Antimicrobial susceptibility, plasmid replicon typing, phylogenetic grouping, and virulence potential of avian pathogenic and faecal Escherichia coli isolated from meat chickens in Australia.

Globally, avian colibacillosis is a leading cause of morbidity and mortality in poultry, associated with economic losses and welfare problems. Here, clinical avian pathogenic E. coli isolates (CEC; n = 50) and faecal E. coli isolates from healthy (FEC; n = 187) Australian meat chickens collected between 2006 and 2014 were subjected to antimicrobial susceptibility testing, phylogenetic grouping, plasmid replicon (PR) typing, multilocus sequence typing, and virulence gene (VG) profiling. Extended-spectrum cephalosporin (ESC)- and fluoroquinolone (FQ)-resistant E. coli isolates underwent further genetic characterization. Significant proportions of CEC and FEC were, respectively, susceptible (13/50; 48/187) or MDR (9/50; 26/187) to 20 tested antimicrobials. Phylogenetic groups A and C, and PR types IncFIB and IncFrep were most represented. Five tested CEC-associated VGs were more prevalent in CEC (≥ 90%) than FEC (≤ 58%). Some isolates (CEC n = 3; FEC n = 7) were resistant to ESCs and/or FQs and possessed signature mutations in chromosomal FQ target genes and plasmid-mediated qnrS, blaCMY-2, and blaDHA-1 genes. Sequence type 354 (n = 4), associated with extraintestinal infections in a broad range of hosts, was prevalent among ESC- and/or FQ-resistant FEC. This study confirmed existence of a small reservoir of ESC- and FQ-resistant E. coli in Australian commercial meat chickens despite absence of use in the industry of these drugs. Otherwise, diversity of VGs and PR types in both FEC and CEC populations was identified. We hypothesize that the source of ESC- and FQ-resistant E. coli is external to poultry production facilities.RESEARCH HIGHLIGHTSLow-level resistance to older and newer generation antimicrobial drugs detected.The most common sequence type (ST) associated with FQ resistance was ST354 (4/10).A small proportion of CEC (n = 3) and FEC (n = 7) were resistant to ESCs and/or FQs.

Virulence-associated genes in faecal and clinical Escherichia coli isolates cultured from broiler chickens in Australia.

A healthy chicken's intestinal flora harbours a rich reservoir of Escherichia coli as part of the commensal microbiota. However, some strains, known as avian pathogenic E. coli (APEC), carry specific virulence genes (VGs) that enable them to invade and cause extraintestinal infections such as avian colibacillosis. Although several VG combinations have been identified, the pathogenic mechanisms associated with APEC are ill-defined. The current study screened a subset of 88 E. coli isolates selected from 237 pre-existing isolates obtained from commercial poultry flocks in Australia. The 88 isolates were selected based on their enterobacterial repetitive intergenic consensus (ERIC) and antimicrobial resistance (AMR) profiles and included 29 E. coli isolates cultured from chickens with colibacillosis (referred to as clinical E. coli or CEC) and 59 faecal E. coli (FEC) isolates cultured from clinically healthy chickens. The isolates were screened for the presence of 35 previously reported VGs. Of these, 34 were identified, with iucA not being detected. VGs focG, hlyA and sfa/foc were only detected in FEC isolates. Eight VGs had a prevalence of 90% or above in the CEC isolates. Specifically, astA (100%); feoB (96.6%); iutA, iss, ompT, iroN and hlyF (all 93.1%); and vat (89.7%). The prevalence of these were significantly lower in FEC isolates (astA 79.7%, feoB 77.9%, iutA 52.5%, iss 45.8%, ompT 50.9%, iroN 37.3%, hlyF 50.9% and vat 42.4%). The odds ratios that each of these eight VGs were more likely to be associated with CEC than FEC ranged from 7.8 to 21.9. These eight VGs may be used to better define APEC and diagnostically detect APEC in Australia. Further investigations are needed to identify the roles of these VGs in pathogenicity.

