Babeh Edi

Transducer Selection

Transducer selection is based on all the F’s. -Frequency
-Format/F.O.V
-Footprint
-Frame Rate
-Focal Characteristic
-Functional Aspects
Frequency- It is best to use the maximum frequency possible to image the region of interest, allowing for adequate penetration to this depth and thus avoiding ‘dropout’ artefact. There are several reasons for this, increasing frequency will; improve axial resolution, produce a better beam shape (longer near field) and increase the return from non-specular interfaces. Transducer frequencies common are 5-15MHz for superficial work and 2-7MHz for deeper areas.
Format- The depth of interest also influences the decision on transducer format used. Linear arrays with their rectangular F.O.V. provide the widest superficial F.O.V and best spatial resolution in the superficial zone. However when imaging to depth a transducer with a radiating scan format is needed to give a useful width to the F.O.V with depth. eg. Convex, Phased and Annular arrays all have this radiating scan format. Convex arrays are the most common today.
Footprint- This refers to the area of the transducer face that contacts the patient. Footprint becomes significant when access is limited (i.e. between ribs or through cranial fontanelle). In these situations phased and annular arrays provide the smallest footprint. In contrast the wide footprint linear and convex arrays provide superior superficial imaging but can be more difficult to keep in complete contact with the skin.
Frame Rate- This refers to how many times a second the image is updated. The frame rate determines the temporal resolution potential of the system and therefore is important when assessing moving interfaces. Cardiac ultrasound is a classic example of a situation where frame rate is very important - multiple focal zones are not used and the line densities used are much lower that for general imaging.
Focal Characteristics- Most systems allow a variable depth of focus by varying the firing delays of the crystal elements across the array. It is important to keep the focal zone(s) to the depth of interest.
Functional Aspects- Will you need to perform colour or PW doppler during the examination and does the transducer have this capability. Not all transducers allow biopsy guides to be attached and specialised transducers need to the used for intracavity exams.