CT Radiation Dose: Philips Perspective
Fulfilling the demand for effective diagnostic and therapeutic information has led to a steady increase in the use of computed tomography (CT). With this trend, CT departments strive to scan with the “As Low As Reasonably Achievable” (ALARA) principle; however, its practice varies significantly among sites and scanners servicing an ever-widening range of clinical indications and patient populations. Philips strategies for simplifying CT dose management are described. Multiple components of the Philips CT imaging chain have been designed to increase volume imaging speed, dose efficiency, and image quality, thereby enabling opportunities for lower dose scan protocols and helping to achieve doses ALARA. In addition, nine seamlessly integrated protocol-driven and patient-adaptive technologies including DoseRight Automatic Current Selection, DoseRight dose modulation, DoseRight Cardiac, Step & Shoot, IntelliBeam Filters, SmartShape Wedge (bowtie) Filters, Eclipse DoseRight collimator, and iDose4 Iterative Reconstruction Technique are described. These combined technologies automatically use the quantity and quality of radiation where and when needed, leading to image quality improvements and dose reductions. Combining Philips’ dose optimized CT imaging chain with automatic dose optimization tools begins a new era where expanding multi-detector CT will be fueled not only by increasing clinical benefits, but also by easily lowering dose to levels not previously possible for broader patient populations.
KeywordsImage Quality Filter Back Projection Iterative Reconstruction Technique Dose Saving Dose Efficiency
During the last decade, technological advances have markedly enhanced and expanded the range of clinical applications of computed tomography (CT) (Boll et al. 2006). Consequently, physicians have ranked CT atop the list of innovations that have improved patient care (Fuchs and Sox 2001). While the benefits of CT have been very well documented, increasing radiation doses to the population drew attention to the need for reducing radiation exposure from CT (Mettler et al. (2000); Brenner and Hall 2007). In response, the radiology community has worked to adhere to As Low As Reasonably Achievable (ALARA) principles in CT imaging (FDA 2001; Frush et al. 2003; Golding and Shrimpton 2002; Kalra et al. 2004). While meeting diagnostic and therapeutic imaging objectives, most CT departments and centers strive to routinely scan with patient radiation doses ALARA. However, techniques for practicing ALARA can vary significantly by clinical indication and patient population (Frush et al. 2002; Technical report 2007). Surveys have shown significant variations between sites and scanners including wide ranges of radiation dose for the same scan indication (Frush et al. 2002), utilization of adult scan protocols for pediatrics (Goske et al. 2008a), and opportunities to increase awareness of CT dose estimates (Lee et al. 2004; Technical report 2007).
Dose management is simplified with Philips Healthcare’s DoseWise philosophy (Morgan 2002) and the advances embodied in modern Philips MDCT scanners. Multiple components of the imaging chain—from the tube to the detectors to the reconstruction of the final CT images—have been enhanced to increase volume imaging speed, dose efficiency, and image quality (Sect. 2), integrated with new dose optimization (Sect. 3) and reporting tools (Sect. 4), thereby enabling opportunities for lower dose scan protocols. The dose optimization tools are integrated with each stage to automatically control the quantity and quality of radiation where and when needed and help to simplify dose management for routine CT scanning. The resulting image quality and dose efficiency improvements help to meet the imaging objectives with doses ALARA.
