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| ANS 96-010B | |||
Accuracy and Application of Real-Time Ultrasound for Evaluation of Carcass Merit in Live Animals
Introduction
Efforts to utilize ultrasound to evaluate carcass characteristics in livestock began in the
late 1950's. Although the relationship between longissimus muscle area (ribeye area) and
carcass cut-out value in beef cattle was known to be low1, measurement was
considered important. The relative value of retail cuts obtained from the loin combined
with its' inclusion by USDA in the beef yield grade equation justified these measurement
efforts.
Ensuing studies using both backfat and loineye area measurements to predict pounds
of lean cuts in swine also showed considerable promise.2 Researchers therefore
continued their pursuit of ultrasonic measurement.
Mode of Action
As with computer technology, early machines were large and cumbersome, limiting their
usefulness in the field. Technicians attempted to completely immobilize animals to overcome
machine limitations and to improve measurement accuracy.
Most early systems employed single crystal transducers that represented distance measurements
with a series of lights. Lighted points further up the scale denoted greater depths of
consistent tissue density. Tissue interfaces and narrow bands of consistent tissue also
excited the light scale.
More complex systems employed a series of scans on the same animal to improve measurement
accuracy. However, any animal movement during the scanning process reduced the usefulness
of measurements.
Technician interpretation of signals required a thorough understanding of
anatomical tissues, as well as awareness of machine limitations. Reasonable success was
demonstrated in fat and muscle measurement in spite of these conditions.3
Dramatic advances in ultrasound technology have occurred since that time. Human medical
research has driven these advances, benefiting both animal researchers and veterinarians.
Modern A-mode systems; single crystal machines more compact and accurate than the lighted
scale machines described above, display readings as numeric measurements. Some units allow
the user to enter animal weights and include prediction equations for carcass leanness. Much
less expensive and more portable than most real-time machines, modern A-mode units are
heavily used by the swine industry for pregnancy detection and backfat measurement and are
becoming more common for total leanness evaluation.
Modern real-time units combine many crystals which fire in sequence emitting high frequency
sound waves. These sound waves bounce off tissue interfaces (such as a change from fat to
muscle) and bounce back to the sending unit. These reflections allow the system to measure
elapsed time and create constantly updated two dimensional images. These images represent a
cross-sectional view of the tissues scanned. Minor anatomical features are visible and can
be assessed for deviations from normal anatomy.
Human obstetricians may use ultrasound for fetal evaluation while animal scientists use the
same equipment to observe and manipulate reproductive functions or to evaluate carcass
characteristics in live animals. Real-time ultrasound systems are quite costly and primarily
used for research purposes. Nonetheless, some individuals and firms offer custom livestock
scanning services with real-time machines.
Accuracy
Measurements made with ultrasound can be influenced by both technician experience and
machine differences.4,5 Inexperienced technicians often confuse shadows and
multiple reflections with anatomical features, leading to inaccurate diagnoses or faulty
measurements. For precise comparisons, quality images must be captured from fixed locations
on each animal and subsequently interpreted. Considerable knowledge of the muscle
structure of tissues scanned is required for accurate assessment. Experience and dedication
are crucial components if precise measurements are to be attained. Technicians must
repeatedly follow scanned animals through the slaughter process to evaluate carcasses and
use this information to improve both image capture and analysis.
Differences in ultrasound equipment can also influence image quality and subsequent
analysis. First generation real-time units required the technician to combine two
separate frozen images to evaluate the entire ribeye muscle in beef cattle. These units
require considerable experience to make accurate measurements.
Second generation real-time machines can accommodate a longer transducer, capable of
imaging the entire beef ribeye on a single screen. This improvement in technology allows
less experienced technicians to improve muscle area measurement.
Technician accuracy is typically evaluated by comparing carcass measurements with
ultrasound measurements of backfat and ribeye or loineye areas. Even though
considerable changes in the fat and muscle arrangement are known to occur as a result of
the slaughter and hanging process,6 carcass measurements are generally
considered standards with which any other measurement technique must be compared.
Certification
The Beef Improvement Federation (BIF) and the National Swine Improvement Federation (NSIF)
have each established guidelines which prospective technicians must meet in order to certify
as qualified ultrasonic technicians. Protocols and accuracy requirements are determined by
each organization. In these programs, prospective technicians scan test animals twice,
interpret their own images, then follow the animals through processing. Carcass data is
then compared with live measurements to develop accuracy and repeatability ratings for
participants. Minimum levels for each area must be attained prior to certification.
(See Table 17,8)
| . | ||||
| Statistic | ||||
| Technician Bias | 12-13th rib fat | 0.12 in. | 10th rib fat | 0.15 in. |
| . | 12-13th rib area | 1.2 sq. in. | 10th rib area | 0.50 sq. in. |
| Standard Error of | 12-13th rib fat | 0.10 in. | 10th rib fat | 0.15 in. |
| Prediction (SEP) | 12-13th rib area | 1.2 sq. in. | 10th rib area | 0.50 sq. in. |
| Standard Error of | 12-13th rib fat | 0.12 in. | 10th rib fat | 0.10 in. |
| Repeatability (SER) | 12-13th rib area | 1.2 sq. in. | 10th rib area | 0.40 sq. in. |
The means or averages of the two measurement techniques are compared to establish a technician bias. For example, if the carcass loineye area measurements averaged 5.5 square inches and ultrasonic loineye measurements averaged 5.8 square inches, the technician bias would be 0.3 square inches. In other words, the technician typically overestimates loineye area by an average of three tenths of a square inch.
