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Sonography belongs to a group of diagnostic methods which have run a stormy course in the last decade. As a noninvasive method, deemed safe for both examiner and patient, sonography involves radiation of nondamaging doses of ultrasound (US) energy only. The major hazard for patients undergoing sonography consists in potential wrong diagnosis. Valid diagnosis in turn depends on the experience of the examiner and quality of the image of the particular sonograph used. Technical developments on the other hand have contributed to the wide range of variable quality equipment available on the market. The technical features of such equipment are not easy to evaluate and urgently needed is a simple, objective and sufficiently accurate method for verifying the quality of the sonograph and thus the image produced. One of the essential criteria determining the accuracy of instruments like this is spatial resolution (SR). This is the criterion now utilised for the evaluation system described below. 

Definition of the problem

Spatial resolution has three components: Axial-, Lateral and Transverse Resolution. Axial SR is influenced mainly by the ultrasound (US) frequency used and very little by the construction of the probe. In contrast, the SR in lateral and transverse planes is dependent on both the construction of the probe, method used for its focusing and the processing of the signal pathway from probe to output as image. The most frequently used device for the evaluation of spatial resolution, is a special measuring phantom which consists of a medium for the predetermined attenuation of US energy, in which different reflecting targets are placed. The image of these reflectors however, leads to more or less subjective judgement of the level of SR of the instrument being evaluated. The accuracy of measurement here can be jeopardised mainly through subjective error and/or nonstandard measurement procedures. Its advantages are that it includes all links in the image chain from probe to monitor and measurement is quick and simple. For the reason we looked for a method for objectifying this process and at the same time maintain its advantages. 


At our institute we have constructed a measuring tank with a point reflector whose position can be changed in all three axes. This ensures a standard input. For evaluation of the output information advantage is taken of the fact that all sonographs are equipped with a standardised videosignal output. With the help of digitalisation of the videosignal we obtain a bitmap picture in which we are able to evaluate the SR in the lateral plane and the signal dynamic. The software used for the above is APROFIL, which is a part of the programme package ARCHIVis APROFIL, a part of ARCHIV program package (supplied by CONDATA s.r.o., Olomouc, Czech Republic) and routinely used in theroutinely used at the Clinic of Gynaecology and Obstetric and Gynaecology Clinics of the Medicals of the Faculty Hospital in Olomouc for archival sonographic images. 
The system function diagram.


The system described fulfills the criteria for solving the outlined problem and enables us now to measure the parameters of both the axial and transversal resolutions. Aditionally we can appreciate dynamic range of  signal, gray scale parameters, the amplitude of videosignal and level of side lobes. 

To evaluate our method we examined four sonographs with different transducers working on the frequencies of 3 and/or 5 MHz. 

Using ball reflectors of different diameter 6 and 0,4 mm we at first checked the independence on the reflector diameter. The results obtained with both reflectors were very similar in the focal area but slightly differed in fare field area. Therefore we used the smaller one for further examinations. 

The measuring tank with point reflector.
Picture 1. The measuring tank with point reflector.

The data extracted from bitmap pictures were interpolated by software MATLAB into 3D graph. The lateral resolution (LR) was estimated from the 3D function for different depth generating the lateral resolution characteristic displayed in the Graph No 2. The position of the focal area declared by producer was compared with the estimated one. The results - value of lateral resolution and focal lengths are shown in the Tab.1 . 


The method gives reliable results for sonographs quality assessment. The work will be continued so that the use of the method can spread and more parameters, e.g. side-lobes amplitude measurement, can be evaluated. 
Graph 1a. Distribution of echo intensity in lateral-transversal plane: transducer #5,
X = lateral axis [mm], Y = transversal axis [step 0.25 mm], Z = echo amplitude [units]
Graph 1b. Spatial distribution of central beam echo intensity: transducer #3,
X = lateral axis [mm], Y = depth [cm], Z = echo amplitude [units]
Graph 2. Characteristics of the lateral resolution (LR) from different transducers.
Transducer number Frequency lateral resolution estimated focal length declared focal length estimated maximal amplitude scanning system
MHz mm mm mm units
1 3.5 3.6 40 30 - 50 184 mechanic
2 5 2.8 40 40 174 mechanic
3 3.5 2.6 80 50 - 70 168 electronic
4 3 6.1 NA 70 238 mechanic
5 5 4.8 NA 50 238 mechanic
 Tab. 1. Review of results (NA - not available)

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