2 Essence Technology: Improved Image Quality and Dose Efficiency
- ClearRay 2D anti-scatter collimation (ASC). Scattered radiation primarily originates from the scanned object and adds to the primary transmitted X-ray intensity which we measure at CT. This deviation from the true attenuation measurements can result in artifacts, inaccuracy in reconstructed CT attenuation (HU) measurements, and degradation of low contrast resolution within an image. The increasing z-axis coverage in latest generation of MDCT scanners requires larger cone angles for X-rays to ensure an increased field of view. This increasing cone angle increases the scatter radiation along the z-axis. Scattering is the dominant, most probable, way that diagnostic X-rays interact with human tissue. Scatter radiation is one of the primary contributors to image quality degradation when it reaches the detector. This amount of scattered radiation that reaches detectors grows with increasing z-axis scanner coverage (Engel et al. 2008). As the amount of scattered radiation has been growing continuously, the scatter rejection technology (1-dimensional anti-scatter grids) has barely undergone any improvements in the wide area MDCT scanners. Starting from 64-slice scanners, this scatter radiation results in large scale inhomogeneities in CT attenuation (HU) values as well as dark streaks between strongly absorbing objects (Joseph and Spital 1982) in an image. In the iCT, the NanoPanel3D detector is spherically shaped so its ClearRay 2D anti-scatter grids (Fig. 2a) can be focused for true 3D cone beam geometry. The resulting scatter reduction can improve low contrast resolution and uniformity by reducing the scatter-to-primary ratio (SPR) to 6% from 18% for the 1D ASC (Vogtmeier et al. 2008). Scans of a Catphan phantom (The Phantom Laboratory, NY, USA) at the same dose show improved low contrast with the 2D ASC compared with the 1D ASC (Fig. 2b) using the same scan protocol.
Fast rotation speed. The new AirGlide gantry uses a frictionless system—as an alternative to ball bearings—that enables rotation speeds of 0.27 s resulting in a substantial improvement in temporal resolution. This speed helps minimize motion artifacts for challenging examinations such as Step & Shoot Cardiac imaging with higher heart rates or when scanning restless children.
These dose efficiency and IQ improvements, such as a reduction in scatter and motion artifact, and others from Essence technology, can provide opportunities to meet imaging objectives with scan protocols adapted for doses ALARA. Advances in Essence technology—integrated with dose optimization tools described in the Sect. 3—show that dose and image quality do not need to be difficult trade-offs.
3 Improved Image Quality and Dose Optimization Technologies
Special pediatric dose saving techniques are then described to show how dose optimization tools are used in concert along with age- and weight-based tube current reduction factors.
The integrated set of dose optimization tools is patient-adaptive and protocol-driven to automatically deploy the optimum component configurations for each scan. A number of dose optimization techniques, such as the SmartShape wedge (bowtie) filters, are designed to optimize both dose and image quality, while others, such as the Eclipse DoseRight collimator, lower exposure without affecting IQ (Philips Healthcare 2009).
3.1 DoseRight Automatic Current Selection
If scan parameters were not adapted for the size of the scan region, larger (smaller) patients would receive less (more) dose per kilogram. This is because for a given projection, the center and peripheral region nearest the detector would typically have lower (higher) beam intensity for a larger (smaller) patient due to attenuation along a larger (smaller) diameter. The summative effect with each tube rotation would result in higher (lower) average noise levels for larger (smaller) patients.
DoseRight ACS automatically suggests tube current settings according to the maximum estimated patient size in the scan region. A default “reference” tube current corresponding to an average patient size is defined for each scan protocol and can be modified by the user. This reference size is defined for each scan type and age group. For example, the reference size for the scan type “body” is 33 cm for an adult, 20 cm for a child, and 16 cm for an infant.
3.2 DoseRight Z-axis Dose Modulation
Average X-ray attenuation can vary significantly along the craniocaudal (z) axis as can be seen on Fig. 4b. For example, the chest has a much lower average attenuation than the abdomen due to air in the lungs and will require a much lower tube current to achieve the desired image quality. As mentioned in Sect. 3.1, WED(z) is measured from the surview for every position along the craniocaudal (z) axis in the scan region. This z-profile can be used to modulate the tube current for each table position. The maximum, average, and minimal tube current levels for Z-DOM are displayed when planning the scan, and the percentage dose reduction is displayed with the reconstructed images. Note that DoseRight ACS and Z-DOM can be switched on separately or used in combination.
Phantom measurements performed on a RANDO phantom (Fluke Biomedical, WA, USA) show that the absorbed organ dose is reduced by 23% in the Thyroid, 19% in the Breast, and 26% in the eyes when Z-DOM is used compared to measurements performed without Z-DOM (Lee et al. 2010). Other unpublished data show that, in multi-region examinations such as chest-abdomen-pelvis (CAP) or head-neck scan protocols, DoseRight Z-DOM has been seen clinically to lower patient dose by about 20–40%.