Risk factors for multidrug-resistant Escherichia coli rectal colonization of dogs on admission to a veterinary hospital.

This study aimed to identify risk factors for intestinal colonization with multidrug-resistant (MDR) E. coli in dogs on admission to a veterinary teaching hospital. Exposures to potential risk factors, including prior treatments, hospitalizations and interventions during the 42 days prior to admission were assessed for 82 case admissions and 82 time-matched controls in a retrospective prevalence-based case-control study of 20 months duration. On multivariable analyses, risk of MDR E. coli colonization on admission was increased with prior hospitalization for 4-7 days and >7 days relative to shorter periods, and in dogs that had prior diagnostic imaging techniques. Univariable analyses indicated that risk was increased following prior treatment with several antimicrobial agents. However, on multivariable analysis, administration of fluoroquinolones was associated with increased risk but risk did not appear to increase following administration of other antimicrobials. These results can inform management of canine patients and infection control procedures to mitigate the risk of clinical disease due to MDR bacteria in hospitalized dogs.

Risk factors for dogs becoming rectal carriers of multidrug-resistant Escherichia coli during hospitalization.

This study aimed to identify risk factors for dogs becoming rectal carriers of multidrug-resistant (MDR) Escherichia coli while hospitalized in a veterinary teaching hospital. Exposures to potential risk factors, including treatments, hospitalization, and interventions during a 42-day pre-admission period and hospitalization variables, were assessed for 90 cases and 93 controls in a retrospective, risk-based, case-control study. On multivariable analyses, hospitalization for >6 days [odds ratio (OR) 2·91-8·00], treatment with cephalosporins prior to admission (OR 5·04, 95% CI 1·25-20·27), treatment with cephalosporins for >1 day (OR 5·18, 95% CI 1·86-14·41), and treatment with metronidazole (OR 7·17, 95% CI 1·01-50·79) while hospitalized were associated with increased risk of rectal carriage of MDR E. coli during hospitalization. The majority of rectal isolates obtained during the study period conformed to MDR E. coli clonal groups previously obtained from extraintestinal infections. These results can assist the development of improved infection control guidelines for the management of dogs in veterinary hospitals to prevent the occurrence of nosocomial clinical infections.

The role of fruit bats in the transmission of pathogenic leptospires in Australia.

Although antileptospiral antibodies and leptospiral DNA have been detected in Australian fruit bats, the role of such bats as infectious hosts for the leptospires found in rodents and humans remains unconfirmed. A cohort-design, replicated survey was recently conducted in Far North Queensland, Australia, to determine if the abundance and leptospiral status of rodents were affected by association with colonies of fruit bats (Pteropus conspicillatus spp.) via rodent contact with potentially infectious fruit-bat urine. In each of four study areas, a 'colony site' that included a fruit-bat colony and the land within 1500 m of the colony was compared with a 'control site' that held no fruit-bat colonies and was >2000 m from the nearest edge of the colony site. Rodents were surveyed, for a total of 2400 trap-nights, over six sampling sessions between September 2007 and September 2008. A low abundance of rodents but a high carriage of leptospires in the rodents present were found to be associated with proximity to a fruit-bat colony. For example, means of 0·4 and 2·3 fawn-footed melomys (Melomys cervinipes) were collected/100 trap-nights at sites with and without fruit-bat colonies, respectively (P<0·001), but the corresponding prevalences of leptospiral carriage were 100% and 3·6% (P<0·001). Such trends were consistent across all of the sampling sessions but not across all of the sampling sites. Leptospires were not isolated from fruit bats by culture, and the role of such bats in the transmission of leptospires to rodents cannot be confirmed. The data collected do, however, indicate the existence of a potential pathway for transmission of leptospires from fruit bats to rodents, via rodent contact with infectious fruit-bat urine. Fruit bats may possibly be involved in the ecology of leptospires (including emergent serovars), as disseminators of pathogens to rodent populations. Stringent quantitative risk analysis of the present and similar data, to explore their implications in terms of disease prevalence and wildlife population dynamics, is recommended.

