Complications of duplex carotid imaging. A case report

Although duplex carotid imaging is a safe diagnostic procedure, 3 neurologic and 2 cardiac adverse events have been associated with its performance in the past twenty-five years. This is a report of a man who had a transient cerebral ischemic attack during duplex imaging. An intraarterial digital subtraction angiogram showed 50% stenosis of the left internal carotid artery and 30% stenosis of the right internal carotid artery. Magnetic resonance imaging showed multiple small infarctions in both cerebral hemispheres and the brainstem. At left carotid endarterectomy, the atherosclerotic plaque removed had several small ulcers.

Is diagnostic ultrasound safe? Current international consensus on the thermal mechanism

OBJECTIVE

To describe the potential risk of heating during ultrasound examinations and to report the international consensus on the safety of ultrasound in medicine.

DATA SOURCES AND DATA EXTRACTION

Literature on the biological effects of hyperthermia and ultrasound.

CONCLUSION

The use of B-mode grey-scale imaging is not contra-indicated on thermal grounds. Some pulsed Doppler equipment has the potential to produce biologically significant temperature increases, specifically at interfaces between bone and soft tissue. Exposures resulting in temperatures less than 38.5 degrees C can be used without reservation.

Current status of research on biophysical effects of ultrasound

This overview of bioeffects of ultrasound presents some key aspects of selected papers dealing with biophysical end-points. Its purpose is to establish a basis for exposure and dosimetric standards for medical ultrasonic equipment. It is intended to provide essential background resource material for the medical/scientific community, and more specifically for scientific working groups. This document was prepared by members of the Safety Committee of the World Federation for Ultrasound in Medicine and Biology. It was produced as a resource document in response to a request for information by Working Group 12 (Ultrasound exposure parameters) of the International Electrotechnical Commission Technical Committee 87, Ultrasonics. IEC TC 87, WG12 is the working group responsible for generating international standards for the classification of equipment by its acoustic fields based on safety thresholds. Our paper is intended to update and supplement information on the thermal mechanism provided in the publication, "WFUMB Symposium on Safety and Standardisation in Medical Ultrasound: Issues and Recommendations Regarding Thermal Mechanisms for Biological Effects of Ultrasound" (WFUMB 1992). It also provides an overview of trends in research into nonthermal mechanisms as a preliminary to the next WFUMB Symposium on Safety of Medical Ultrasound when this subject will be examined in detail by a select group of international experts. The WFUMB-sponsored workshop will take place in Utsunomiya, Japan during 11-15th July, 1994. The purpose of the meeting is to evaluate the scientific literature and to formulate internationally accepted recommendations on the safe use of diagnostic ultrasound that may be endorsed as official policy of the WFUMB. It should be noted that the current publication is not intended for review or endorsement as an official WFUMB document. It is produced as a scientific paper by individuals who are members of the WFUMB Safety Committee, and it therefore represents the opinions of the authors. Nevertheless, during the preparation of this document, contributions were received from members of the International Electrotechnical Commission Technical Committee 87 as well as many other individual experts, and the authors sincerely acknowledge their support.

Ultrasound heating in a tissue-bone phantom

Temperature rise generated by focused ultrasound beams was tested on semipermanent tissue-bone phantoms. The phantoms were capped (sealed) plastic hollow cylindrical containers filled with tissue-mimicking material (TMM), in which were imbedded 25 microns diameter copper-constantan thermocouples (TC) and a piece of compact human or cow bone. The acoustic frequency specific attenuation coefficient of TMM was adjusted to be 0.3 dB cm-1 MHz-1 as specified by the FDA for a frequency range of 1-5 MHz. A high density 0.318 cm thick polyethylene sheet was chosen as the material to make caps of the phantoms. A formula developed to estimate the upper limit of temperature rises at tissue-bone interfaces generated by focused ultrasound has been proved to be appropriate experimentally using the semipermanent phantoms.