3.3 DoseRight 3D Dose Modulation
3.4 DoseRight Cardiac: Dose Modulation for Retrospectively-Gated Helical Scans
3.5 Step & Shoot Cardiac and Complete: Prospective Gating
Imaging an organ over multiple physiological phases (e.g., over the cardiac cycle) can provide both anatomical and functional information (see Sect. 3.4); however, for many indications it is only necessary to image—and expose—a single phase. With the Step & Shoot Cardiac and Complete techniques, the tube current can be prospectively modulated to image only a desired phase in an axial scanning mode, thus enabling prospectively gated axial cardiac scanning during the “quiet” phase of the cardiac cycle. Step & Shoot Cardiac scans can result in a dose savings of up to 80% compared with a retrospectively-gated helical technique, while maintaining optimum image quality (see Fig. 7b). Furthermore, larger detector width can be further leveraged by increasing the axial shot coverage for decreasing field of view. This enables larger step lengths with minimum cone beam overlap.
3.6 IntelliBeam Filters
The IntelliBeam filter selection is configured with the scan protocols, and is used automatically in combination with SmartShape wedges (see Fig. 9) and the X-ray tube’s intrinsic filtration to optimize both low contrast resolution and dose.
Default scan protocol
HVLd (mm AI equiv)
Adult head and body
The filters are designated “full”, “half”, and “off-center enabled”.1 The full IntelliBeam filter used with all adult head and body protocols reduces dose by about 30% at 120 kVp and 46% at 80 kVp relative to half filter. The half filter is used with infant scan protocols to enhance low contrast resolution. This helps manage dose in combination with the small wedge and intrinsic filtration (as shown in Table 1).
3.7 SmartShape Wedge (Bowtie) Filters
Wedges, also known as bowtie filters, can increase both dose efficiency and image quality. This double advantage is achieved by filtering the beam’s intensity according to the patient’s shape. Each wedge provides more filtering from medial—where the patient thickness is greatest—to lateral, thereby facilitating a uniform dose and noise distribution as the tube rotates. Three SmartShape wedges are provided: (a) small: infants 0–8 months, (b) medium: cardiac, and (c) large: adult head and body (see Fig. 9a).
A medium size phantom is used in Fig. 9b to illustrate the importance of wedge. It shows a green shaded beam intensity in the center region for all three wedge sizes. When a large (small) wedge is used on the medium phantom, as shown on the right (left), the wedge’s peripheral attenuation thickness is too small (large), the peripheral beam intensity is too high (low), resulting in a higher (lower) peripheral dose. When appropriately sized, the wedge’s curvature facilitates a uniform exit beam intensity and image quality throughout the scanned region. They are used in conjunction with the IntelliBeam filters, and are automatically selected based on the protocol being used.
Infant wedge. CTDI(vol) is reduced by approximately 14 and 22% as measured with the standard FDA head (16 cm) and body (32 cm) phantoms, respectively, compared with the large wedge. A dose savings of 10–14% can therefore be estimated for properly centered infant body examinations since the head phantom better approximates infant body size (less than 18 months).
Cardiac wedge. Body CTDI(vol) and CTDI(peripheral) is reduced by approximately 11 and 13%, respectively, relative to the large wedge. These results can be used to estimate a lung dose savings of 11–13% for properly centered cardiac examinations.
