An in vivo rat model simulating imaging of human kidney by diagnostic ultrasound with gas-body contrast agent

Contrast Agent-Free 3D Renal Ultrafast Doppler Imaging Reveals Vascular Dysfunction in Acute and Diabetic Kidney Diseases

To combat the irreversible decline in renal function associated with kidney disease, it is essential to establish non-invasive biomarkers for assessing renal microcirculation. However, the limited resolution and/or vascular sensitivity of existing diagnostic imaging techniques hinders the visualization of complex cortical vessels. Here, a 3D renal ultrafast Doppler (UFD) imaging system that uses a high ultrasound frequency (18 MHz) and ultrahigh frame rate (1 KHz per slice) to scan the entire volume of a rat's kidney in vivo is demonstrated. The system, which can visualize the full 3D renal vascular branching pyramid at a resolution of 167 µm without any contrast agent, is used to chronically and noninvasively monitor kidneys with acute kidney injury (AKI, 3 days) and diabetic kidney disease (DKD, 8 weeks). Multiparametric UFD analyses (e.g., vessel volume occupancy (VVO), fractional moving blood volume (FMBV), vessel number density (VND), and vessel tortuosity (VT)) describe rapid vascular rarefaction from AKI and long-term vascular degeneration from DKD, while the renal pathogeneses are validated by in vitro blood serum testing and stained histopathology. This work demonstrates the potential of 3D renal UFD to offer valuable insights into assessing kidney perfusion levels for future research in diabetes and kidney transplantation.

Assessing Polycystic Kidney Disease in Rodents: Comparison of Robotic 3D Ultrasound and Magnetic Resonance Imaging

Polycystic kidney disease (PKD) is an inherited disorder characterized by renal cyst formation and enlargement of the kidney. PKD severity can be staged noninvasively by measuring total kidney volume (TKV), a promising biomarker that has recently received regulatory qualification. In preclinical mouse models, where the disease is studied and potential therapeutics are evaluated, the most popular noninvasive method of measuring TKV is magnetic resonance imaging (MRI). Although MRI provides excellent 3D resolution and contrast, these systems are expensive to operate, have long acquisition times, and, consequently, are not heavily used in preclinical PKD research. In this study, a new imaging instrument, based on robotic ultrasound (US), was evaluated as a complementary approach for assessing PKD in rodent models. The objective was to determine the extent to which TKV measurements on the robotic US scanner correlated with both in vivo and ex vivo reference standards (MRI and Vernier calipers, respectively). A cross-sectional study design was implemented that included both PKD-affected mice and healthy wild types, spanning sex and age for a wide range of kidney volumes. It was found that US-derived TKV measurements and kidney lengths were strongly associated with both in vivo MRI and ex vivo Vernier caliper measurements (R ²=0.94 and 0.90, respectively). In addition to measuring TKV, renal vascular density was assessed using acoustic angiography (AA), a novel contrast-enhanced US methodology. AA image intensity, indicative of volumetric vascularity, was seen to have a strong negative correlation with TKV (R ²=0.82), suggesting impaired renal vascular function in mice with larger kidneys. These studies demonstrate that robotic US can provide a rapid and accurate approach for noninvasively evaluating PKD in rodent models.

Histological and blood chemistry examination of the rodent kidney after exposure to flash-replenishment ultrasound contrast imaging