Ultrasound-induced lung hemorrhage in the monkey

Studies with the mouse have shown that lung hemorrhage can result from exposure to ultrasound at a peak pressure of approximately 1 MPa at 4 MHz (Mechanical Index [MI] approximately 0.5). In order to determine whether a comparable outcome could occur in a larger animal with characteristics similar to humans, studies were performed with monkeys using a clinical scanner under maximum output conditions (imaging + pulsed and color Doppler; derated pr of 3.7 MPa [4.5 MPa, measured in water], 4 MHz; MI approximately 1.8) (N = 57). Monkeys ranged in age from 1 day of life to 16 years with exposures limited to the right lung lobes (5 min cranial, 5 min caudal; N = 41 exposed, N = 12 sham-exposed controls, N = 4 colony controls). Results showed that animals ranging in age from 3 months to 5 years (mean age of 2.5 years) had a greater propensity for the occurrence of multiple well-demarcated circular hemorrhagic foci (0.1-1.0 cm), which were not observed in either control group. These lesions were characterized by marked congestion of alveolar capillaries with accumulation of red blood cells within the alveolar spaces, and were unassociated with major vessels or respiratory bronchioles. Further studies will be required in order to determine the relevance of these findings to the human, although it was concluded that ultrasound-induced lung hemorrhage in the monkey is of a significantly lesser degree when compared to the mouse.

Diagnostics of malignant lymphomas with ultrasound guided 1.2 mm biopsy-gun

In a retrospective analysis of 129 ultrasound-guided biopsy-gun biopsies (USGB) from patients with known or suspected malignant lymphoma, a histopathological diagnosis was obtained in 101 (78%) instances and no further procedures for histological verification were required. In the 28 cases with initially non-diagnostic results, 14 new USGBs were performed and a diagnosis was obtained in 11. Thus, a total success rate of 87% was achieved. The correct diagnosis was confirmed with either surgery, autopsy, or radiological or clinical follow-up (median 40 months). The diagnoses were categorised as Hodgkin's disease and high-grade or low-grade non-Hodgkin's lymphoma. Further subtyping of the lymphoma was possible in a few cases only. Immunohistochemistry was utilised only in a minor proportion of the cases (25/129), but refined the diagnosis in several instances. The biopsy-gun method was safe and minor adverse effects were seen in two patients only.

A test of the hypothesis that diagnostic ultrasound disrupts myelination in neonatal rats

Neonatal rats were exposed or sham exposed for 30 min to pulsed ultrasound [2.25 MHz carrier frequency, 1 microsecond pulse length, 50 Hz pulse repetition frequency (PRF), 50 W/cm2 Imax, 2 mW/cm2 ITA], euthanised and prepared for electron microscopic analysis of the nodes of Ranvier of the dorsal and ventral roots of the spinal cord. There was also a cage control. All materials were processed and scored blindly, evaluating whether perinodal myelin was normal. Rats from all regimens had areas of disrupted myelination. There was no statistically significant difference among the regimens for absence of myelination. The results did not confirm an earlier report that diagnostic ultrasound disrupts myelination in neonatal rats.

Ultrasonography as a tool for monitoring in ovo chicken development. 2. Effects of eggshell alteration and ultrasonography on embryonic and posthatch development

The influence of creating and closing acoustic windows on embryonic and posthatch development for the purpose of chicken embryogenesis monitoring by real-time ultrasonography (US) was evaluated at 2, 6, 9, 14, and 17 days of incubation. Acoustic windows were closed using either a porous dialysis film and tape (FM) or an eggshell allograft attached with collodion (CP). Results from eggs closed in each manner with and without concurrent nonsterile US examination were compared with two control groups. Window creation reduced hatchability. The hatchability reduction was caused primarily by bacterial contamination. Contamination was more common in the FM eggs than in the CP eggs and was greater in eggs that also underwent US. Hatchability increased and contamination decreased when US was performed closer to hatch. Egg weight loss was increased after Day 6 of incubation in FM + US eggs treated on Day 2 and after Day 9 in all eggs with windows except CP eggs treated on Day 9 and CP + US eggs treated on Days 6 and 9. Hatch weight decreased in chicks from eggs that had windows, particularly in FM eggs.

