Quantifying the effect of slice thickness on the imaging performance of ultrasound scanners
By Scott Inglis, NHS Lothian
Objective
The resolution integral (R) is a technique combining resolution at different depths into a single figure-of-merit. Overall imaging performance is quantified by calculating R using the Edinburgh Pipe Phantom (EPP), which contains nine anechoic pipes of diameters (d) between 0.4-7.9mm [1]. The depth (L) over which each pipe is visualised is plotted against 1/d, and the area under the curve is equal to R. From this curve, the depth of field (LR) and the characteristic resolution (DR) are calculated. LR is the length of the region of optimum imaging, and DR is the typical resolution within this region.
Aim
To develop a method for calculating R only in the slice thickness (ST) dimension (R(ST)), and compare results with R measured using the EPP (R(EPP)).
Methods
A slice thickness phantom (CIRS ATS538NH) was used to obtain continuous profiles of ST versus depth for 3 curvilinear (Siemens 4C1 and 6C1, GE C1-5-D), 2 linear (GE 9L-D, Siemens 18L6HD) and 3 multi-row linear transducers (Siemens 14L5 and 9L4, GE ML6-15-D). Assuming that a pipe cannot be imaged if a beam’s ST is wider than its diameter, L equals the depth range over which the ST is less than d. Values of L were obtained from which R(ST), DR(ST) and LR(ST) were calculated.
Results
R(ST) values (range 26 to 41) were 25-60% of the R(EPP) values, and were similar to single-element transducers (previously reported range 22 to 37). There was no clear relationship between R(EPP) and R(ST) but LR and DR were strongly correlated: r=0.99 and 0.86 respectively.
Conclusion
Slice thickness is an important limiting factor in the quality of grey scale ultrasound imaging. The performances
of the clinical transducers tested, including three modern multi-row transducers, were similar to single
element transducers in the slice thickness dimension.