Image analysis – Histogram processing
Reexamination Certificate
1998-09-15
2002-09-10
Tran, Phuoc (Department: 2621)
Image analysis
Histogram processing
C382S169000
Reexamination Certificate
active
06449390
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method adapted for use in an image processing apparatus for converting the gradation characteristics of plural images taking a same portion of an object, and more particularly to an image processing apparatus and an image processing method adapted for use, for example in the medical radiology field or the like, in a subtraction image processing apparatus for processing plural radiological images obtained by radiological image takings of an object under different image taking conditions.
2. Related Background Art
The X-ray radiological images for medical purpose have long been obtained by converting the intensity distribution of X-ray transmitted by the human body into the intensity distribution of fluorescent light of a fluorescent member and recording such light intensity distribution directly on a silver halide-based film, but, in recent years, the radiological images are being read in the form of electrical signals by a method of forming and reading a latent energy image representing the X-ray intensity distribution in a photostimulable fluorescent member, a method of directly reading the distribution of fluorescent light generated on a fluorescent member by X-ray or a method not depending on the fluorescent light, and a digital image is obtained by digital conversion of such electrical signals.
The use of such digital image allows to improve the efficiently of filing, to implement remote diagnosis, and to achieve improvements in the diagnostic technologies and efficiency. In addition, such digital image enables various image processing, thus realizing various changes in the method of diagnosis.
Among such image processing, most common is the gradation processing. As an example, a regular conversion of the pixel values within the image allows to enhance a pattern that is not easily recognizable in the original image, into a more easily recognizable form. Also even in case the condition of radiological image taking is inappropriate or unstable, the image that would be unusable on the conventional X-ray film can be rectified to stable gradation characteristics by appropriate gradation processing.
Among such gradation processing, the averaging of histogram (frequency distribution of pixel values) is simplest and most commonly employed, but this method may be unsuitable for diagnostic purpose since it only executes straight-forward averaging by broadening concentrated portions of the frequency distribution of the pixel values in the entire image and narrowing or uniting less dense portions of the frequency distribution. The image used for diagnosis has been developed over a long history based on the conventional silver halide-based X-ray film, and, for the radiologists, the gradation characteristics (appearance of image) similar to those of the conventional X-ray film are better for the efficiency and accuracy of diagnosis. More specifically, the histogram of the image of the conventional silver halide-based X-ray film is naturally not flat but characteristically describes the area of disease by the image taking technology developed over many years, and the radiologists have been so trained as to identify the diseases based on such form of image presentation.
It is therefore desired, also in the gradation processing of the digital image, to effect conversion into such image enabling easy diagnosis in stable manner. Such objective has been attained by processing the image, obtained by a digital image taking device with characteristics significantly different from those of the conventional silver halide-based film, through a considerably complex process including empirical methods, but a first drawback lies in a fact that such process inevitably involves a generally very long processing time and cumbersome operations.
A large advantage of image digitization lies in the ease of filing and retrieval of the image. For example, it is rendered possible to promptly investigate the change in the disease in a same patient by retrieving and comparing the image filed in the past. If the image taken in the past was obtained under different image taking conditions, the difference in such image taking conditions can be canceled by applying a gradation processing. However, there is encountered a second drawback that a cumbersome process is required for substantially matching the gradation of the past image with that of the current image.
Also in case of obtaining an image corresponding to the distribution of transmittance for a radiation such as X-ray thereby showing the internal structure of an object, there are conventionally been known so-called energy subtraction process of utilizing two radiations of different energy distributions to obtain two images corresponding to the distribution of transmittance, based on a fact that a specified constituent in the interior of the object has specific radiation transmittance (or absorbing characteristics for the energy of radiation), then digitizing these images and effecting a calculation between these images to extract or erase the specified constituent in the interior of the object. In the processing of medical X-ray images, such process is considered extremely useful for diagnostic purpose, such as erasure or extraction of a bone portion.
FIG. 1
shows a conventional energy subtraction apparatus, in which provided are a tube
101
for generating X-ray; a human body
102
constituting the object; a CCD
103
constituting a solid-state image pickup device for separating the distribution of radiation into pixels thereby obtaining an electrical signal; an A/D converter
104
for converting an analog electrical signal into digital values; an offset/gain correction unit
105
for correcting the fluctuation in the offset-gain of the CCD
103
; a logarithmic conversion unit
106
for converting digital values, proportional to the obtained radiation intensity, into logarithmic values; memories
107
,
108
serving to store the image data and connected to a signal bus
111
; a central processing unit (CPU)
109
capable of access to the memories
107
,
108
through the bus
111
; and a program memory medium
110
such as a FD, a HD or a MOD in which the process sequence is stored as a program. The final image obtained after energy subtraction is stored in a memory
112
connected to the bus
111
.
In the following there will be explained the function of the apparatus described above. At first the X-ray tube
101
emits X-ray with a first energy distribution, and the obtained image is converted in the CCD
103
into an electrical signal, which is converted by the A/D converter
104
into digital values. After the correction in the offset/gain correction unit
105
and the conversion into values corresponding to logarithmic values in the logarithmic conversion unit
106
, the digital values are once stored in the memory
107
. Then the X-ray tube
101
emits the X-ray with a second energy distribution, and the image obtained by logarithmic conversion in a similar process is stored in the memory
108
.
Then considered is a pixel in the logarithmically converted two images obtained in the above-described manner, and such pixel is assumed to receive X-ray that has transmitted a bone and a soft tissue in the human body.
In the first energy distribution, the intensity H
1
of transmitted radiation is given by:
H
1
=I
1
exp(−&mgr;
1
t
1
)exp(−&mgr;
2
t
2
) (1)
wherein &mgr;
1
, &mgr;
2
are radiation transmittances of respectively bone and soft tissue, t
1
, t
2
are thicknesses thereof, and I
1
is the intensity of first incident radiation in this portion.
Similarly the intensity H
2
of transmitted radiation in the second energy distribution is given by:
H
2
=I
2
exp(−&mgr;′
1
t
1
)exp(−&mgr;′
2
t
2
) (2)
wherein &mgr;′
1
, &mgr;′
2
are radiation transmittances of respectively bone and soft tissue, t
1
, t
2
are thicknesses thereof, and I
Morgan & Finnegan L.L.P.
Tran Phuoc
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