Radiation image displaying method and apparatus

Method and apparatus for displaying images obtained by reading out stimulable phosphor screens wherein radiation images are stored. A mosaic type image is composed by means of a plurality of read-out images and said mosaic type image is displayed on a preview monitor or on one of a plurality of preview monitors.

FIELD OF THE INVENTION. 
The present invention is in the field of digital radiography. The invention 
more specifically relates to a method and an apparatus for displaying (a) 
radiologic image(s). 
BACKGROUND OF THE INVENTION 
In the field of digital radiography a wide variety of image acquisition 
techniques have been developed such as computerised tomography, nuclear 
magnetic resonance, ultrasound, detection of a radiation image by means of 
a CCD sensor or a video camera, radiographic film scanning etc. 
Still another technique has been developed wherein a radiation imager for 
example x-rays transmitted by an object, is stored in a screen comprising 
a photostimulable phosphor such as one of the phosphors described in 
European patent application 503 702 published and in U.S. Ser. No. 
07/842,603, now U.S. Pat. No. 5,340,661. 
The technique for reading out the stored radiation image consists of 
scanning the screen with stimulating radiation, such as laser light of the 
appropriate wavelength, detecting the light emitted upon stimulation and 
converting the emitted light into an electric representation for example 
by means of a photomultiplier. This technique further comprises digitizing 
and processing said electric signal and applying it to a recorder for 
recording a hard copy for example on film. This hard-copy can be viewed on 
a lightbox for diagnosic purposes. 
After read-out of the image stored in the photostimulable phosphor screen 
one disposes of an electric image representation that can be applied to a 
monitor for display of the corresponding visual image. 
Apparatus for performing the above-described image acquisition methods are 
commonly accompanied by a preview monitor to which the image signal is 
applied before being sent to an output device, i.e. a recorder or a 
workstation. 
The image can be then be evaluated either on the recorded hard-copy or on 
the display unit of the workstation or on the preview monitor. 
However, before the image is available at the output of the hard copy 
recorder, some time passes due to the duration of the recording process 
including the development of the film and occacionally due to formation of 
a queue of image signals waiting to be reproduced by the recorder. 
Also when the image is shown and evaluated at the workstation some 
processing time is to be taken into account before the visible image is 
available. 
Inspection on the preview monitor on the other hand can be performed almost 
immediately after acquisition and hence provides for early feedback to the 
operator so that corrections can be performed in case the acquisition went 
wrong. 
In hospitals that dispose of several radiology rooms it is possible that a 
preview monitor is installed in each radiology room in addition to a 
central preview monitor that is provided in the vicinity of an image 
acquisition apparatus. 
Immediately following acquisition, the acquired image is shown on the 
central monitor and eventually also on monitors that are locally provided 
in the radiology rooms so that the evaluation can be made by the operator 
who is occupied in each of the rooms. Evaluation can be made very fast 
after image acquisition so that in case of mis-acquisition the error can 
be immediately corrected. 
Sequentially acquired images are sequentially shown on the monitor(s). This 
mode of operation allows fast evaluation but has the disadvantage that the 
period of time of which the operator disposes to make an evaluation 
depends on the period of time between the display of a first image and the 
moment on which data regarding a subsequent image are available. Typically 
this is for example in a system wherein an image is read-out from a 
photostimulable phosphor screen about 1 minute. So, it may happen that an 
image is already removed from the display and a subsequent image is 
displayed while the operator did not yet have the opportunity to evaluate 
the former image on the display unit. 
This problem is partially solved by providing the acquisition apparatus 
with an interaction modality for example implemented by a wait, cancel and 
proceed function. Upon activation of the wait function for example in a 
system wherein images are stored in photostimulable phosphor screens, the 
operator can interrupt the process of successively reading out a sequence 
of phosphor screens so that he can study and evaluate a read out image on 
the preview monitor until he activates the cancel or proceed function. 
Upon activation of the proceed function the acquired image signal is sent 
further to the output device whereas upon activation of the cancel 
function, the image signal is no longer retained. After activation of 
either of these functions, the interruption is terminated and the 
acquisition apparatus starts acquiring a next image. 
