This invention relates generally to imaging, and more particularly to determining the mass and the volume of soft matter in reconstructed images and, for medical images, determining a risk metric for a patient when the soft matter is soft plaque.
Configurations of the present invention are particularly useful in medical and diagnostic computed tomographic (CT) applications for quantification of calcification and/or lesions, but the present invention is not limited to medical applications or to CT.
In some known CT imaging system configurations, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as an “imaging plane”. The x-ray beam passes through an object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated radiation beam received at the detector array is dependent upon the attenuation of an x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam intensity at the detector location. The intensity measurements from all the detectors are acquired separately to produce a transmission profile.
In third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged such that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two-dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units” (HU), which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time, a “helical” scan may be performed. To perform a “helical” scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.
Reconstruction algorithms for helical scanning typically use helical weighing algorithms that weight the collected data as a function of view angle and detector channel index. Specifically, prior to a filtered backprojection process, the data is weighted according to a helical weighing factor, which is a function of both the gantry angle and detector angle. The weighted data is then processed to generate CT numbers and to construct an image that corresponds to a two-dimensional slice taken through the object.
To further reduce the total acquisition time, multi-slice CT has been introduced. In multi-slice CT, multiple rows of projection data are acquired simultaneously at any time instant. When combined with helical scan mode, the system generates a single helix of cone beam projection data. Similar to the single slice helical, weighting scheme, a method can be derived to multiply the weight with the projection data prior to the filtered backprojection algorithm.
Cardiovascular related deaths constitute more that 500000 annually in the USA, and much more globally, according to the American Heart Association. A major portion of them is attributed to coronary artery disease, where the chief culprit is the build up of plaque, specifically soft-plaque and its ruptures. In x-ray or non-contrasted CT, soft plaque is not easily detectable. Calcified plaque has been used as a surrogate for the presence of soft plaque on the basis that calcified plaque is the by product of ruptured plaque. Coronary plaque has been classified into 6 stages according to the Stary scale. According to general consensus, it is crucial to determine plaque in stages 4 and 5 as such plaque merits the critical label “vulnerable plaque.” Vulnerable plaque can lead to plaque rupture or dislodging, causing blockages that lead to myocardial infarction (MI). The so-called “gold standard” for determining plaque and its constituency is intravascular ultrasound (IVUS), however, IVUS is only performed on symptomatic patients due to its invasive nature. Symptomatic patients are already at an advanced stage and past non-invasive therapy options.
With the arrival of cardiac volume computed tomography (cardiac VCT) and its ever increasing spatial and temporal resolution and the impending arrival of high definition (HD) cardiac VCT, it has become possible to image a contrasted study of the heart gated to mitigate heart motion. Using these images, it is possible to distinguish plaque from vessel and from calcification, however it is still not possible to do so in an automated manner to assist in standardization and to achieve productivity improvements for radiologists and cardiologists.