High-resolution melt-curve analysis of random amplified polymorphic DNA (RAPD-HRM) for the characterisation of pathogenic leptospires: intra-serovar divergence, inter-serovar convergence, and evidence of attenuation in Leptospira reference collections.

High-resolution melt-curve analysis of random amplified polymorphic DNA (RAPD-HRM) is a novel technology that has emerged as a possible method to characterise leptospires to serovar level. RAPD-HRM has recently been used to measure intra-serovar convergence between strains of the same serovar as well as inter-serovar divergence between strains of different serovars. The results indicate that intra-serovar heterogeneity and inter-serovar homogeneity may limit the application of RAPD-HRM in routine diagnostics. They also indicate that genetic attenuation of aged, high-passage-number isolates could undermine the use of RAPD-HRM or any other molecular technology. Such genetic attenuation may account for a general decrease seen in titres of rabbit hyperimmune antibodies over time. Before RAPD-HRM can be further advanced as a routine diagnostic tool, strains more representative of the wild-type serovars of a given region need to be identified. Further, RAPD-HRM analysis of reference strains indicates that the routine renewal of reference collections, with new isolates, may be needed to maintain the genetic integrity of the collections.

High-resolution melt-curve analysis of random-amplified-polymorphic-DNA markers, for the characterisation of pathogenic Leptospira.

A new test for pathogenic Leptospira isolates, based on RAPD-PCR and high-resolution melt (HRM) analysis (which measures the melting temperature of amplicons in real time, using a fluorescent DNA-binding dye), has recently been developed. A characteristic profile of the amplicons can be used to define serovars or detect genotypes. Ten serovars, of leptospires from the species Leptospira interrogans (serovars Australis, Robinsoni, Hardjo, Pomona, Zanoni, Copenhageni and Szwajizak), L. borgpetersenii (serovar Arborea), L. kirschneri (serovar Cynopteri) and L. weilii (serovar Celledoni), were typed against 13 previously published RAPD primers, using a real-time cycler (the Corbett Life Science RotorGene 6000) and the optimised reagents from a commercial kit (Quantace SensiMix). RAPD-HRM at specific temperatures generated defining amplicon melt profiles for each of the tested serovars. These profiles were evaluated as difference-curve graphs generated using the RotorGene software package, with a cut-off of at least 8 'U' (plus or minus). The results demonstrated that RAPD-HRM can be used to measure serovar diversity and establish identity, with a high degree of stability. The characterisation of Leptospira serotypes using a DNA-based methodology is now possible. As an objective and relatively inexpensive and rapid method of serovar identification, at least for cultured isolates, RAPD-HRM assays show convincing potential.

Introduction of new multidrug-resistant Salmonella enterica strains into commercial dairy herds.

A longitudinal observational study of 59 dairy herds was conducted in Washington State to estimate the rate of introduction of new multidrug-resistant (MDR) Salmonella enterica strains onto commercial dairy herds. Samples were collected on these herds over 7 visits separated by intervals of 2 to 4 mo over a period of 15 to 21 mo. Samples were cultured for Salmonella spp. and serogroup, serovar, and antimicrobial susceptibility patterns were identified for MDR Salmonella isolates. Fingerprinting generated by pulsed-field gel electrophoresis (PFGE) using XbaI restriction enzyme digestion generated genotyping profiles for all MDR isolates identified in the study. The rate of new MDR Salmonella strain introduction was 0.9 per herd-year (95% confidence interval: 0.6-1.4). The rates for the most commonly introduced MDR Salmonella serovars were 0.4/herd-year for Typhimurium, 1.2/herd-year for Newport, and 0.1/herd-year for Dublin. Thirty-three of 59 herds (56%) had at least one new MDR Salmonella introduction during the study period. The number of new MDR Salmonella strains acquired by dairy herds ranged from zero to 8. Thirteen of the 59 herds had a history of clinical salmonellosis. Among these 13 herds, 6 herds acquired new MDR Salmonella strains, although these strains were different than historical clinical strains. These data indicate that acquisition of new MDR Salmonella strains by dairy herds was a common event in participating herds, although the number of strains introduced varied greatly among herds.