By using these functions the operator can dispose of a longer period of 
time to make a first evaluation on the preview monitor. However, this 
procedure decreases the throughput of the read-out apparatus. 
In case more than one monitor is provided and every acquired image is sent 
to each of the monitors, it may occur that an operator who is occupied in 
one radiology room and makes his evaluation on the locally provided 
monitor needs to let pass the images taken in other rooms. Since these 
images are of no interest to him, this brings about a waste of time and 
even demands from the operator increased attention to detect among the 
displayed images those images made in the radiology room of interest. 
U.S. Pat. No. 5,015,854 issued May 14, 1991 discloses a configuration of a 
workstation (not preview monitor) to be interfaced with a signal gathering 
apparatus. 
The disclosure deals in particular with the retrieval of particular images 
out of said large number of images stored in a storage device. 
The retrieval is performed with the aid of outline images. 
When a stored image is to be retrieved by the operator of the workstation, 
a number of outline images is displayed simultaneously on the display 
device so that the operator can select the image of interest on the basis 
of low detailed pictorial information displayed on the monitor screen. On 
the basis of this selection he can order display of the complete 
non-reduced image. 
This method is generally referred to as "pictorial index" and has been 
described extensively by Th. Wendler et al. in Pictorial Information 
Systems in Medicine, published in Nato ASI Series, Vol. F19. 
OBJECTS OF THE INVENTION 
It is an object of the invention to provide a display system that enables 
quasi immediate evaluation after image acquisition and still gives the 
operator a reasonable amount of time to make an evaluation on the basis of 
this display without retarding the operation and without decreasing the 
throughput of the acquisition apparatus. 
It is another object of the present invention to provide such a display 
system adapted to a situation of multiple radiology rooms and a central 
image acquisition system. 
It is a further object is to provide a display system wherein the image 
quality of the displayed image is such that the displayed image is 
sufficiently similar to the final image to allow image evaluation. 
It is still a further object to provide such a display system to be used in 
connection with an image acquisition system wherein a radiation image that 
was stored in a photostimulable phosphor screen is read-out by scanning 
said screen with stimulating radiation, detecting the light emitted upon 
evaluation and converting the detected light into a digital signal 
representation. 
Further objects will become apparent from the description hereinafter. 
SUMMARY OF THE INVENTION 
The objects of the present invention are achieved by a method of displaying 
on a display device radiation images each represented by a digital signal 
representation characterised by the steps of 
deducing reduced image signals from said digital signals, said reduced 
signals representing reduced images comprising less pixels than said 
radiation images, 
forming a composed signal representing a mosaic type image by means of a 
number of reduced image signals, 
applying said composed signal to said display device, 
as a new reduced signal is deduced, amending said composed signal by means 
of said new signal so that at least one of the reduced images in the 
displayed image is replaced by the image represented by said new reduced 
signal and applying said amended signal to said display device. 
For application in case more than one display devices is provided, the 
invention provides a method of displaying radiation images each being 
represented by a digital signal representation on one of a plurality of 
display devices comprising the steps of 
associating a code identifying one of said plurality of display devices 
with each of said signals, and 
applying each of said signals to the identified display device for display. 
The invention further provides a method of displaying radiation images each 
represented by a digital signal representation on one of a plurality of 
display devices comprising the steps of 
associating with each of said digital signals a code identifying one of a 
plurality of display devices, 
deducing reduced image signals representing reduced images from said 
digital signals, 
forming composed signals representing mosaic type images by means of a 
number of reduced image signals originating from digital signals with an 
identical associated code, 
applying each of said mosaic type image representing signals to an 
identified display device, 
as a new reduced signal is deduced, amending a composed signal composed 
with signals with the same associated code by means of said new reduced 
image signal so that at least one of the reduced images in the displayed 
image is replaced by the image represented by said new reduced signal and 
applying said amended signal to the display device identified by said 
associated code. 
By the term "mosaic-type" image is meant an image composed of a 
two-dimensional array of individually distinguishable images that together 
cover the entire addressable area of a display device. Such-like 
"mosaic-type images" can be compared with the mosaic-type image that is 
used in television broadcasting, wherein an image is displayed that is 
composed of an array of smaller images each showing the program 
broadcasted on a particular channel. 
