Preliminary Ultrasound Imager Quality Test “UNIFORMITY” J. Satrapa, TCC

Primary Task Large part of ultrasound scan-heads today are linear or curved arrays. The primary condition for high quality of ultrasound images may be defined as “properties equality” of all array elements – or uniformity. The first step in assessment of probe quality is to find the variations of element sensitivity. For such test the probe must be connected to ultrasound imager in order to avoid the eventual defect inside of imager.

Preliminary Probe Test For the first test of any probe the special tool is not necessary. The test is based on scanning the hand as shown on the image. During scanning at the imager screen appears the moving pattern of the random distributed scatter contained in the hand tissue and … … fix pattern of probe. The fix pattern is created from defects and non uniform sensitivity of the probe piezo electric elements.

In the Case of Fix Pattern Visibility … … the additional analysis of images is necessary Such special test will show quantitatively if the probe is applicable for diagnostic purposes. Special care must be taken with probes used in breast imaging where the quality of the probe plays serious role. Fix pattern

In the C-Image… … taken in near field the visibility of dark lines is much better. The “C-image” can be produced from sequence of B-images. The dark lines in C- image comes from disconnected array elements or from their reduced sensitivity

The gray level profile… … shows defective elements but it is unknown how large are the sensitivity variations. For quantitative evaluation of variations given from elements sensitivity it is necessary to introduce the reference. The reference can be compared with the given gray level variations and the sensitivity drop can be measured. Unfortunately the sensitivity variations can be very large and almost achieve the level of disconnected elements.

Interpretation It can be expected that half of the probe covered with high attenuation 5x tape (TESA 4651) will produce the gray level as shown schematically. The tape doesn’t cut abruptly the image of random distributed scatter of the hand. Some steepness of the edge can be expected. The equal steepness can be expected by disconnected elements with typical wedge shape.

Simulation The image shows linear array probe with tape. The tape cover the half of probe and two narrow strips cover additional several probe elements. Such arrangement simulate the possible probe defect. Difference to disconnected elements is the reverberation in near fields caused from tape.

Probe With Reference Tapes 1)The probe is by narrow, a little bit wider and half of probe tape covered as already descriebed. 2)B-image of given arrangement. 3)C-image of fix pattern. 4)Gray level distribution of tape covering. The image correspond to given interpretation of disconnected elements. The disconnected elements give very similar image but simulation of element disconnection made by covering with tape is much simpler to do with no special tool. 1) 2) 3) 4)

Example With Linear Array With special 3D filter it is possible to separate the fix pattern from moving part in all images of image sequence. The array defect is shown with blue profile. From shadow alone is not possible to conclude if there is an element disconnection or only the drop of sensitivity Already B-image shows the „shadows“ from disconnected elements. Better visible are the shadows in C-image Probably disconnected

Example With Linar Array+Reference The reference shows the typical slope as disconnected part of array. Although the typical slope appear surely can be concluded that more elements are disconnected with some elements with reduced sensitivity. The same B-image and C- image taken with reference. Several slices of the tape produce strong shadow.

Example With Defect Curved Array Strong damaged curved array revel all types of appearances met in uniformity analyses. 1)Disconnection of single element 2)Disconnection of more elements one near other 3)Sensitivity reduced elements Curved array needs special processing with “near field rectification”

Example With Curved Array+Reference Real disconnected elements show in the near field only the dark shadow. Not the case with tape reference, which shows in near field the reverberation. The reverberation doesn’t influence the typical reference slope as visible by disconnected elements. Identical slopes

Follows of Element Disconnection and Reduced Sensitivity Image shows the cross sections of Point Spread Function (PSF) from partly damaged probe. Shown probe generated side lobes and grating lobes. The side lobes is follow of disturbed apodization in azimuthal direction.

Visual Presentation of Side lobes… … gives the image of 3D point spread function (PSF). The secondary side lobes are visible coming from rectangular structure of active transducer elements (not visible in cross sections). ELEVATIONAL SIDE LOBES GRATING LOBES AZIMUTHAL SIDE LOBES SECUNDARY SIDE LOBES

Side Lobes Influence the Signal to Noise Ratio (SNR) Bad uniformity produce extra speckle noise in the image. The cyst doesn’t appear “dark” as expected but are filled with echoes, which are coming from side lobes.

Quality Criteria of Imager Due to Uniformity Defect The uniformity defect is almost always the problem of probe and probe cable.. If the uniformity defect is visible in B-image and in C- image as dark trace than it can be suggested to repair the probe if it is possible or exchange the probe. By repair it must be guaranteed the quality of the probe. In comparison with new one the quality must be comparable. Most of repaired probes will be supplied with new lens. In such a case the probe must be identical with new one in slice thickness. It can be simple measured only with 3D phantom.