Quantitative phantomless calibration of computed tomography scans
Inventors
Kopperdahl, David L. • Lee, David Choen • Keaveny, Tony M.
Assignees
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Publication Number
US-9936934-B2
Publication Date
2018-04-10
Expiration Date
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
A phantomless computer-implemented method measures a calibrated value of an equivalent-density parameter for a region of interest in a CT scan of a patient by converting CT numbers expressed as Hounsfield Unit values. The method does not use an external calibration phantom. It uses a set of pre-determined values of the equivalent-density parameter for air and one internal reference tissue to convert HU-values for the region of interest into calibrated equivalent-density-parameter values.
The calibration is based on measuring HU-values for the internal reference tissue and for air from the CT scan. The predetermined values include values for the internal reference tissue and for air, and the conversion is performed using the measured HU-values and the predetermined values. The predetermined values can be conditioned using scanner and scan-acquisition parameters, patient factors, and specified effective energy.
A further embodiment calibrates a CT scan for a current patient by measuring HU-values for one or more internal reference tissues and referencing a set of predetermined equivalent-density parameter values for those internal reference tissues. The set of predetermined values is obtained previously for a plurality of prior patients without the use of a calibration phantom, and calibration is performed responsive to the measured HU-values and corresponding assigned equivalent-density values. Another embodiment provides calibrated bone mineral density values from HU values for a bone region of interest without a calibration phantom by converting measured HU-values using predetermined equivalent-BMD values for air and an internal reference tissue.
Claims Coverage
The independent claims cover three computer-implemented phantomless calibration approaches: conversion to an equivalent-density parameter using air and one internal reference tissue, calibration for a current patient using predetermined equivalent-density values derived from prior patients, and conversion to calibrated bone mineral density values using air and an internal reference tissue.
Phantomless equivalent-density calibration using air and one internal reference tissue
A computer-implemented method for measuring a calibrated value of an equivalent-density parameter for a region of interest in a CT scan without a calibration phantom, using predetermined values for air and one internal reference tissue, by specifying predetermined equivalent-density values for the internal reference tissue and for air, measuring HU-values for the internal reference tissue and for air, and converting the HU-values for the region of interest into values of the equivalent-density parameter.
Patient calibration using prior-patient predetermined equivalent-density sets without a calibration phantom
A method for calibrating a CT scan for a current patient without a calibration phantom that measures HU-values for one or more internal reference tissues, references a set of predetermined equivalent-density parameter values for the one or more internal reference tissues previously obtained for a plurality of prior patients without a calibration phantom, assigns equivalent-density values to each internal reference tissue from the predetermined set, and calibrates the scan responsive to the measured HU-values and the corresponding assigned equivalent-density values.
Phantomless calibrated BMD conversion using air and one internal reference tissue
A computer-implemented method for providing a set of calibrated values of bone mineral density for a region of interest of a bone in a CT scan without a calibration phantom, by specifying particular predetermined equivalent-BMD values for an internal reference tissue and for air, measuring an HU-value for the internal reference tissue and obtaining an HU-value for air, measuring a set of HU-values for the region of interest, and converting the measured HU-values into calibrated BMD values responsive to the HU-value measured in the internal reference tissue, the HU-value obtained for air, and the particular predetermined values.
Across the independent claims, the inventive coverage is directed to phantomless CT calibration by converting measured HU-values using predetermined equivalent-density or equivalent-BMD values anchored to air and internal reference tissues, including an approach that leverages predetermined values derived from prior patients without a calibration phantom.
Stated Advantages
Removes the need for an external calibration phantom.
Provides calibrated equivalent-density-parameter values and calibrated bone mineral density values from CT scans without a calibration phantom.
Documented Applications
Monitoring osteoporosis drug treatments over time using calibrated BMD values.
Pre-operative orthopaedic surgical planning for hip, spine, or knee procedures, including total joint replacement, spinal fusion, pedicle screw fixation, and fracture fixation of long bones, using calibrated BMD values.
Assessing the degree of fracture healing or bone fusion using calibrated BMD values.
Providing calibrated gray-to-white matter ratios, and mixture-component tissue equivalents, as described in the patent summary.
Assessing bone apparent density/BMD, marrow/yellow marrow and fat/liver fat, and vessel mineral content, as described in the patent summary.
Hepatic fat content assessment and vascular mineral content assessment, as described in the patent summary.
Coronary calcification/Agatston score context is mentioned in the patent summary.
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