The number of reduced images (represented by the above-defined reduced 
signals) that together compose a mosaic type image depends on the number 
of addressable pixels in the display device and on the number of pixels in 
each of the reduced images. 
In one embodiment the radiation image is stored in a photostimulable 
phosphor screen and the digital signal representation of said radiation 
image is obtained by scanning said screen with stimulating radiation, 
detecting the light emitted upon stimulation and converting the detected 
light into a digital signal. 
In the following the invention will be explained with reference to such an 
image acquisition system. It will be clear that this invention is not 
limited to this kind of image acquisition system and that alternatives may 
be envisioned. 
The invention is particularly advantageous for display of radiographic 
images on a preview monitor. However, the invention is not limited to 
display on a preview monitor and in the following whenever the term 
"preview monitor" is used, it can be replaced by another kind of display 
device. 
By applying the method of the present invention to the images that are 
displayed on the preview monitor the operator disposes of a longer period 
of time to make a first evaluation of the recorded image. 
The embodiment of the method of the present invention relating to the 
display of mosaic type images on a plurality of monitors can provide that 
only those images that relate to exposures made in one radiology room are 
sent to the preview monitor installed in said room and that, if a mosaic 
type image is displayed at the local preview monitor, this mosaic type 
image is only composed of these component images. 
Reduced image signals can be deduced from the digital signal representation 
obtained by reading out a photostimulable phosphor screen by subsampling, 
i.e. by sampling the digital signal at every nth position in a row and 
every mth position in a column of the array of pixel elements representing 
the image. 
Alternatively a reduced image signal can be obtained by interpolation of 
the pixel values. 
In European patent application 527 525 filed 14 August 1991 and published 
on 22 February 1993 a contrast enhancing method is disclosed which 
comprises the steps of receiving an original digital image represented by 
an array of pixel values, processing said original image and recording the 
processed image on a recording medium or visualising it on a display 
monitor, said processing comprising the steps of 
a) decomposing said original image into a sequence of detail images at 
multiple resolution levels and a residual image at a resolution lower than 
the minimum of said multiple resolution levels, 
b) modifying the pixel values of said detail images to yield pixel values 
of a set of modified detail images by means of at least one non-linear 
monotonically increasing odd mapping function with a slope that gradually 
decreases with increasing argument values and 
c) computing said processed image by applying a reconstruction algorithm to 
the modified detail images and the residual image, the reconstruction 
algorithm being such that when applied to the detail images and the 
residual image said original image or a close approximation thereof would 
be obtained. 
In a specific embodiment of the invention described in the above European 
patent application the original image is decomposed into a so-called 
pyramidal sequence of detail images, i.e. successively formed detail 
images in the set of multiresolution detail images have a reduced number 
of pixels. 
For example, the multiresolution representation after decomposition may 
have a pyramidal structure such that the resolution level of the detail 
images differs by a factor of 2, and the detail images at each resolution 
level are calculated by filtering the original image with the difference 
of two low-pass filters and by subsampling the resulting image by a factor 
2. 
The used filter preferably has a two dimensional Gaussian distribution. 
This procedure can be implemented as follows. The original image is 
filtered by means of a low pass filter as described above, and subsampled 
by a factor of two, which is implemented by computing the resulting low 
resolution approximation image g.sub.1 only at every other pixel position 
of every alternate row. 
A detail image b.sub.0 at the finest level is obtained by interpolating the 
low resolution approximation g.sub.1 with doubling of the number of rows 
and columns, and pixelwise subtracting the interpolated image from the 
original image. 
The interpolation is effectuated by an interpolator, which inserts a column 
of zero values every other colunm, and a row of zero values every other 
row respectively, and next convolves the extended image with a low pass 
filter. The subtraction is done by an adder. 
The same process is repeated on the low resolution approximation g.sub.1 
instead of the original image, yielding an approximation of still lower 
resolution g.sub.2 and a detail image b.sub.1. 
