Practical Aspects of the Use of Urethane Test Objects for Ultrasound Quality Control Nicholas J Dudley* & Nicholas M Gibson Medical Physics Department, Nottingham University Hospitals, Nottingham, UK *Present address: Medical Physics Department, Grantham & District Hospital, Grantham, UK *Address for correspondence: Medical Physics Department, Grantham & District Hospital, 101 Manthorpe Road, Grantham NG31 8DG, UK. Tel. 01476 464288; email.
[email protected] The final, definitive version of this paper has been published in Ultrasound, 18(2), 5/2010 by RSM Press. All rights reserved. © The British Medical Ultrasound Society. http://ult.rsmjournals.com
The temperature dependence of urethane test objects has been previously demonstrated. As temperature increases, speed of sound and attenuation decrease. Our aims were to confirm these findings and to assess the implications in routine practice. The test object was cooled to 40C, scanned immediately and images captured for the measurement of speed of sound, resolution and low contrast penetration. Further images were captured and analysed as the test object warmed until the speed of sound reached that specified by the manufacturer. Our urethane test object is stored in a laboratory which is poorly temperature controlled. Biannual performance measurements made in summer and winter were retrospectively compared for 13 scanners. Speed of sound, low contrast penetration and resolution were compared. Images were also captured, with the test object at room temperature, using spatial compound imaging to look for blurring. Experimentally, the speed of sound decreased and low contrast penetration increased as the test object warmed, confirming the temperature dependence of these variables. In the retrospective study speed of sound was generally lower in summer than winter but resolution measurements were not affected; where data were available, low contrast penetration was greater in summer than winter where a significant difference in speed of sound had been found. Targets are blurred when using spatial compound imaging. Urethane test objects should be stored and used in a temperature-controlled environment or should be stabilised before use. Spatial compound imaging should be disabled during Quality Control testing. Keywords: Ultrasound Quality Control, Urethane, Acoustic Velocity
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Introduction Urethane
test
objects
are
widely
used
in
ultrasound
performance
measurement as part of a Quality Control (QC) programme. They have an advantage over some other test objects in their durability and long lifetime. However, a potential disadvantage is that their sound propagation speed is approximately 1450 m s-1, compared to 1540 m s-1 assumed in image construction in ultrasound scanners.
This results in errors in axial
measurements, errors in lateral measurements with curved or sector probes and defocusing.1,2,3,4
These factors are probably not significant when
comparing serial QC measurements, provided that the speed of sound (SoS) remains constant.
The SoS may also have an effect when using spatial
compound imaging during QC measurements; we hypothesise that there will be beam angle dependent axial and lateral displacement of images of targets in a urethane test object, resulting in blurring. Investigation of the acoustic properties of tissue mimicking materials has shown that the SoS and attenuation in urethane are temperature dependent.5,6 A cooler urethane test object has a faster SoS and is more attenuating. This may have an impact on serial QC measurements. The aims of this study were firstly to experimentally assess the effects of using a urethane test object, the ATS 539 (ATS Laboratories Inc., Bridgeport, CT, USA), at the incorrect temperature, then to assess the practical implications in routine QC, where measurements of calliper accuracy, resolution and low contrast penetration (LCP; the maximum depth at which speckle is seen) may be affected by seasonal variations in test object temperature.
Our third aim was to investigate the hypothesis that spatial
2
compound imaging may lead to blurring of target images in a urethane test object.
Methods Estimation of Speed of Sound SoS was estimated from the imaged separation of nylon filaments over the largest visible distance, assuming an imaged separation of 10 mm between successive filaments at 1454 m s-1, the specified SoS (at 230C) for the test object.
The dominant source of uncertainty in estimates with the probe
clamped in place between measurements is the quantisation error due to pixel size, where the 95% confidence interval (CI) is approximately +/-80% of the pixel size.7 For measurements between targets separated by 160 mm and a pixel size of 0.43 mm, as is the case in the experimental work below, this represents a SoS uncertainty of approximately +/-3 m s-1. When comparing measurements made on separate occasions uncertainties are larger, in our experience up to +/-15 m s-1, due to factors such as reproducibility of probe position. Distance measurements were made using the Nottingham Ultrasound QC software7 and the SoS estimated as follows.
