Quantitative phantomless calibration of computed tomography scans

Inventors

Kopperdahl, David L. • Lee, David Choen • Keaveny, Tony M.

Assignees

O N Diagnostics LLC

Abstract

An apparatus, method, and computer program product for calibrating a CT scan without the use of an external calibration phantom, to enable quantitative assessment of internal body tissues and organs and additionally for any application that would benefit from a calibration of the scan attenuation data, such as viewing CT images in a consistent fashion. Embodiments are described with applications to quantitative assessment of bone density in the spine and hip, mineral content in blood vessels, hepatic-fat content in the liver, and gray-to-white matter ratio in the brain. The primary advantages of the method are that it does not require the use of an external calibration phantom, it is robust across different CT machines and scanner settings, and it is also highly precise, lending itself to a high degree of automation.

Core Innovation

The disclosed invention provides phantomless quantitative computed tomography (CT) calibration by executing a patient-specific method on a processor that calibrates a set of HU-values for a CT scan without the use of a calibration phantom. The method specifies a value for an effective energy of the CT scan, measures patient-specific HU-values from a CT scan for a set of internal reference tissues, and uses predetermined non-patient-specific values of X-ray attenuation that are expressed as a function of effective energy. Calibrated HU-values are then generated by converting the measured HU-values using the specified effective energy and the specified attenuation values, and the calibrated HU-values are communicated to memory for storage.

A key concept is that the X-ray attenuation for internal reference tissues is specified responsive to predetermined non-patient-specific values of X-ray attenuation for the set of internal reference tissues, including an expression as a function of effective energy. The method thereby links patient HU measurements to attenuation-energy relations through an effective-energy specification, and converts a set of measured HU-values into a set of calibrated HU-values. In disclosed embodiments, internal reference tissues and tissue regions are used to estimate calibration-relevant mapping from HU to calibrated outcomes while avoiding a calibration phantom.

Two alternative calibration approaches are disclosed for deriving calibrated values. An Effective-Energy approach estimates scan effective energy from scanner/patient parameters or from HU of reference tissues, and uses attenuation-based mixture modeling to map HU to calibrated outcomes, including corrected HU-values and quantitative tissue or material measures such as bone density/volume fraction, hepatic fat fraction, muscle fat content, and mineral in vessels, as well as gray-to-white matter ratios/volume fractions. An Equivalent-Density approach predefines equivalent-density values for air and a single internal reference tissue, optionally tabulated versus scanner/patient factors, and derives an HU-to-equivalent-density mapping for voxels of interest.

Claims Coverage

The independent claims cover patient-specific phantomless CT calibration and calibrated quantitative measurements derived from calibrated HU-values, including at least three inventive-feature groupings: effective-energy-based calibration without an external phantom, calibrated BMD determination using internal reference tissues and effective-energy specification, and region-of-interest tissue quantity measurement using calibrated HU-values derived from internal reference tissues.

Across the independent claims, the shared inventive theme is phantomless quantitative CT calibration and measurement by specifying an effective energy, measuring HU-values for internal reference tissues (and for voxels of interest or a bone/region of interest), using predetermined non-patient-specific attenuation values expressed as a function of effective energy, converting/calibrating HU-values, and communicating calibrated quantitative results such as calibrated BMD or tissue quantity.

Stated Advantages

Documented Applications