A sequence of detail images b.sub.i, i=0 . . . L-1 and a residual low 
resolution approximation g.sub.L are obtained by iterating the above 
process L times. 
The finest detail image b.sub.0 has the same size as the original image. 
The next coarser detail image b.sub.1 has only half as many rows and 
columns as the first detail image b.sub.0. At each step of the iteration 
the maximal spatial frequency of the resulting detail image is only half 
that of the previous finer detail image, and also the number of columns 
and rows is halved, in accordance with the Nyquist criterion. After the 
last iteration a residual image g.sub.L is left which can be considered to 
be a very low resolution approximation of the original image. In the 
extreme case it consists of only 1 pixel which represents the average 
value of the original image. 
When this kind of image decomposition is applied, it is advantageous to use 
one of the intermediate low resolution images that are generated during 
the decomposition of the image into a set of detail images and a residual 
image as the reduced image when composing the mosaic type image. 
According to the image processing method described in the above mentioned 
European application EP 527 525 the pixel values of said detail images are 
modified to yield pixel values of a set of modified detail images. 
Preferably, the modification is performed according to at least one 
non-linear monotonically increasing odd mapping function with a slope that 
gradually decreases with increasing argument values. 
Finally a processed image is computed by applying a reconstruction 
algorithm to the residual image and the modified detail images. The above 
mentioned European application EP 527 525 describes such-like 
reconstruction algorithms. 
In one embodiment of such a reconstruction algorithm the residual image is 
first interpolated by interpolator to twice its original size and the 
interpolated image is next pixelwise added to the detail image of the 
coarsest level b'.sub.L-1, using an adder. The resulting image is 
interpolated and added to the next finer detail image. If this process is 
iterated L times using the unmodified detail images b.sub.L-1 . . . then 
an image equal to the original image will result. If at the other hand the 
detail images are modified before reconstruction, then a processed image, 
for example a contrast enhanced image will result. The interpolators are 
identical to those used in the decomposition section. 
The reduced image used for the composition of the mosaic type image may 
then consist of an image generated during the reconstruction process, said 
reconstruction process being limited up to some intermediate resolution 
level which is lower than the original resolution. 
The above-described embodiments are advantageous because the reduced image 
obtained by application of the above decomposition or reconstruction 
process has a better image quality than a subsampled image, and this high 
quality image is obtained in a fast way, the method does not require any 
additional computational effort since during the image processing the 
above intermediate images are already available. 
According to the present invention the mosaic type image representing 
signal is amended each time a new reduced image signal is available so 
that at least one of the reduced images in the displayed mosaic-type image 
is replaced by the image corresponding with said new reduced signal. 
The position of the replacement image in the mosaic type image can be 
selected. 
It is preferred that the reduced image component of the mosaic type image, 
that was first available for the composition of the mosaic type image is 
replaced by the new image since the former was visible on the screen 
during the longest period of time. However, this is not essential of the 
present invention, it is merely a matter of design choice and alternative 
embodiments might be implemented. 
Also for the component images in the mosaic type image that are not 
overwritten alternative embodiments are possible. 
The method can be implemented so that when a new reduced image becomes 
available, the reduced image represented by the reduced image signal that 
was first available is overwritten and that the location of the other 
component images remains unchanged. 
Alternatively the new reduced image may come in the place of the reduced 
image represented by the former reduced image signal. Then the image 
corresponding with the reduced signal component that was first available 
may be dropped and the other images may be shifted in a predetermined 
order to cover the locations in the mosaic image therein between. 
Still alternative embodiments may be envisioned. 
In the embodiment of the present invention incorporating a plurality of 
monitors the selection of the images that are to be sent to a specific 
monitor comprising the steps of after image signal acquisition, 
associating a code identifying a monitor with an acquired image signal and 
displaying said image on the identified monitor. 
In case of composition of a mosaic type image on one of a plurality of 
monitors, a reduced image signals to which an identical code was 
associated will add to the composition of a mosaic type image to be 
displayed on the monitor identified by means of said code. 