Firstly the software was
calibrated against the scanner’s on-screen scale markings in the captured images, calibrating over the largest distance possible to minimise errors, giving a calibration independent of the urethane test object.
All scanner
calibrations had previously (at acceptance and following any software changes) been confirmed as accurate using a test object with SoS of 1540 m s-1. The software was then used to measure the imaged separation of the
3
targets in the urethane test object.
The real separation of the targets is
approximately 9.4 mm in order to give the appearance of a 10 mm separation when imaged (at 1454 m s-1 in the urethane) on a scanner designed for 1540 m s-1. For any other SoS in the test object the imaged distance between 2 targets is proportional to the ratio of 1454 m s-1 to the actual SoS, e.g. if the SoS in the test object is increased to 1480 m s-1 the imaged distance between adjacent targets will be reduced to 9.8 mm (10 mm x 1454/1480).
Experimental Study The test object was cooled overnight in a refrigerator to 40C. It was scanned immediately, using a Philips HDI 5000 (Philips Medical Systems, Eindhoven, Netherlands) with a curved array probe (model C5-2) with a single transmit focus set at 67 mm. The probe was clamped in position, and images of the vertical column of filaments captured for the measurement of SoS, resolution and LCP. Output was increased and gain adjusted to maximise LCP at the start of the experiment.
It is our normal practice to capture images with speckle at
approximately mid-grey. In order to achieve this and to mitigate the effect of potentially increasing brightness (as attenuation decreased) on LCP, overall gain was reduced to maintain a visually assessed constant grey level. As a retrospective assessment, the grey-scale histogram was sampled in a 25 mm wide region extending from 55 mm to 80 mm deep in a target free area and the median speckle grey-level recorded. Five image sets captured over the first 15 minutes of the experiment were used to estimate the reproducibility of measurements. Further images were
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captured and analysed as the test object warmed until the SoS reached that specified by the manufacturer, i.e. the average imaged separation between targets reached 10 mm. Images were analysed using the Nottingham Ultrasound QC software.7 Data were plotted against time. It was not possible to reliably measure test object temperature non-invasively. The test object has a temperature indicator on the outside of the housing; this will indicate current ambient temperature but may not reliably indicate the temperature of the urethane, particularly if the ambient temperature is unstable. It is acknowledged that the test object may have warmed inhomogeneously, which could have been prevented by stabilising the temperature in a water bath before each measurement. However, the experiment was designed to assess the impact on measurements of cooling the test object, rather than rigorously determining temperature dependence, which has been reported in other studies.6
Retrospective Study The urethane test object is stored in our ultrasound laboratory, which is inadequately heated in winter and has no air conditioning to reduce temperatures in the summer (this was a retrospective study and so no laboratory temperature measurements were available). Although scanners were tested in their normal clinical locations, measurements were generally completed within 30 minutes of leaving the laboratory, making it unlikely that a hot or cold test object would stabilise at the ambient temperature of the clinical room. Biannual QC measurements made in the most recent summer 5
and winter were compared for a single probe on each of 13 scanners (those normally tested in December to February and June to August) from a range of manufacturers; 5 probes were capable of harmonic imaging and these results were compared in addition to fundamental mode.
Eleven probes were
curvilinear arrays and 2 were phased arrays. The SoS, LCP and resolution were compared.
95% CIs were calculated from 5 sets of measurements
taken at acceptance testing of each scanner.
Spatial Compounding Acceptance tests have been performed on a scanner with compound imaging capabilities,
a
Toshiba
Aplio
SSA-790A
Corporation, Tochigi-ken, Japan).
(Toshiba
Medical
Systems
Images with and without spatial
compounding were acquired, using a model 6C1 probe with a single transmit focus, and the images were compared.
