June 2015

Moving towards Low Dose CT Scans

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iDose4 iterative reconstruction technique is a breakthrough in image quality and dose reduction with the 4th generation of reconstruction, says Raveendran Gandhi, Senior Director – Radiology, Philips

Recent technological advances have markedly enhanced and expanded the clinical application of computed tomography (CT). While the benefits of CT have been well documented and supported, many aspects of modern healthcare, increasing radiation doses to the population have raised attention to the need for reduction of radiation exposure from CT. In response, the radiology community (radiologists, physicists and manufacturers) has worked to adhere to ALARA (As-Low- As-Reasonably-Achievable) principles in CT imaging. Dose management is simplified with Philips Healthcare’s Dose Wise philosophy and the advances embodied in CT platforms. Multiple components of the imaging chain have been enhanced to increase volume imaging speed, dose efficiency, and image quality, thereby enabling opportunities for lower dose scan protocols. As the performance of the imaging chain was increased, the limitations of image quality resulting from conventional filtered back projection (FBP) reconstruction algorithms— especially at lower doses — became apparent.

Raveendran Gandhi, Senior Director – Radiology,

Raveendran Gandhi, Senior Director – Radiology,

This is an in-depth review of an innovative, 4th generation iterative reconstruction technique. iDose4— the latest addition to Dose Right tools — that provides significant improvements in image quality combined with dose reduction capabilities. Benchmarking tests relative to alternate technologies help demonstrate the benefits of this 4th generation iterative reconstruction technique in preventing photon starvation artefacts (streaks, bias) prior to image creation and in maintaining image texture to overcome the artificial or “plastic” look of images that have been frequently reported when using previous-generation iterative reconstruction techniques. Evidence from phantom tests and rigorous clinical evaluations with clinical collaborators demonstrate the potential of iDose4 to improve image quality and lower radiation dose levels beyond those previously achievable with conventional, routine-dose acquisitions, filtered back projection reconstructions.

iDose4 is a 4th generation reconstruction technique that provides significant improvements in image quality and radiation dose reduction. It provides an innovative solution, in which iterative processing is performed in both the projection and image domains. There construction algorithm starts first with projection data where it identifies and corrects the noisiest CT measurements – those with very poor signal to noise 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 are preserved. This process ensures that the gradients of underlying structures are retained, thus preserving spatial resolution while allowing a significant noise reduction. In doing so, this process prevents the primary cause of low signal streaks. Also, since the corrections are performed on the acquisition data (unlogged projections); this method successfully prevents bias error. The noise that remains after this stage of the algorithm is propagated to the image space; however, the propagated noise is now highly localised and can be effectively removed to support the desired level of dose reduction.

The next major component of the iDose4 algorithm deals with subtraction of the image noise while preserving the underlying edges associated with true anatomy or pathology. This subtraction begins with an estimate of the noise distribution in the image volume. This estimate is used to reduce the noise while preserving the true structure. This estimate also allows the preservation of the image noise power-spectrum characteristic of a higher-dose acquisition and FBP reconstruction. Following this, a selector chooses among noiseless structural models, and the model that best fits the local topology of the image volume is chosen. Once the best model is chosen, it is used to reduce the noise in the image volume. To ensure uniform noise removal at all frequencies, multi-frequency noise removal is performed.

iDose4 is a sophisticated and complex reconstruction algorithm that demands enormous computational power. 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 would result in clinically unacceptable reconstruction times. The new RapidView IR reconstruction engine was designed from the ground up to benefit from not only higher performance computational cores but also the number of cores. The architecture is highly parallel and the design enables there constructor to scale with the latest multiplecore processors and state-of-theart massively parallel, high-density computing devices. The high-density computing device on RapidView IR processes and transfers huge amounts of data. The latest generation PCI express bus offers substantially higher bandwidth, and Intel 6-core processors are utilised to address the additional computing requirements. As a result, the new re-constructor is able to deliver exceptional reconstruction performance with iDose4, thus providing reconstruction speeds similar to those previously achievable with FBP on conventional re-constructors. An additional benefit of the Rapid View IR is that FBP reconstruction speeds on this enhanced hardware are significantly higher than previously achievable.

Detailed phantom studies demonstrate that, for routine dose acquisitions, the spatial resolution can be improved by up to 68 per cent

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 paediatric imaging where radiation dose reduction is paramount, iDose4 enables significantly lower radiation dose while maintaining diagnostic image quality. In other cases, where image quality (e.g., spatial resolution) is of higher priority than the dose reduction, such as in the assessment of coronary stent patency, iDose4 enables significantly improved spatial resolution. Intermediate levels of dose reduction and spatial resolution improvement can be applied in combination for other clinical scenarios.

iDose4 can be used to significantly improve the spatial resolution of any acquisition, regardless of the dose with, which it was acquired. It iDose4 can be used to improve the spatial resolution, the contrast-to-noise ratio, or both beyond that has been traditionally achievable. Detailed phantom studies demonstrate that, for routine dose acquisitions, the spatial resolution can be improved by up to 68 per cent.

Routine clinical practice often requires the combination of dose reduction and image quality improvement benefits relative to routine-dose acquisitions and FBP reconstruction; hence, iDose4 provides the functionality to combine these benefits in proportions best suited to the clinical indication. Routine clinical practice often requires the combination of dose reduction and image quality improvement benefits relative to routine-dose acquisitions and FBP reconstruction; hence, iDose4 provides the functionality to combine these benefits in proportions best suited to the clinical indication.

In tube – power – intensive acquisitions, such as those faced in imaging obese patients, the maximum power of a CT scanner tube may not be sufficient to provide the desired image quality for a given indication. The artifact prevention and noise reduction enabled through iDose4, provides image quality that is equivalent to that associated with a significantly higher-dose acquisition without having to actually irradiate the patient with the higher dose. In such scenarios, the effective tube power is increased and can overcome either tube limits (e.g., bariatric imaging) or skin dose concerns associated with higher-dose acquisitions.

To summaries, iDose4 iterative reconstruction technique is a real breakthrough in image quality and dose reduction with the 4th generation of reconstruction and paves the way for moving towards low dose CT scans.

 

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