The purpose of this work is to investigate whether imaging sequences of flash-replenishment contrast enhanced ultrasound (CEUS) of the kidney result in chronic or acute bioeffects. Kidneys of female Fischer 344 rats were imaged using the flash-replenishment technique. Animals were separated into four groups (N = 31). Imaging was conducted with a 4C1 probe, driven by an Acuson Sequoia system with Definity microbubbles as the ultrasound contrast agent. During the flash phase of the imaging sequence, one kidney in each animal was exposed to either a mechanical index (MI) of 1.0 or 1.9. For each MI, half of the animals were sacrificed shortly after imaging (4 h) or after 2 weeks. A blinded veterinary nephropathologist reviewed the histopathology of both the imaged and control (non-imaged) kidney. Blood urea nitrogen (BUN) was measured for each animal prior to imaging and at the time of necropsy. Histopathology assessments in both the 1.0 and 1.9 MI groups revealed no signs of hemorrhage at either the 4-h or 2-week time point. BUN showed minor but statistically significant elevations in both the 1.0 and 1.9 MI groups, but no significant difference was present at the 2-week time point in the 1.0 MI group. All BUN levels (at both time points) remained in the normal range. In conclusion, CEUS with flash-replenishment imaging sequences did not result in kidney bioeffects observable with histology at early or late time points. Increases in BUN levels were observed after imaging, but were minimized when using a moderate MI (1.0).

Capillary Hemorrhage Induced by Contrast-Enhanced Diagnostic Ultrasound in Rat Intestine

Contrast-enhanced diagnostic ultrasound (CEDUS) can lead to microvascular injury and petechial hemorrhage by the cavitational mechanism of ultrasonic bioeffects. Capillary hemorrhage has been noted in the heart and kidney, which are common targets of CEDUS examination. CEDUS has also become useful for monitoring intestinal inflammation. In the 1990s, the risk of intestinal microvascular hemorrhage was investigated both for incidental exposure by lithotripter shockwaves and for contrast agent microbubbles acting as cavitation nuclei with laboratory pulsed ultrasound systems. This study was initiated to update the risk assessment for intestine exposed to diagnostic imaging simulating CEDUS. The abdomens of anesthetized rats were scanned by a 1.6 MHz phased array probe during infusion of microbubble suspensions simulating Definity ultrasound contrast agent. Dual image frames were triggered intermittently, and the output power was varied to assess the exposure response. Petechiae counts in small intestine mucosa and muscle layers increased with increasing trigger interval from 2 s to 10 s, indicative of a slow refill after microbubble destruction. The counts increased with increasing output above a threshold of 1.4 MPa peak rarefactional pressure amplitude. Petechiae were also seen in Peyer's patches, and occult blood was detected in many affected segments of intestine. These results are consistent with early laboratory pulsed-ultrasound results.

Hepatocyte Injury Induced by Contrast-Enhanced Diagnostic Ultrasound

OBJECTIVES

Contrast-enhanced diagnostic ultrasound (US) has a potential to induce localized biological effects. The potential for contrast-enhanced diagnostic US bioeffects in liver were researched, with guidance from a report by Yang et al (Ultrasonics 2012; 52:1065-1071).

METHODS

Contact and standoff scanning was performed for 10 minutes with a diagnostic US phased array at 1.6 MHz during bolus injection or infusion of a contrast agent at a high dose. The impact of the imaging on rat liver was investigated by measuring enzyme release, microvascular leakage, and staining of injured hepatocytes.

RESULTS

The results showed liver enzyme release at 30 minutes, indicating liver injury, and elevated extraction of Evans blue dye, indicating microvascular leakage. In addition, Evans blue and trypan blue vital-staining methods revealed scattered stained cells within the US scan plane. For the Evans blue method, fluorescent cell counts in frozen sections were greatest for standoff exposure with contrast infusion. The count decreased strongly with depth for bolus injection, which was probably reflective of the high attenuation noted for this agent delivery method.

CONCLUSIONS

The results qualitatively confirmed the report by Yang et al and additionally showed hepatocyte vital staining. Research is needed to determine the threshold for the effects and the contrast agent dose response.