Results Experimental Study Table 1 shows the time of image capture, receiver gain (noted from the proximal figure on the displayed time gain compensation curve), image median grey-level, measured target separation (specified separation 160 mm) and estimated speed of sound for each image acquired whilst the test object was warming. The data show an increase in apparent target separation as the test object warmed, indicating a reduction in SoS.
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Image number Time elapsed (h:min)
1
2
3
4
5
6
7
8
00:00 00:03 00:05 00:07 00:15 01:12 02:01 02:55
Gain (no units displayed)
38
38
38
38
38
34
32
32
Median Grey (pixel values)
118
119
122
125
124
119
125
122
Target separation (mm) Estimated SoS (m s-1)
157.0 157.0 156.8 156.8 157.0 157.9 159.2 160.2 1482
1482
1484
1484
1482
1473
1461
Table 1. Time of image capture, receiver gain, image median grey level, target separation (specified separation 160 mm) and estimated speed of sound. The 95% Confidence Interval on median grey (from 5 measurements on one image) is +/-4.
Figure 1 shows the latest 4 SoS estimates.
Figure 1 also shows
corresponding LCP measurements which demonstrate attenuation decreasing as the test object warmed.
Figure 2 shows the apparent deterioration in
lateral resolution as test object warmed; this was significant between image 5 and the later images but not between images 6, 7 and 8.
7
1452
1490
185
1485
180
1480
175
1475
170
1470
165
1465
160
1460
155
1455
150
1450
145
1445
140
1440 0
5
10
15
20
25
SoS (m/s)
LCP (mm)
190
LCP SoS
30
Estimated Temperature (C)
FWHM (mm)
Figure 1 Estimated SoS and measured LCP versus estimated test object temperature. Error bars on LCP show the 95% Confidence Interval calculated from the first 5 images (assuming no change in attenuation during the first 15 minutes). Error bars on SoS show the 95% Confidence Interval of the quantisation error due to pixel size (0.43 mm), which is the dominant error with the probe fixed in position.
10 9 8 7 6 5 4 3 2 1 0
10C 15C 20C 25C
0
50
100
150
200
Depth (mm) Figure 2 Resolution, represented by the full-width-half-maximum (FWHM) of target image profiles, versus target depth for estimated temperatures from 100C to 250C. Error bars show the 95% Confidence Interval on the mean of the first 5 measurements. 8
Retrospective Study Table 2 shows the estimated SoS for each probe in summer and in winter. 7 of the 13 SoS results are significantly slower in summer than in winter; harmonic results are consistent with fundamental results. SoS was generally less in the summer when the test object was likely to be warmer. Scanner
Fundamental
95% CI
Winter Summer
Harmonic
95% CI
Winter Summer
Aloka SSD-1700
1472
1470
±15
GE Vivid 5 (P)
1426
1420
±8
Hitachi EUB-525
1456
1435*
±7
Kretz SA9900
1436
1430
±7
Philips HDI 5000 (P)
1438
1412*
±4
1427
1419*
±5
Philips HDI 5000
1435
1425*
±8
1446
1431*
±8
Sonosite 180+
1436
1421*
±11
Sonosite Titan
1434
1438
±10
Toshiba Aplio 1
1441
1438
±5
1435
1434
±5
Toshiba Aplio 2
1443
1425*
±3
1438
1425*
±10
Toshiba Eccocee
1458
1449
±15
Toshiba Nemio
1437
1412*
±6
1437
1415*
±5
Toshiba Powervision
1454
1432*
±8
Table 2. Estimated speed of sound (m s-1) in the test object in summer and winter for 13 probes. Probes were curved arrays except for those marked (P), which were phased arrays. The 95% CI was calculated from the 5 sets of images taken at baseline assessment of the probe. (* - significantly different, p180 mm; N/A = mode not available; * summer baseline, no 95% CI available as LCP beyond the bottom of the test object). Probes were all curved arrays.