In one embodiment of the present invention the radiation image is stored in 
a photostimulable phosphor screen and the identification code is witten 
into an electrically erasable programmable read only memory (EEPROM) that 
is provided for identification purposes on a cassette conveying the 
stimulable phosphor screen. The provision of a such-like EEPROM chip on a 
cassette has been described extensively in German patent application 37 31 
204. 
Data relating to the exposed object, such as the patient name, and to the 
exposure, such as the name of the radiologist and the type of examination 
to be performed, are written onto the EEPROM by means of an identification 
camera. Supplementary to these identification data a code identifying the 
preview monitor to which the read-out image is to be sent can also be 
provided on this EEPROM device. 
These data are read-out in the radiation image read out apparatus and 
accompany the image representing digital signal through the image 
processing and image recording step. 
In a practical situation an identification station is often provided in 
each of the radiology rooms. The station itself is identified by a code, 
so that when the code of the identification station is associated with the 
screen during the identification procedure (for example written on the 
EEPROM on the cassette), this code automatically identifies the radiology 
room where the record was made and the preview monitor provided in said 
room. In this way, no additional identification needs to be performed. 
Alternative ways of identification of the preview monitor to which the 
read-out image signal is to be sent are enumerated hereinbelow, this list 
being non-exhaustive. These data or part thereof can also be written on an 
identification card such as a magnetic card or on an optically readable 
card, it can be provided in the form of a bar code or it can be entered 
manually, for example by means of a keyboard input. 
The present invention further discloses an apparatus for performing the 
method of the present invention. The apparatus comprises 
means for deducing a reduced image signal from said digital signal, 
means for forming a composed signal representing a mosaic type image by 
means of a number of said reduced image signals, 
a display device to which said composed signal is applied, 
control means providing that when a new reduced image signal is deduced, a 
new composed signal is formed so that at least one of the component images 
in the mosaic-type image is replaced by the image represented by the new 
reduced signal and that said new composed signal is fed to said display 
device. 
For application in a system comprising a plurality of display devices the 
present invention provides an apparatus comprising 
means for determining an identification code identifying a display device, 
said code being associated with an acquired image, 
means for applying said digital signal to the display device identified by 
said code. 
The invention further discloses an apparatus for displaying radiation 
images each being repesented by a digital signal representation on one of 
a plurality of display devices comprising 
means for determining an identification code associated with an image, said 
code identifying one of said plurality of display devices, 
means for deducing a reduced image signal from a digital signal, 
a plurality of display devices each being identified by an identification 
code and each comprising means for forming a composed signal representing 
a mosaic type image by means of a number of said reduced image signals to 
which the same identification code was associated, and each being provided 
with control means providing that when a new reduced image signal with the 
same identification code is deduced, a new composed signal is formed so 
that at least one of the component images in the mosaic-type image is 
replaced by the image represented by the new reduced signal and that said 
new composed signal is fed to said display device. 
Preferably said control means comprise a serial memory capable of storing 
at least said N reduced image signals, clock signal generating means for 
timing the read-out of said serial memory at regular intervals and for 
timing the application of the reduced signals into said means for forming 
the composed signal. 
In a particular embodiment said radiation image is stored in a 
photostimulable phosphor screen, and a digital signal representation of 
said image is obtained by an image acquisiiton apparatus comprising 
means for scanning a photostimulable screen wherein a radiation image is 
stored with stimulating irradiation, 
means for detecting the light emitted upon stimulation and 
means for converting said detected light into a digital signal. In this 
particular embodiment the means for determining an identification code 
comprise means for reading out a code from an electronic memory device 
provided on a cassette conveying the photostimulable phosphor screen 
wherein a radiation image has been stored. 
In the above described embodiments the display apparatus can additionally 
be provided with means for selecting either display of a single 
non-reduced image or display of a mosaic type image.

FIG. 1 generally shows an apparatus in which the method of the invention 
can be applied. 
A radiation image of an object was recorded on a photostimulable phosphor 
screen (3) by exposing (2) said screen to x-rays transmitted through the 
object (not shown). The stimulable phosphor screen was conveyed in a 
cassette (3) provided with an electrically erasable programmable read only 
memory (EEPROM). In an identification station 4 various kinds of data, for 
example patient identification data (name, date of birth) and data 
relating to the exposure and/or to the signal processing were written into 
the EEPROM. 