The role of animal movement, including off-farm rearing of heifers, in the interherd transmission of multidrug-resistant Salmonella.

Fifty-nine commercial dairy farms were sampled 7 times over 15 to 21 mo to determine the role of animal movement, including off-farm rearing of heifers, in the interherd transmission of multidrug-resistant (MDR) Salmonella spp. Farm management data were collected by on-site inspections and questionnaires on herd management practices before and after the study. Forty-four percent (26/59) of herds did not acquire any new MDR Salmonella strains. The number of newly introduced MDR Salmonella strains acquired by the remaining 56% (33/59) of herds ranged from 1 to 8. Logistic regression models indicated that off-farm heifer raising, including contract heifer raising where heifers commingle with cattle from other farms [commingled heifers, odds ratio (OR) = 8.9, 95% confidence interval (CI): 2.4, 32.80], and herd size per 100-animal increment (herd size, OR = 1.04, 95% CI, 1.01, 1.05) were significantly associated with the introduction of new MDR Salmonella strains. The negative binomial regression similarly revealed that commingled heifers [relative risk (RR) = 2.3, 95% CI: 1.1, 4.7], herd size per 100 animals (RR = 1.02, 95% CI, 1.01, 1.03), and a history of clinical salmonellosis diagnosed before the study (RR = 2.5, 95% CI, 1.3, 5.0) were significantly associated with the number of new MDR Salmonella strains that were introduced. Factors not associated with the introduction of new MDR Salmonella strains were housing of heifers and cows in the same close-up pen, a common hospital-maternity pen, and the number of purchased cattle. This study highlights the role of animal movement in the interherd transmission of MDR Salmonella spp.

Associations between bovine, human, and raw milk, and beef isolates of non-O157 Shiga toxigenic Escherichia coli within a restricted geographic area of the United States.

A survey for Shiga toxigenic Escherichia coli in raw milk and beef was conducted within a defined geographic region of the United States. Prevalence rates based on detection of Shiga toxin gene (stx) were 36% for retail beef, 23% for beef carcasses, and 21% for raw milk samples, which were significantly higher than were Shiga toxigenic E. coli isolation rates of 7.5, 5.8, and 3.2%, respectively. Seasonal prevalence differences were significant for stx positivity among ground beef and milk samples. Distribution of stx subtypes among isolates varied according to sample type, with stx1 predominating in milk, stx2 on carcasses, and the combination of both stx1 and stx2 in beef. Ancillary virulence markers eae and ehx were evident in 23 and 15% of isolates, respectively. Pulsed-field gel electrophoresis demonstrated associations between food isolates and sympatric bovine fecal, and human clinical isolates. These data demonstrate that non-O157 Shiga toxigenic E. coli is present in the food chain in the Pacific Northwest, and its risk to health warrants critical assessment.

Multidrug-resistant E. coli and enterobacter extraintestinal infection in 37 dogs.

BACKGROUND: Extraintestinal infections caused by multidrug-resistant (MDR) Escherichia coli and Enterobacter are becoming more common in veterinary medicine. OBJECTIVE: To generate hypotheses for risk factors for dogs acquiring extraintestinal infection caused by MDR E. coli and Enterobacter, describe antimicrobial resistance profiles and analyze treatment and clinical outcomes. ANIMALS: Thirty-seven dogs diagnosed with extraintestinal infection caused by MDR E. coli and Enterobacter spp. between October 1999 and June 2006. METHODS: Retrospective case series assembled from hospital records data, including clinical history before 1st MDR isolation and treatment outcome. Identity and antimicrobial susceptibility profiles were confirmed by standard microbiological techniques for 57 isolates. RESULTS: Most dogs had an underlying disease condition (97%), received prior antimicrobial treatment (87%), were hospitalized for >or =3 days (82%), and had a surgical intervention (57%). The urinary tract was the most common infection site (62%), and urinary catheterization, bladder stasis, or both were common among dogs (24%). Some dogs were treated with high doses of co-amoxyclavulanate (n = 14) and subsequently recovered even though the isolates showed in vitro resistance to this antimicrobial. Other dogs were successfully treated with chloramphenicol (n = 11) and imipenem (n = 2). CONCLUSION AND CLINICAL IMPORTANCE: Predisposing disease condition, any prior antimicrobial use rather than a specific class of antimicrobial, duration of hospitalization, and type of surgical procedure might be risk factors for acquiring MDR extraintestinal infections. Whereas culture and sensitivity results can indicate use of last-resort antimicrobials such as imipenem for MDR infections, some affected dogs can recover after administration of high doses of co-amoxyclavulanate.

