulted from very noisy measurements using a model that includes the true photons statistics. Through an iterative diffusion process, the noisy data is penalised and edges preserved. This process ensures that the gradients of underlying structures are retained, thus, preserving spatial resolution whilst allowing a significant noise reduction.

Fig. 4: Philips 128 Slice Ingenuity CT scanner with iDose4 technology
Fig. 4: Philips 128 Slice Ingenuity CT scanner with iDose4 technology

The noise that remains after this stage of algorithm is propagated to the image space. however, the propagated noise is now highly localised and can be effectively removed to support desired level of dose reduction. Then the iDose4 algorithm deals with subtraction of the image noise whilst preserving the underlying edges associated with true anatomy or pathology.

Challenges faced. With a healthy innovation comes obstacles that need to be tackled. Talking about the challenges the Philips team faced whilst coming up with iDose4, Raveendran Gandhi says, “While transitioning from the existing, widely accepted FBP technology to IR technology through our basic first-generation iDose platform, and its innovative journey to graduate into the current fourth generation of iDose4, optimising and improving the product in every stage, we have surpassed several challenges.” Radiologists appreciate images, which have a natural appearance, devoid of all artefacts. “As each CT patient study may contain acquisitions of several hundred cross-sectional images of the body, those images were expected to be reconstructed under a minute to match with the busy patient workflow of a radiology department. Such ultrafast reconstruction requires very high computational power. Philips achieved reconstruction speed of 20 images per second through its Rapidview IR hardware. The other biggest challenge was to improvise the existing methods to significantly improve spatial resolution in order to drastically improve image quality. Despite these challenges, iDose4 acquired up to 68 per cent improvement in resolution,” added Raveendran Gandhi.

Hardware involved
“iDose4 is a sophisticated and complex reconstruction algorithm that demands enormous computational power,” says Raveendran Gandhi. The interaction of information between the projection and image domains requires the support of elegant software and hardware architectures. Running iDose4 on the prior generation of reconstruction hardware (RapidView) would result in clinically unacceptable reconstruction times.

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Fig. 5: Philips 256 Slice Brilliance iCT scanner with iDose4
Fig. 5: Philips 256 Slice Brilliance iCT scanner with iDose4

The RapidviewIR reconstruction engine was designed from the ground to benefit from not only higher performance computational cores but also the number of cores. The architecture is highly parallel, and the design enables the reconstructor to scale with the latest multiple-core processors and state-of-the art massively parallel, high-density computing.

Raveendran Gandhi informs that the high-density computing device on Rapid View IR processes and transfers huge amount of data. The latest generation PCI express bus offers substantially higher I/O bandwidth and Intel 6-core processors are utilised to address the additional computing requirements. As a result, the new Philips RapidView IR reconstructor is able to deliver exceptional reconstruction performance with iDose4, thus providing reconstruction speeds similar to those previously achievable with FBP conventional reconstructors.

The iDose4 advantage
iDose4 enables significant dose reduction whilst preserving equivalent diagnostic image quality to a corresponding full-dose scan. Optimising the implementation of iDose4 on the Philips CT scanner platforms has enabled the additional clinical benefit of being able to adapt the spatial resolution and dose-reduction benefits to the specific clinical indication. “For example, for pediatric imaging, where radiation dose reduction is paramount, iDose4 enables significantly lower radiation dose whilst maintaining diagnostic quality. In other clinical scenario, such as coronary stent patency imaging, where image quality (spatial resolution) is of higher priority, iDose4 can be used to improve image quality by significantly improving spatial resolution. Intermediate levels of dose reduction and spatial resolution improvement can be applied in combination for other clinical scenario as well,” informs Raveendran Gandhi.

Something similar to iDose4?
“Most of the CT vendors are working towards technology which results in reducing radiation dose to patients. The performance efficiency of the desired product should be determined by its ability to adapt and integrate into the routine clinical work flow of CT department, its speed of image reconstructions, image quality and ease for technical use,” says Raveendran Gandhi. Traditionally, Philips always worked to adhere to as low as reasonably achievable principle in CT Imaging and consistently brought out solutions in practical low-dose technology and implemented in its CT product ranges.

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In the future
“Our primary objective is to address the radiation dose issues mentioned earlier and provide the real benefit of low-dose, high-quality images to patients and radiologists. Our focus is to create and spread awareness about low-dose technology available with Philips for each and every CT investigation, routinely incorporated for every patient study,” says Raveendran Gandhi. He further adds, “We would like to enhance our brand and differentiate ourselves as a medical device manufacturing company, which is fully adapted to patient-first approach and care, and has truly proven its stand with this innovative technology.”

iDose4 is a fourth-generation product of iterative reconstruction technology. When asked if there would be plans in the future to incorporate additional features, Raveendran Gandhi says, “We do not anticipate further changes with iDose4 product technology. However, Philips is also progressing with unique horizons of low-dose technology, which is totally based on model-based reconstructions,” says Raveendran Gandhi. On asking him if iDose4 can be utilised with other CT scanners in the market, he says, “No, it will work only on Philips CT.”

The author is a senior technical correspondent at EFY Bengaluru