In a radiation image read-out apparatus 1 the image stored in the 
photostimulable phosphor screen was read-out by scanning the phosphor 
screen with stimulating rays emitted by a laser. The stimulating rays were 
deflected into the main scanning direction by means of galvanometric 
deflection. The subscanning was performed by transporting the phosphor 
screen in the subscanning direction. The stimulated emission was directed 
onto a photomultiplier for conversion into an electrical image 
representation. Additionally the information stored in the EEPROM was 
read. 
The subsequent data flow is illustrated in FIG. 2. The output signal of the 
photomultiplier was converted into a logaritmic quantity log E (E being 
the exposure value), and next the signal quantised. This quantised image 
signal, called the raw image signal, was sent to the image processing 
module of the read-out apparatus (FIG. 1, numeral 7) where it was stored 
in an internal buffer. 
From the image processor the image was sent to the preview monitor (FIG. 1, 
numeral 5). 
Without any modifications it was also sent from the image processor to an 
image workstation where it was temporarily stored on a hard disc. This 
back up ensured that the signal was never lost, even not when any of the 
components of the apparatus would fail and that the signal could be 
retrieved for any kind of later processing, for example processing with 
different parameter setting. This feature could be used when the result of 
the on-line processing was unsatisfactory due to bad exposure conditions 
or inadequate selection of the processing parameters. 
The latitude of the raw image is normally too large to be printed on film 
or to be displayed on a monitor. Therefore the latitude was confined to 
the diagnostically relevant region (requantization). The result hereof 
returned a 10 bit image, representing an image proportional to log 
exposure, where the grey levels below and above the diagnostically 
relevant region were clipped to zero and 1023 respectively. 
Then the signal was fed to a circuit for forming a composed signal 
representing a mosaic type image, the output signal of this circuit was 
further applied to the frame buffer of the preview monitor for display. 
It was also possible to display a single non-composed image. 
The composition of the mosaic type representing signal is explained with 
reference to FIG. 3. 
The number of pixels read-out by the apparatus in the present application 
was 2048 pixels per line multipied by 2048 lines per image, the number of 
lines that is scanned being a function of the format of the screen in 
wherein the image was stored. 
A reduced signal was extracted from the image signal by subsampling. The 
number of pixels was reduced by a factor 16 to 512.times.512 pixels. 
Then the reduced signal was written into a first in, first out memory 
comprising 4 memory locations. 
The above-described procedure was repeated under control of appropriate 
control and clock signals (not indicated in the figure) for subsequently 
read-out image signals so that the fifo was completely filled. 
Next the fifo was read-out and the read-out signals were fed into a 
subsampling ciruit wherein each second pixel was retained. By means of the 
resulting signals, a mosaic type image as shown in FIG. 4 was composed of 
4 individually recognizable images corresponding with the subsampled 
signals and together occupying the total number of addressable elements in 
the monitor. 
As a new image was read-out the corresponding reduced signal was written 
into the first location of the fifo and the data in the other locations 
were shifted, the first read-in signal was dropped. Then a new signal 
representing a new mosaic type image was composed, fed to the frame buffer 
of the preview monitor and displayed. 
The explanation relating to the operation of the formation of the 
mosaic-type image was given hereinbefore for a single monitor. In case 
more than one monitor is coupled to a single read-out device, the code 
identifying the monitor where the display of a particular image is desired 
was written into the EEPROM provided on the cassette wherein the image 
storing phosphor is conveyed and read-out in the read-out apparatus as 
described higher. 
The apparatus then additionally comprises switching means controlled in 
function of the read-out identification code so as to feed the reduced 
signals to the identified monitor and associated first in, first out 
memory and composition circuitry. A mosaic type image is then composed by 
means of signals with the same associated code. 
Display of the images on the preview monitor(s) was followed by mapping of 
signal values (in this case log exposure) values into corresponding 
density values according to a specified gradation mapping curve and the 
image signal was passed to a laser recorder for hard copy recording. 
Occacionally this mapping can be preceded by additional signal processing 
such as contrast enhancement processing.