Survey of rabies vaccination status of Queensland veterinarians and veterinary students.

BACKGROUND: To determine the rabies vaccination status of Queensland veterinarians and veterinary students and their perception of zoonotic risk from Australian bat lyssavirus (ABLV). DESIGN: Cross-sectional questionnaire surveys. METHODS: Questionnaires were sent by post in 2011 to veterinary surgeons registered in Queensland, to final-year veterinary students at James Cook University via SurveyMonkey® in 2013 and to final-year veterinary students at James Cook University and University of Queensland via SurveyMonkey® in 2014. RESULTS: The response rate for registered veterinarians was 33.5% and for veterinary students 33.3% and 30% in 2013 and 2014, respectively. Of the 466 registered veterinary surgeons, 147 (31.5%) had been vaccinated, with 72 (15.5%) currently vaccinated. For veterinary students the rabies vaccination rate was 20.0% (4/20) and 13.0% (6/46) in the 2013 and 2014 surveys, respectively. More than 95% of veterinary students had received the mandatory Q fever vaccine. Both veterinarians and students regarded bats and horses as high-risk species for zoonoses. CONCLUSIONS: Queensland veterinarians and veterinary students have low levels of protection against ABLV. Although incidents of ABLV spilling over from a bat to a domestic mammal are likely to remain rare, they pose a significant human health and occupational risk given the outcome of infection in humans is high consequence. Principals of veterinary practices and veterinary authorities in Australia should implement a policy of rabies vaccination for clinical staff and veterinary students.

Phenotypic and genotypic profiling of antimicrobial resistance in enteric Escherichia coli communities isolated from finisher pigs in Australia.

OBJECTIVE: To assess herd-to-herd variation in antimicrobial resistance phenotypes and associated antimicrobial resistance genes (ARGs) in faecal commensal Escherichia coli communities isolated from Australian slaughter-age pigs. METHODS: Hydrophobic grid-membrane filtration (HGMF) was used to screen populations of E. coli isolated from faecal samples obtained from pigs prior to or at slaughter. Multiplex PCRs were applied to the pooled DNA extracted from the samples to identify specific ARGs. METHODS: Pooled faecal samples from 30 finishers, from 72 different Australian pig farms, produced 5003 isolates for screening. HGMF techniques and image analysis were used to confirm E. coli resistance phenotypes to four antimicrobial agents (ampicillin, gentamicin, florfenicol and ceftiofur) using selective agars. Multiplex PCRs were performed on DNA from pooled samples for 35 ARGs associated with seven chemical classes. RESULTS: The prevalence of E. coli isolates showing no resistance to any of the drugs was 50.2% (95% confidence interval (CI) 41.8-58.6%). Ceftiofur resistance was very low (1.8%; CI 0.8-3.9%) and no ARGs associated with 3rd-generation cephalosporin resistance were detected. By contrast, ampicillin (29.4%, CI 22.8-37.0%), florfenicol (24.3%, CI 17.8-32.3%) and gentamicin (CI 17.5%, 10.7-27.2%) resistance prevalence varied greatly between farms and associated ARGs were common. The most common combined resistance phenotype was ampicillin-florfenicol. CONCLUSION: The use of registered antimicrobials in Australian pigs leads to the enteric commensal populations acquiring associated ARGs. However, despite a high intensity of sampling, ARGs imparting resistance to the critically important 3rd-generation cephalosporins were not detected.