The Dependence of Glomerular Capillary Hemorrhage Induced by Contrast Enhanced Diagnostic Ultrasound on Microbubble Diameter

A recently proposed two-criterion model for cavitational bioeffects in tissue with microbubbles (MBs) was tested. The glomerular capillary hemorrhage bioeffect was observed in rat kidney for contrast agent MB suspensions with mean diameters of 1.6, 3.1 and 5.5 µm. A diagnostic ultrasound machine was used at 3.6 MHz and 5.5 MHz for intermittent scans at power settings 2 dB apart. Petechial hemorrhage counts scored on the surface of the kidneys, and glomeruli were scored in histology. Thresholds for the petechial hemorrhage measurements were the same for the large and medium MB suspensions but substantially higher for the small MBs. For the histology, the medium MBs gave a higher threshold than the large MBs at 5.5 MHz. The pressure amplitude thresholds are in approximate agreement with theory, and the optimum MB size counterintuitively increased for increasing ultrasound frequency, as predicted. The two-criterion model of MB-associated capillary hemorrhage is supported.

In Vivo Assessment of the Potential for Renal Bio-Effects from the Vaporization of Perfluorocarbon Phase-Change Contrast Agents

Low-boiling-point perfluorocarbon phase-change contrast agents (PCCAs) provide an alternative to microbubble contrast agents. Although parameter ranges related to in vivo bio-effects of microbubbles are fairly well characterized, few studies have been done to evaluate the potential of bio-effects related to PCCAs. To bridge this gap, we present an assessment of biological effects (e.g., hemorrhage) related to acoustically excited PCCAs in the rodent kidney. The presence or absence of bio-effects was observed after sonication with various perfluorocarbon core PCCAs (decafluorobutane, octafluoropropane or a 1:1 mixture) and as a function of activation pulse mechanical index (MI; minimum activation threshold, which was a moderate MI of 0.81-1.35 vs. a clinical maximum of 1.9). Bio-effects on renal tissue were assessed through hematology and histology including measurement of blood creatinine levels and the quantity of red blood cell (RBC) casts present in hematoxylin and eosin-stained kidney tissue sections after sonication. Short-term (24 h) and long-term (2 and 4 wk) analyses were performed after treatment. Results indicated that bio-effects from PCCA vaporization were not observed at lower mechanical indices. At higher mechanical indices, bio-effects were observed at 24 h, although these were not observable 2 wk after treatment.

Influence of temperature, needle gauge and injection rate on the size distribution, concentration and acoustic responses of ultrasound contrast agents at high frequency

This paper investigated the influence of needle gauge (19G and 27G), injection rate (0.85ml·min(-1), 3ml·min(-1)) and temperature (room temperature (RT) and body temperature (BT)) on the mean diameter, concentration, acoustic attenuation, contrast to tissue ratio (CTR) and normalised subharmonic intensity (NSI) of three ultrasound contrast agents (UCAs): Definity, SonoVue and MicroMarker (untargeted). A broadband substitution technique was used to acquire the acoustic properties over the frequency range 17-31MHz with a preclinical ultrasound scanner Vevo770 (Visualsonics, Canada). Significant differences (P<0.001-P<0.05) between typical in vitro setting (19G needle, 3ml·min(-1) at RT) and typical in vivo setting (27G needle, 0.85ml·min(-1) at BT) were found for SonoVue and MicroMarker. Moreover we found that the mean volume-based diameter and concentration of both SonoVue and Definity reduced significantly when changing from typical in vitro to in vivo experimental set-ups, while those for MicroMarker did not significantly change. From our limited measurements of Definity, we found no significant change in attenuation, CTR and NSI with needle gauge. For SonoVue, all the measured acoustic properties (attenuation, CTR and NSI) reduced significantly when changing from typical in vitro to in vivo experimental conditions, while for MicroMarker, only the NSI reduced, with attenuation and CTR increasing significantly. These differences suggest that changes in physical compression and temperature are likely to alter the shell structure of the UCAs resulting in measureable and significant changes in the physical and high frequency acoustical properties of the contrast agents under typical in vitro and preclinical in vivo experimental conditions.

