X-ray or gamma ray systems or devices – Specific application – Absorption
Reexamination Certificate
2003-03-17
2004-09-14
Bruce, David V (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Absorption
C378S051000, C378S098200
Reexamination Certificate
active
06792071
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to computer-assisted radiological measurements on radiographic images.
BACKGROUND OF THE INVENTION
In musculoskeletal radiology, it is current practice to use a wide range of imaging modalities to determine skeletal disorders and abnormalities.
In this discipline diagnosis is often based on quantified radiological findings of geometrical quantities.
At least 50% of all radiological examinations today are conventional exposures of thorax and the skeleton. 80% of skeletal exposures leads to the correct diagnosis based on the radiographs.
A plurality of textbooks in the X-ray diagnostics from world-renowned radiological and orthopaedic experts make a contribution in this area.
However, to refine the diagnosis, to conduct better differential diagnosis, to assess severity of change, to plan and control therapy, to conduct treatment follow-up, to establish physical ability in sports medicine, labour medicine and military medicine, verbal descriptions based on skeletal radiographs are insufficient in many respects.
Better diagnosis can be achieved by quantifying radiological findings.
Geometrical quantities measured on radiological images must be checked against normal values. These normal values have been collected from measurements of a representative sample of the normal healthy population and are tabulated in the above-mentioned text books.
Geometrical measurements in digital images comprise linear and angular measurements. Linear measurements in 2 dimensions and 3 dimensions may be supplemented with distance along a curvilinear path. Angular measurements are considered in the plane of the image, in a world plane or in 3D space. Geometrical areas are considered in the image plane, or more generally of surface patches in 3D images. Volumes are computed in 3D images but may be based on planar measurements. Geometrical indices are clinical quantities based on image measurements. In any of these categories, the measurand is defined as the physical parameter being quantified by measurement.
Today, radiological measurements on X-ray images are either made on film using conventional measuring devices (such as a ruler, a caliper or a rubber band to measure lengths, and a square or goniometer to measure angles) or in a digital image displayed on screen using cursor controlled points (such as a pair of points to measure Euclidean distance between).
The current measurement procedure thus involves 4 distinct media:
1. An X-ray film comprising the anatomical sites to be measured, displayed on a light box. With the emergence of digital radiography modalities (film digitisation, computed radiography, digital radiography sensors), the digital image may be displayed on a computer display. However, such electronic medium still is physically different from the other components described hereafter.
2. A measurement atlas comprising the measurement scheme: imaging technique, graphical template and description of the measurements covered by the scheme (nomenclature, clinical significance, and normative tables, sometimes interchangeably represented by curves)
3. An analogue measurement device (ruler, square . . . ) to perform geometrical measurements,
4. Pencil/Paper to note the measurement quantity according to the appropriate medical nomenclature and the measurement value.
A calculator device may be needed to compute indices from a collection of measurements, or to convert measured values to true quantities using calibration measures. Alternatively, electronic spreadsheets may be used in conjunction with a database to store the measurements and indices.
The use of different media asks for repeated focusing of attention between the atlas and the radiological image.
Moreover, in the absence of an atlas scheme the position of the measurement objects is not defined and hence different users may locate a given anatomical landmark differently.
Because there is no link between the measurement template in the atlas and its associated measurement entities, there is no systematically imposed consistency of the naming of measured quantities, and therefore exchange or collection of measurement values of different clinicians (e.g. for the purpose of cross-refereeing) is fundamentally hampered.
Another major drawback of the prior art method to perform geometrical measurements is increased measurement error or measurement uncertainty.
The error of measurement is the result of a measurement value minus the (true) value of the measurand. Measurement error is due to different sources, basically falling into one of two classes: systematic and random errors.
Systematic or bias errors arise from consistent and repeatable sources of error (like an offset in calibration).
Systematic errors can be studied through inter-comparisons, calibrations, and error propagation from estimated systematic uncertainties in the sensors used. Systematic error is defined as the mean that would result from an infinite number of measurements of the same measurand carried out under repeatability conditions minus the (true) value of the measurand. This source of error can be reduced by better equipment and by calibration.
Random errors also referred to as statistical errors, arise from random fluctuations in the measurements. In particular, digitisation noise (e.g. geometric digitisation: finite pixel size; intensity digitisation: quantisation of grey levels) and the errors introduced by counting finite number of events (e.g. X-ray photon count) are examples of random errors in the context of digital X-ray images. Random error is defined as the result of a measurement minus the measurement that would result from an infinite number of measurements of the same measurand carried out under repeatability conditions. Particularly this source of error is prevailing in the prior art of performing measurements on X-ray images.
Inter-observer and intra-observer variance on measurement values contribute to this source of error, and has its origin in several forms of ambiguity in defining the measurand.
Lack of unambiguous definition of the measurand with respect to the imaged patient anatomy and lack of knowledge of the geometrical pose of the patient with respect to source and detector are the main source of random error.
Repeatability and reproducibility of a measurement require that the random errors involved in the measurement procedure are low. Although random errors are reduced when a measurement is repeated many times and the results averaged together, this can rarely be achieved in clinical practice.
It is an object of the present invention to provide a user-friendly radiological measurement method that overcomes the drawbacks of the prior art.
SUMMARY OF THE INVENTION
The above-mentioned objects are achieved by a method as set out in claim
1
.
The method of the present invention is described with regard to geometrical measurements performed on radiological images of humans. It will be clear that the invention is not limited to human beings and can also be applied in other fields, for example in veterinary applications.
In the context of the present application the term ‘activation’ refers to loading a measurement scheme from memory and measurements to be performed according to the loaded scheme.
A measurement scheme or measurement template is a pattern of measurements to be performed. The measurements to be performed are grouped in the form of a measurement procedure wherein the sequence, the inter-dependence and method of measurements are defined. Such a measurement scheme can be noted and stored in a computer in standard notation XML (Extensible Mark-up Language).
In general the measurement scheme comprises a graphical part (also called graphical model) and an internal part (also called internal model).
The graphical part represents the geometric relation between measurement entities (objects and operators) and the anatomy in the type of image on which the measurements are to be performed. Measurement objects are e.g. points, lines, circles etc. The measurement objects are defined relative
AGFA-Gevaert
Bruce David V
Hoffman, Warnick & D'Alessandro LLC
Merecki John A.
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