Errors and artifacts of Doppler flowmeters and their solution.

Continuous-wave Doppler ultrasound flowmeters are essential instruments for the vascular surgeon. Unfortunately, when used for quantitative purposes, they yield a flow velocity waveform that is substantially in error. Current directional continuous-wave Doppler velocimeters used a O-crossing detector to convert the Doppler signal to a waveform. Unfortunately, there are several inherent problems with this technique, and velocity waveforms are produced that have significant artifacts or errors. In this report, it is shown that a multifilter system is a simple, inexpensive alternative to systems using a O-crossing detector, and furthermore, permits real-time display of the Doppler waveform. The severity of the underlying arterial disease can be quantified by the calculation of pulsatility index. The pulsatility index was calculated from the Doppler waveforms recorded from peripheral arteries in 155 limbs and showed an excellent correlation with the severity of atherosclerosis as assessed by arteriography.

A longitudinal study of Shiga-toxigenic Escherichia coli (STEC) prevalence in three Australian diary herds.

Over a 12 month period, 588 cattle faecal samples and 147 farm environmental samples from three dairy farms in southeast Queensland were examined for the presence of Shiga-toxigenic Escherichia coli (STEC). Samples were screened for Shiga toxin gene (stx) using PCR. Samples positive for stx were filtered onto hydrophobic grid membrane filters and STEC identified and isolated using colony hybridisation with a stx-specific DNA probe. Serotyping was performed to identify serogroups commonly associated with human infection or enterohaemorrhagic Escherichia coli (EHEC). Shiga-toxigenic Escherichia coli were isolated from 16.7% of cattle faecal samples and 4.1% of environmental samples. Of cattle STEC isolates, 10.2% serotyped as E. coli O26:H11 and 11.2% serotyped as E. coli O157:H7, and the E. coli O26:H11 and E. coli O157:H7 prevalences in the cattle samples were 1.7 and 1.9%, respectively. Prevalences for STEC and EHEC in dairy cattle faeces were similar to those derived in surveys within the northern and southern hemispheres. Calves at weaning were identified as the cattle group most likely to be shedding STEC, E. coli O26 or E. coli O157. In concurrence with previous studies, it appears that cattle, and in particular 1-14-week-old weanling calves, are the primary reservoir for STEC and EHEC on the dairy farm.

Characterisation and clonal relationships of Shiga-toxigenic Escherichia coli (STEC) isolated from Australian dairy cattle.

A total of 136 Shiga toxin-producing Escherichia coli (STEC) isolated during a longitudinal survey of three Australian dairy farms were examined to determine their virulence factors, serotype and genomic relationships. This study aimed to assess the potential of these STEC to cause disease in humans and to analyse the on-farm ecology of STEC. Virulence factors (stx, eae, ehxA) were used as determinants of potential to be enterohaemorrhagic E. coli (EHEC) and were examined using polymerase chain reaction (PCR). Among the cattle groups tested, calves, both before and during weaning, shed the most putative EHEC and were the main source of serotypes commonly associated with human disease. E. coli O157:H7 and E. coli O26:H11 represented 9.4 and 7.8% of cattle STEC isolates respectively, with other putative EHEC serotypes reported for the first time from cattle. Based on serotype and virulence factors, 20% of STEC were putative EHEC. Pulsed-field gel electrophoresis (PFGE) was used to compare the genomic profiles of STEC from dairy farms. Isolates common to cattle and the farm environment were identified. Multiple strains of STEC with high clonal turnover were detected in the faeces of cattle, and isolates appeared to be specific to individual farms. To fully assess the pre-slaughter EHEC risk factors on-farm, examination of STEC virulence is as important as determination of STEC prevalence.

A comparative study and assessment of Doppler ultrasound spectral estimation techniques. Part I: Estimation methods.