3.8 Eclipse DoseRight Collimator
The Eclipse DoseRight Collimator reduces unnecessary exposure during spiral scanning without affecting image quality. Dose reduction is proportionally higher in scans with wider collimation, larger pitch, and shorter scan lengths such as those used in cardiac, pediatric, and brain exams
Typical scan length (cm)
Dose savings on DLP
3.9 iDose4 Iterative Reconstruction Technique
Filtered back projection (FBP) has been the industry standard for CT image reconstruction for decades (Radon 1917). 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 (e.g., scans of morbidly obese individuals). 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 (Whiting et al. 2006). When these very high levels of noise are propagated through the reconstruction algorithm, the result is an image with significant artifacts and high quantum mottle noise. Over time, incremental enhancements were made to FBP to overcome some of its limitations. These improvements continued until recently, when a completely different approach to image reconstruction was explored through the clinical implementation of iterative reconstruction (IR) techniques. IR techniques attempt to formulate image reconstruction as an optimization 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 image. Hence, IR techniques treat noise properly at very low signal levels, and consequently reduce the noise and artifacts present in the resulting 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. While IR techniques have been used for many years in PET and SPECT imaging, the sampling density and the data set sizes in CT have historically caused IR techniques to perform extremely slowly when compared to FBP. However, recent innovations in hardware design and algorithm optimizations have permitted the clinical use of an IR technique in CT.
iDose4, a 4th generation reconstruction technique, is the latest addition to Philips’ scanners that has been integrated with Essence technology and that provides significant improvements in image quality as well as noise reduction (Philips Healthcare 2011).
iDose4 provides an innovative solution in which iterative processing is performed in both the projection and image domains. The reconstruction 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 penalized 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 localized and can be effectively removed to support the desired level of image noise. 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 lower noise 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, multifrequency noise removal is performed.
From the workflow perspective, iDose4 reconstruction is triggered by changing the reconstruction type from “standard” (FBP) to “iDose”. iDose4 reconstruction can be either selected prospectively (before the scan) or performed retrospectively on datasets for which the projection (raw) data is still available on the scanner. Scan protocols may utilize iDose4 by default. An additional parameter—iDose4 level (scale: 1–7) is used to define the strength of the iterative reconstruction technique in reducing image quantum mottle noise (range 11–55% noise reduction relative to a corresponding FBP reconstruction). The chosen level does not affect the reconstruction time (up to 20 images per second) and can be defined independent of the radiation dose with which an acquisition is performed.
3.10 Pediatric Scan Protocols
These pediatric protocols are used in combination with dose reducing tools, such as the infant SmartShape wedge filter and the Eclipse DoseRight Collimator, to provide multiple advantages and a comprehensive solution for pediatric dose management.
4 Dose Reporting
Advances in Philips’ CT platforms also include intuitive reporting and recording of estimated dose, dose reduction, and dose efficiency. This feedback enables intra- and inter-scan assessments and comparisons of estimated dose levels. It can also be used in conjunction with image quality reviews to optimize scan protocols.
The advances of Philips’ CT platforms were designed to increase dose efficiency and optimize image quality, thereby helping to achieve doses ALARA for ever-widening patient populations. This was achieved through Essence technology and by integrating dose optimization technologies in each stage of the imaging chain. These advances simplify the delivery of the right quantity and quality of exposure when and where needed for routine as well as challenging examinations.
The DoseWise philosophy also includes ongoing investments in innovative research and development and clinical collaboration. Further dose reductions will be achieved: X-ray beams will be dynamically shaped, filtered, modulated, and localized according to the reason for scanning and the tissue to be scanned. Dose estimates will be reported and recorded more accurately, taking into account patient size, shape, and tissue sensitivity. An array of new low dose CT applications will emerge such as those low enough to replace the radiograph. New partnerships and alliances will be forged to update ALARA practice standards, including education and reporting, and to provide a forum for sharing low dose scan protocols and techniques.
Together in clinical partnership, the modern Philips MDCT platforms and the DoseWise strategies embolden a vision of a new era of expanding MDCT use, fueled by increasing clinical benefits and decreasing doses. This vision shows the way to routine detection of disease before the onset of symptoms and provides timely information to improve treatment decisions, leading to better patient care.
We would like to thank our colleague Mark Olszewski, from Philips HealthCare CT Product Management, for taking the time to assess this manuscript. His comments were greatly appreciated. We also would like to thank our colleagues Efrat Shefer and Leon de Vries, from Philips CT Clinical Science for their careful reading and corrections to this chapter. All clinical images presented in this chapter are courtesy of Professor E. Coche, from the Cliniques Universitaires Saint-Luc (UCL) at Brussels in Belgium.
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