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NHLBI SBIR/STTR

068 Multilayer-coated Gratings for Phase-Contrast Computed Tomography

NHLBI SBIR/STTR Contract Topic

(Fast-track proposals will not be accepted)

Number of anticipated awards: 1-2

Background

In the United States computed tomography (CT) currently accounts for approximately 15% of all diagnostic imaging procedures. The number of CT scans has seen a threefold increase in the two decades leading to 2007 due to its unique capabilities. A CT scan produces high resolution, volumetric rendition of internal organs in a short period. CT scanners are relatively compact and economic to operate and maintain. Thus, they are widely available even in difficult environments. However, CT technology offers relatively low visibility (contrast) of soft tissue structures when compared to magnetic resonance imaging, and at the cost of exposing the patient to substantial ionizing radiation.

In the past few years there has been intense development of the concept of phase-contrast CT, a promising strategy to improve soft-tissue visibility and lower radiation dose by tenfold or higher over conventional absorption-based technology. To translate the phase-contrast concept to clinical applications, the technology must meet a number of requirements, including the ability to work with compact x-ray tubes, fast imaging speed and large field of view, and compact instrumentation. Full-field imaging techniques that utilize x-ray transmission gratings are prime candidates for eventual clinical applications due to their speed and adaptability, but they have not delivered the theoretically possible performance to-date. The main obstacle has been the inability to produce gratings of less than 1 micrometer periods for hard x-rays.

A promising technology to overcome the above limitation is vapor deposition of alternating high and low atomic number layers with layer thicknesses in the sub micrometer to nanometer range. The resulting multilayer structure serves as transmission gratings of extremely small periods when used in the transmission configuration. The concept has been proven effective in x-ray focusing optics. An additional advantage of this technology is that the grating depth-to-period ratio is unlimited in principle, making it effective for hard x-rays employed in animal and human CT. However, current multilayer x-ray transmission optics are fabricated on smooth substrates, while phase-contrast CT presents the need for deposition on patterned substrates in order to achieve sufficient grating areas for CT scanners. Therefore, the aim of this solicitation is to develop fabrication techniques of multilayer structures on patterned substrates to effect large-area, hard x-ray transmission gratings.

Specifications

This solicitation encourages the development of a directional deposition technology of multilayers of alternating high and low atomic number materials on periodic echelle (staircase like) substrates to form large-area x-ray gratings. Deposition rates of 200 nanometer per minute or higher are desired to permit total multilayer thicknesses of tens of micrometers in reasonable deposition time. The packing density of the layers should approach bulk materials for the purpose of effectively modulating the intensity and phase of hard x-rays used in animal and human CT scanners. The thickness and geometry of individual layers should be precisely controlled and uniform over the desired grating area. The directionality of the technology should be sufficient to target specific surfaces of the patterned substrates while avoiding deposition on other surfaces. Deposition protocols for material pairings of tungsten/silicon, chromium/silicon and titanium/silicon should be developed. Proper adhesion of the multilayer structure to the substrate should be achieved without spontaneous delamination. Individual layer thicknesses of approximately 50 nanometers and structurally uniform coated areas of 60mm x 60mm should be developed for small animal CT at 35 keV. Ultimately the technology should be extended to large area gratings for human CT scanners at 60 – 100 keV.

Deliverables

Phase I proposals should focus on the development of multilayer deposition technology of material pairings of tungsten/silicon for absorption gratings and chromium/silicon and titanium/silicon for phase gratings. The targeted x-ray energy is 35 keV for small animal CT scanners. The proposals should aim to achieve individual layer thicknesses of 50 nanometers, total deposition thickness of up to 100 micrometers, and structurally uniform coated areas of 60 mm x 60 mm. Techniques should be developed to selectively deposit on specific facets of the patterned substrate while minimizing material deposition on other surfaces. The substrates for coating tests will be provided by Division of Intramural Research, National Heart, Lung, and Blood Institute, National Institutes of Health (DIR/NHLBI/NIH). The proposals should also include evaluation of the multilayer structures with cross- sectional electron microscopy either independently by the contractor or with the help of DIR/NHLBI/NIH.

Phase II effort should focus on scaling up the technology developed in Phase I to larger grating areas and total deposition thicknesses of over 100 micrometers for human computed tomography.

For more information, see the FY2012 Contract Solicitation or contact OTAC.

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Last Updated December 2011

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