When compared to the classical Discrete Fourier Transform (DFT) or Fast Fourier Transform (FFT) approach, modern estimation methods offer the potential for achieving significant improvements in estimating the power density spectrum of Doppler ultrasound signals. Such improvements, for example, might enable minor flow disturbances to be detected, thereby improving the sensitivity in arterial disease assessment. Specifically, reduction in the variance and bias can be achieved, and this may enable disturbed flow to be detected in a more sensitive manner. The approach taken here, is to consider spectral estimation methods as a problem of fitting an assumed model to the Doppler signal. The models described assume that the signal is stationary. Since the Doppler signal is generally nonstationary, it is assumed that a short enough time window interval can be chosen over which the signal can be considered stationary. We shall review the various methods and when appropriate, relate them to the nature of the Doppler signal.

On the relation between aggregation, packing and the backscattered ultrasound signal for whole blood.

Previous studies have shown that the backscattered ultrasound (US) power from blood depends on the manner in which red blood cells (RBCs) are packed and, in particular, on spatial variations in the red blood cell number density (i.e., the RBC concentration variance). Experimental measurements have also shown that the backscattered US power depends on the degree of RBC aggregation, and it has been hypothesized that this is primarily due to the effect of RBC aggregation on the concentration variance. An initial simulation study of the relationship between RBC aggregation and packing statistics is presented, in which the effects of hematocrit, aggregate size, shape and size distribution on concentration variance are investigated. Both two-dimensional (2-D) and 3-D samples of aggregated and disaggregated RBCs were simulated; these enabled the concentration variance to be calculated. In agreement with theoretical predictions and experimental US results, the concentration variance for disaggregated RBCs is shown to be lowest at low and high hematocrits, and to peak at intermediate hematocrits. The concentration variance is shown to be particularly sensitive to changes in aggregate size and size distribution, and less sensitive to the shape of small aggregates. The results of this study provide a foundation for relating the state of aggregation in a blood sample to the manner in which RBCs are packed and, therefore, to the backscattered US power.

Effects of transducer beam geometry and flow velocity profile on the Doppler power spectrum: a theoretical study.

A theoretical model is used to show how the Doppler spectrum for various axisymmetric velocity profiles is affected by beam misalignment and incomplete insonation. Results are presented for both circular and square beam geometries. Moreover, a closed-form expression is derived for the power spectral density received by an on-axis transducer with a Gaussian beam profile. It is shown that the error incurred in measuring the mean Doppler frequency with such a profile will generally be bounded by the results for the circular and square beam geometries. The effects of an ideal high-pass filter on the mean Doppler frequency and the backscattered Doppler power are examined. It is shown that such a filter can introduce large differences in the measured systolic to diastolic power ratios. Finally, theoretical expressions and results are presented for the spectral broadening index (SBI), normalized spectral variance (NSV), coefficient of kurtosis (CK), the coefficient of skewness (CS) as functions of the axisymmetric velocity profile shape assuming complete uniform insonation.

Quantitative study of steady flow using color Doppler ultrasound.

The use of color Doppler flow mapping systems for quantitative in vitro studies of flow fields is examined and illustrated. A 5-MHz color Doppler system was used, and the resolution was determined by comparing the results of flow-field measurement for steady parabolic pipe flow with calculated values. The velocity accuracy was about 6% of the velocity corresponding to half the pulse repetition frequency, and the spatial resolution was better than 1 mm. Frame frequency limitations permitted only partial tracking of fast temporal changes in the flow field. However, detection of vortices downstream from a small cylinder placed in the flow tube was significantly enhanced by synchronizing the frame frequency with the vortex shedding frequency and using a velocity-variance mode. Color Doppler aliasing was found to be useful to define streamlines and determine whether the flow was laminar or turbulent. The color Doppler system clearly imaged Poiseuille, transitional and turbulent flow and vortex shedding in vitro. It is concluded that color Doppler ultrasound flow mapping can enable large, complex flow fields to be quantitatively studied in vitro.