Experimental validation of predicted temperature rises in tissue-mimicking materials

Increasingly, it is recognised that diagnostic ultrasound is capable of causing temperature rises sufficient to damage tissue. Predictions of this heating are often based on simplified models of both the in vivo conditions and the relevant physical effects. Few measurements have been made to verify these predictions, however, particularly for the tightly focused beams often employed in diagnosis. Furthermore, non-linear effects in both the acoustic field and the surrounding medium have largely been ignored in calculations. To provide an alternative way to estimate the heating, NPL has developed a measurement system to determine directly the temperature rise in tissue-mimicking materials. If necessary, the measurement results can be processed to model the effects of blood perfusion. The temperature is measured using thin-film thermocouples, which have essentially no interaction with the ultrasound or thermal fields. Measurements were made on transducers operating in the frequency range 2-10 MHz and with focal beam-widths from 1 to 3 mm. The measurements agree with theoretical predictions that use either the measured beam-profile or a simplified (Gaussian) model. The results are also compared with the standard soft-tissue models developed by the National Council on Radiation Protection and Measurements (NCRP) and jointly between the American Institute of Ultrasound in Medicine and the National Electrical Manufacturers Association (AIUM/NEMA). On average, the predictions of the NCRP formula are 15% higher than the measurements, confirming its validity as a worst-case model. The predictions based on the AIUM/NEMA formula, however, are typically 30% lower than the measured values.

Effect of in utero exposure to diagnostic ultrasound on the postnatal survival and growth of mouse

Pregnant Swiss albino mice were exposed to 3.5 MHz diagnostic ultrasound for 10 min (upper limit for ISPTP = 1 W/cm2 and for ISATA = 240 mW/cm2, acoustic power = approximately 65 mW) on day 3.5 (preimplantation period), 6.5 (early organogenesis period), 11.5 (late organogenesis period), or 14.5 (early fetal period) of gestation. The offspring were observed for changes in litter size at birth, and sex ratio at 4 weeks of age, and postnatal mortality and growth retardation up to 6 weeks of age. No significant difference from control in litter size or sex ratio was observed in the offspring exposed to ultrasound on any of the gestation days studied. Exposure at the early organogenesis period produced a marginally significant increase in the postnatal mortality. A significant number of ultrasound exposed animals showed lesser body weights than the control group. However this growth retardation was transient and the normal growth pattern was restored by 6 weeks of age. It is concluded that the early stages in mouse gestation may be sensitive to the lethal effects of ultrasound than the later stages of pregnancy. Even though a small increase in the postnatal mortality was observed in the ultrasound exposed animals, the normal growth was not affected in the surviving animals except for a transient growth retardation. Low birth weight may be a characteristic effect of exposure only at the preimplantation period.

Lysis of erythrocytes by exposure to CW ultrasound

The threshold for lysis of erythrocytes suspended at concentrations of 0.5-1% in saline or plasma in rotating cylindrical exposure vessels is approximately spatial peak intensities of 2 W/cm2 at 1 MHz continuous wave (CW). Results of a series of experiments in which cell concentration, viscosity and gas composition of the suspending medium and rotation speed of the exposure vessel were varied combined with observations of sonoluminescence are all consistent with a hypothesis that cells are lysed by inertial (transient) acoustic cavitation. For the proposed mechanism to operate in cell suspensions, it is necessary that bubbles be brought into contact with the cells. Rotation of the chamber recycles bubbles that are driven by radiation forces to the far wall of the chamber in a matter of milliseconds. The physical and chemical properties of the wall of the chamber appear to be important as stabilizing sites for nuclei that serve as seeds for cavitation events.

