|
What we test and why Sensitivity:
Element Sensitivity is a measure of the relative response of the individual
crystals within the array. There should only be minor variations in signal
amplitude of any given crystal within a fully functional array. Ideally, the
impulse response of each transducer element of an array should be identical.
Identical array element response would give rise to the best spatial impulse
response afforded by a particular beam former design. Unfortunately, perfectly matched array elements
cannot be constructed. Therefore variations will exist as shown The
sensitivity is displayed as a bar graph of the returning echo intensity. The
sensitivity graph should display a uniform intensity across the array.
Variations are not, however uncommon for older transducers. Signal reductions of
more that 20% are indications of weak elements. These probes will produce lower
quality Doppler and Color Flow results.
Capacitance:
This is a measure of the electrical performance of each individual element’s
circuit. The acoustic array is, by nature, a capacitor. Higher frequency probes
such as 7.5 MHz and above are usually smaller arrays and thus, have lower
capacitance, (approx. 50 pF). Lower
frequency probes such as 3.5 Curved Arrays usually have larger elements, and
thus higher capacitance, (approx 350 pF). The
cable also has capacitance. The standard ultrasound cable used on probes over 2
years old was typically 120 pF per meter. The newer probe designs often make use
of “low-cap” cable that is typically 85 pF per meter. It is important to
know how to read the capacitance graph in order to diagnose a broken cable,
cracked array element, or connector problem. Very high capacitance is a sign of
a short circuit. A short can occur anywhere along the length of the cable or in
the array itself. Pulse-Width:
The length of the returned echo pulse is an indication of the solvency of the
acoustic stack bonding. Longer transducer pulse-lengths can be the source of
poor axial resolution. The longer pulse lengths can also cause a mis-registration
of the color flow overlay. Center Frequency – The center frequency is the mid-point of the Pulse Spectrum. The center frequency graph should be uniform across the array. In a narrow-band array the center frequency is the highest point located on the particular transducers response curve. In broadband probes the center frequency is better calculated using the formula (as used by FirstCall2000Ô): FC=FL+FU/2, where FL is the lower –3dB point, and FU the upper –3dB point Fractional
Bandwidth-
Modern transducers have bandwidth specification of >50%. This allows a user
to optimize the image for either resolution or penetration. It also improves
Doppler sensitivity. Low Fractional Bandwidth readings can be attributed to the
poor design of a clone probe.
|
|
Home FAQ Phantom Comparison Test Parameters Report Example Hidden Cost Email: info@redrocksmed.com Copyright 2002. All rights reserved |