10
40
LCP difference (mm)
35 30
r = 0.84
25 20 15 10 5 0 -30
-25
-20
-15
-10
-5
0
SoS difference (m/s)
Figure 3. Difference in LCP against difference in SoS (summer minus winter). Open symbols are harmonic results and closed symbols are fundamental results.
Spatial Compounding Figure 4 shows images of anechoic, circular cross-section targets with and without spatial compounding, where compounding has caused blurring of the targets. In Figure 4a (no compounding) each beam is formed along a single axis at 900 to the probe face; any axial displacement due to the low SoS is in the same direction for a given target, resulting in a consistent image.
In
Figure 4b (compounding on reception) for each transmitted beam several receive beams are formed along several axes over a range of angles; the low SoS results in lateral as well as axial displacement at beam angles other than 900 to the probe face, resulting in apparent lateral blurring when the beams are combined in a single image.
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(a)
(b)
Figure 4. Images of the urethane test object (a) without spatial compounding on reception, and (b) with spatial compounding on reception.
Discussion We have confirmed the earlier findings5,6 that SoS and attenuation in urethane decrease as its temperature increases and have shown that this can affect resolution and LCP measurements.
Our results together with those of
Browne et al.6 suggest that for every 10C change in temperature SoS and LCP change by 2 m s-1 and 2 mm respectively. Assuming that scanner distance calibrations remained constant, SoS in the test object was significantly slower in summer than in winter for 7 of the 13 scanners in the study. The test object is designed to allow assessment of calliper accuracy; these seasonal differences, if truly related to temperature
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induced changes in SoS introduce an artefactual error, in our worst case, of 1.8%, which would be considered a calliper accuracy test failure. Where LCP data were available, these correlate with the SoS results, confirming a difference in the test object between summer and winter. A number of LCP results were outside our tolerance for routine QC but discussion of the follow up is beyond the scope of this study. Figure 2 is consistent with previous results showing deterioration in resolution with a reduced SoS.2
However, there were no significant changes in
resolution between summer and winter and so these SoS differences do not have a practical significance in this respect. The acoustic properties of urethane test objects appear to be sensitive to the temperature changes encountered in our hospital laboratory between summer and winter, although in this study the variations were insufficient to cause erroneous resolution results. Such test objects should be stored and used in a temperature-controlled environment. If exposed to warm or cool conditions, e.g. during transport, they should be allowed to stabilise before use; this may take several hours. The use of spatial compounding when imaging a test object with SoS different to the design speed of the scanner results in image blurring, as shown in Figure 4b; in these circumstances spatial compounding should be disabled during QC testing. The clinical efficacy of spatial compounding cannot be assessed using a urethane test object.
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References 1. Goldstein A. The effect of acoustic velocity on phantom measurements. Ultrasound Med Biol 2000; 26:1133-1143 2. Dudley NJ, Gibson NM, Fleckney MF, Clark PD. The effect of speed of sound in ultrasound test objects on lateral resolution. Ultrasound Med Biol 2002;28:1561-1564 3. Goldstein A. Beam width measurements in low acoustic velocity phantoms. Ultrasound Med Biol. 2004;30:413-416 4. Chen Q, Zazebski JA. Simulation study of effects of speed of sound and attenuation on ultrasound lateral resolution. Ultrasound Med Biol. 2004;30:1297-1306 5. Iball GR, Metcalfe SC, Evans JA. Can you trust your phantoms? Abstracts from the 13th EUROSON Congress 33rd BMUS Annual Scientific Meeting at the EICC, Edinburgh, Scotland 11th – 14th December 2001. Eur J Ultrasound 2002;15 (Suppl.1):S36-37 6. Browne JE, Ramnarine KV, Watson AJ, Hoskins PR. Assessment of the acoustic properties of common tissue-mimicking test phantoms. Ultrasound Med Biol 2003;29:1053-1060 7. Gibson NM, Dudley NJ, Griffith K. A computerized ultrasound quality control testing system. Ultrasound Med Biol 2001;27:1697-1711
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