Temperature elevation in focused Gaussian ultrasonic beams at various insonation times

Transient solution of the thermal conductivity equation for the three-dimensional case of the Gaussian ultrasonic focused beam was derived and applied for cases relevant to medical ultrasonography. Quantitative results for the case of a homogeneous medium with constant values of thermal coefficients and constant absorption as well as for the two-layer tissue model used in obstetrics were presented for various diagnostic probes used in ultrasonography. The possible effects of perfusion and nonlinear propagation were neglected. The results obtained are in agreement with results of other authors when considering the steady-state and the infinitely short insonation time. The computations show the influence of the insonation time on the temperature elevation, thus making it possible to introduce its value as a factor in limiting the possible harmful effects in ultrasonography. This has been shown in diagrams presenting the temperature distribution along the beam axis of 6 different diagnostic probes for various insonation times and demonstrating the corresponding temperature decrease when limiting the insonation time to 5 and 1 min. For instance, the highest temperature elevation (for probe number 1, see Table 1) decreases 2.6 and 5 times with respect to the steady-state temperature when the insonation time equals 5 and 1 min, respectively.

Ultrasonic gas body activation in Elodea leaves and the mechanical index

Membrane damage resulting from ultrasonic gas body activation was investigated in leaves of the aquatic plant Elodea using pulse-mode exposures from 0.745-15 MHz. The frequency response was similar to that previously observed for continuous exposures. Cell death thresholds were higher for the pulse modes; for example, at 6 MHz the threshold was 166 W/cm2 spatial-peak, pulse-average (SPPA) intensity for 1 microsecond pulses and 1 kHz pulse repetition frequency (PRF) for 60 s exposures, compared to 12 W/cm2 for 60 ms continuous exposure. Increasing the PRF for 2.5 MHz, 3 microseconds pulses from 10 Hz to 5 kHz resulted in a gradual decrease in the threshold. Increasing the pulse duration from 1 microsecond to 30 microseconds and PRF from 33 Hz to 1 kHz only weakly influenced the threshold, for the constant 60 ms total on-time. Decreasing the pulse duration for single 6 MHz pulses increased the threshold up to 1,540 W/cm2 at 30 microseconds. The gas body activity was modeled with linear theory for oscillation of the gas channel walls, and intracellular shear stress, which causes lethal damage to the plasma membranes, was modeled with theory for acoustic microstreaming. Theory and observation indicate that the minimum shear stress threshold as a function of resonance frequency has essentially the same form as the Mechanical Index (MI) in the 0.5 to 20 MHz range. Specifically, for 3 microseconds pulses with 0.2-2 kHz PRF and 60 ms total on-time, the pressure-amplitude threshold divided by the square root of the frequency was approximately constant at 0.95 MPa/MHz1/2.

Morphological effects of pulsed ultrasound in the lung

We have previously described the induction of subcapsular hemorrhage in the murine lung by extracorporeal shock wave lithotripsy at exposures of 2 MPa (Hartman et al. 1990) and pulsed ultrasound (Child et al. 1990). Since extravasation of erythrocytes and alveolar flooding are prominent, we proposed to determine whether or not the injury was progressive, by continuing to develop following termination of exposure, and by localizing where the injury was developing. Mice were exposed to 10 microsecond impulses at 1.6 MPa for 3 min and sacrificed either immediately or 5 min following exposure. When observed with both light and transmission electron microscopy, there was no gradation in lung injury, with a sharp demarcation of the hemorrhagic area. Moreover, both type I pneumocytes and capillary endothelial cells were injured, causing direct continuities between vessel lumina and alveolar spaces. In the absence of extravasation, the tissue appeared normal. There was no evidence that injury increased in severity during the first 5 min after exposure.

A comparison of hemolytic and sonochemical activity of ultrasonic cavitation in a rotating tube