A second measure of accuracy is known as the standard error of prediction (SEP). This statistic establishes an amount we would expect individual ultrasonic measurements to differ from carcass measurements. For a technician with a SEP for backfat of .08 inches, we would expect two thirds of his ultrasonic backfat measurements to be within .08 inches of the carcass measurements.
A third measure of technician accuracy is known as the standard error of repeatability (SER). This statistic establishes an amount we expect repeated measurements on the same animal to differ. A technician with a SER for ribeye area of 1.0 square inch would be expected to arrive at two measurements on the same animal within one square inch of each other, at least two thirds of the time.
Implications
Recent research efforts using ultrasonic measurements to predict weights of lean in pork carcasses have show considerable promise.9 When a model including backfat, loineye area and carcass weight was used, approximately 80 percent of the variation in carcass leanness was predicted. Similar accuracy's have been demonstrated using live swine measurements.10 Overall swine leanness is somewhat easy to predict because ultrasonic backfat measurements are quite accurate and swine lean yield is relatively fat dependent.
Estimating weight of lean in cattle with similar measurements has been more difficult. The amount of total body lean in cattle is less fat dependent than similar leanness in swine, thus our predictions for cattle must place more emphasis on less accurate muscle area measurement. Although Brethour has demonstrated reasonable accuracy in predicting yield grade in cattle with this technology,11 more work needs to be completed comparing ultrasonic estimates with actual product yield.
Beef researchers at Iowa State have developed a large database of ultrasonic information on breeding cattle. However, relating this information to it's impact on slaughter weight of offspring is just beginning. A recent Colorado State study summarized ultrasound research relating backfat thickness and ribeye area in both yearling bulls and/or heifers and in slaughter steers.12
Interestingly, most of the data shows a positive genetic correlation between the two measurements in yearling breeding animals but a negative correlation in slaughter steers. Because of these relationships, they surmised that selection for lower backfat in breeding animals may actually increase backfat in slaughter progeny. They, like many others, conclude that selection for larger ribeye area without consideration of accurate quality grade measurement could have unfavorable consequences for breeders.
Researchers at several universities and private firms are now attempting to quantify marbling differences in beef cattle and swine with ultrasound. Current ultrasound equipment is capable of detecting small flecks of intramuscular fat, or marbling, but quantifying these amounts is more difficult. Technicians cannot visually differentiate between signals reflected by marbling from those reflected by other structures, such as connective tissue, blood veins and arteries, which permeate muscle tissue.
Research efforts have concentrated on statistical analysis of ultrasonic images. Gray scale differences (the level of brightness of pixels in ultrasonic display screens) between images of loin muscles of different animals have been evaluated as well as pattern differences between images. The accuracy of these systems to quantify marbling in yearling breeding cattle and in feedlot cattle is not well documented. Although recent work from Kansas shows promise relating ultrasound speckle to marbling scores,11 the Colorado State study demonstrates flaws in certain processes.12
As with most new technology, businesses arise to capitalize on advancements. Custom sonographers typically carry from $25,000 to $30,000 worth of equipment to perform these services. Most groups charge a per head fee in addition to reimbursement of travel expenses. Fees are based on the number of animals to be scanned. Per head charges range from $4.00/head to $15.00/head, depending on the species, type of measurements made, and number of animals scanned.
As with most services, quality of the information received varies with the experience and ability of the technician. National organizations publish lists of certified technicians that have met minimum requirements for certification, and may be able to provide some additional information on available technicians.
Summary
The use of real-time ultrasound has had a considerable impact in reducing excess fat in swine herds as major breeding companies have employed the technology in their selection process. Similar advances in total lean production in beef cattle, due to the use of ultrasonic measurements or any other selection tool, have not materialized. Many breeders of both cattle and swine have used ultrasonic information for promotional purposes, with a poor understanding of measurement accuracy or implication for selection decisions. Without validated research on the ramifications of selection, valuable use of this information is limited.
When one considers the structural changes between the live animal and the hanging carcass, combined with the margin of error in ultrasonic measurements, discrepancies between the two measurement techniques are expected. An experienced ultrasound technician can accurately rank contemporaries on carcass differences. However, the ability to differentiate between individuals on relatively minor muscle size differences is beyond the current capability of ultrasound technology.
To date, the accuracy of ultrasound speckle scores to estimate marbling in cattle or swine lacks adequate support in the scientific data. Some technologies have demonstrated reasonable accuracy while others failed. These differences should be considered prior to substantial expenditures by animal breeders. Further development in equipment and analysis techniques, substantiated by published data, is needed. Regardless of this predicament, ultrasound may still be the most accurate system available for evaluating the live animal for carcass merit.
References
Reviewed by:
Darwin G. Braund, Ph.D., Raymond W. Harvey, Ph.D., and Charles S. Stanislaw, Ph.D.,
Department of Animal Science, North Carolina State University.
Since June 1, 2000