Biological effects of in vitro ultrasonic exposure may result from mechanical and from sonochemical mechanisms related to ultrasonic cavitation. Mechanical cell lysis by 1.61 MHz ultrasonic cavitation was assessed in a rotating tube exposure system by hemolysis measurements. Free radical generation was assessed by the terephthalic acid dosimeter, calibrated by gamma-ray dosage. Sonochemical production was assessed by measuring residual hydrogen peroxide using the sensitive isoluminol method. Exposure conditions were similar for all tests, except longer durations were needed for the free radical and hydrogen peroxide tests. The sonochemical mechanisms were relatively more important for increasing intensity, and increasing temperature. Increasing rotation speed or bubbling with argon before exposure enhanced all cavitation activity. Stopping the rotation reduced all cavitation activity. Burst-mode ultrasound (10.5 microseconds bursts, 1:1 or 1:3 duty cycle) reduced cavitation activity, but gave relatively greater sonochemical activity under some conditions. These results indicate that the mechanical and sonochemical mechanisms can be separately favored to some extent by varying exposure conditions. The observed trends should be helpful for selecting exposure conditions favorable for studying bioeffects of the sonochemical mechanism.

Intracoronary ultrasound imaging: correlation of plaque morphology with angiography, clinical syndrome and procedural results in patients undergoing coronary angioplasty

OBJECTIVES

This study was designed to establish the relation between ultrasound-derived atheroma morphology and the clinical, procedural and angiographic features of patients presenting for coronary angioplasty.

BACKGROUND

Intracoronary ultrasound imaging provides accurate dimensional information regarding arterial lumen and wall structures. Atheroma composition may also be assessed by ultrasound; however, only limited studies have been performed in patients.

METHODS

In 65 patients a diagnostic ultrasound imaging catheter or a combination imaging-angioplasty balloon catheter was used during coronary angioplasty to image both the lesion and the vessel segment just proximal to it (reference segment). Ultrasound images were analyzed for lumen, total vessel and plaque areas and were classified into five morphologic subtypes (soft, fibrous, calcific, mixed plaque and concentric subintimal thickening). These data were compared with angiographic morphologic features, procedural results and clinical angina pattern (stable vs. unstable).

RESULTS

Morphologic analysis of the ultrasound images obtained from the lesion correlated well with the clinical angina syndrome. Compared with patients with stable angina, patients with unstable angina had more soft lesions (74% vs. 41%), fewer calcified and mixed plaques (fibrotic, soft or calcific components in one or more combinations [25% vs. 59%]) and fewer intralesional calcium deposits (16% vs. 45%) (all p < 0.01). There was no correlation between ultrasound and angiographic lesion morphologic characteristics for either the reference segment or the lesion. Ultrasound demonstrated greater sensitivity than angiography for identifying unstable lesions (74% vs. 40%). Dimensional analysis demonstrated a large plaque burden in the reference segments (45 +/- 15% of total vessel area). Postangioplasty plaque burden was also high (62 +/- 9%). There was a significant, but only fair correlation between lumen area determined by angiography and ultrasound for both the reference segment (r = 0.70, p < 0.001) and the postangioplasty lesion (r = 0.63, p < 0.05).

CONCLUSIONS

Morphologic plaque classification by ultrasound is closely correlated to clinical angina but has little relation to established angiographic morphologic characteristics. Intracoronary ultrasound imaging during angioplasty identifies a large residual plaque burden in both the reference segment and the lesion. In the future, determination of plaque composition by intracoronary ultrasound may be important in selecting or modifying interventional therapeutic options.

A comparison of ultrasound exposure in therapy and pulsed Doppler fields

A detailed comparison of the ultrasound exposure in water from a therapeutic beam and a pulsed Doppler beam was carried out. A significant overlap in acoustic power was found between therapy intensity levels used clinically and the upper end of the diagnostic range, between approximately 100 mW and 200 mW. In addition, pulse pressure amplitudes in the range 0.5-1.0 MPa were measured close to the transducer on both units. It is common to use physiotherapy equipment at pulse average intensities of 0.5 W/cm2 or less, and at these levels exposures of similar magnitude may be obtained with beams currently defined as therapeutic and those available from pulsed Doppler equipment.

Pulmonary embolism during compression US of the lower extremity

To the author's knowledge, occurrence of pulmonary embolism during performance of compression ultrasound (US) of the lower extremity has not previously been reported in the radiology literature. The author describes a case in which pulmonary embolism was witnessed during compression US of venous thrombosis in a superficial femoral vein of a 64-year-old man. The embolism was documented with perfusion lung scanning.