Frequency Dependence of Petechial Hemorrhage and Cardiomyocyte Injury Induced during Myocardial Contrast Echocardiography

Myocardial contrast echocardiography (MCE) for perfusion imaging can induce microscale bio-effects during intermittent high-Mechanical Index scans. The dependence of MCE-induced bio-effects on the ultrasonic frequency was examined in rats at 1.6, 2.5 and 3.5 MHz. Premature complexes were counted in the electrocardiogram, petechial hemorrhages with microvascular leakage on the heart surface were observed at the time of exposure, plasma troponin elevation was measured after 4 h and cardiomyocyte injury was detected at 24 h. Increasing response to exposure above an apparent threshold was observed for all endpoints at each frequency. The effects decreased with increasing ultrasonic frequency, and the thresholds increased. Linear regressions for frequency-dependent thresholds indicated coefficients and exponents of 0.6 and 1.07 for petechial hemorrhages, respectively, and 1.02 and 0.8 for cardiomyocyte death, compared with 1.9 and 0.5 (square root) for the guideline limit of the mechanical index. The results clarify the dependence of cardiac bio-effects on frequency, and should allow development of theoretical descriptions of the phenomena and improved safety guidance for MCE.

Do Anesthetic Techniques Influence the Threshold for Glomerular Capillary Hemorrhage Induced in Rats by Contrast-Enhanced Diagnostic Ultrasound?

OBJECTIVES

Glomerular capillary hemorrhage can be induced by ultrasonic cavitation during contrast-enhanced diagnostic ultrasound (US) exposure, an important nonthermal US bioeffect. Recent studies of pulmonary US exposure have shown that thresholds for another nonthermal bioeffect of US, pulmonary capillary hemorrhage, is strongly influenced by whether xylazine is included in the specific anesthetic technique. The objective of this study was to determine the influence of xylazine on contrast-enhanced diagnostic US-induced glomerular capillary hemorrhage.

METHODS

In this study, anesthesia with ketamine only was compared to ketamine plus xylazine for induction of glomerular capillary hemorrhage in rats by 1.6-MHz intermittent diagnostic US with a microsphere contrast agent (similar to Definity; Lantheus Medical Imaging, Inc, North Billerica, MA). Glomerular capillary hemorrhage was measured as a percentage of glomeruli with hemorrhage found in histologic sections for groups of rats scanned at different peak rarefactional pressure amplitudes.

RESULTS

There was a significant difference between the magnitude of the glomerular capillary hemorrhage between the anesthetics at 2.3 MPa, with 45.6% hemorrhage for ketamine only, increasing to 63.2% hemorrhage for ketamine plus xylazine (P < .001). However, the thresholds for the two anesthetic methods were virtually identical at 1.0 MPa, based on linear regression of the exposure response data.

CONCLUSIONS

Thresholds for contrast-enhanced diagnostic US-induced injury of the microvasculature appear to be minimally affected by anesthetic methods.

Effects and Mechanism of Combination of Rhein and Danshensu in the Treatment of Chronic Kidney Disease

Traditional Chinese medicine (TCM) plays a systemic role in disease treatment, targeting multiple etiological factors simultaneously. Based on clinical experience, rhubarb and Salvia miltiorrhiza are commonly prescribed together for the treatment of chronic kidney disease (CKD) and have been proven to be very effective. However, the rationale of the combination remains unclear. The major active ingredients of these two herbs are rhein (RH) and danshensu (DSS), respectively. The aim of this paper is to investigate the renoprotective effects of RH and DSS in vitro and in vivo, and the underlying mechanism. A total of 5/6 nephrectomy rats and HK-2 cells were subjected to chronic renal injury. The combination of RH and DSS conferred a protective effect, as shown by a significant improvement in the renal function, blood supply, and fibrotic degree. Proinflammatory cytokines and adhesion molecules were suppressed by RH and DSS through NK-κB signaling. The combination also inhibited apoptosis by up-regulating Bcl-2 and down-regulating Bax. Inhibiting the TGF-β/Smad3 pathway was at least in part involved in the antifibrotic mechanism of the combination treatment of RH and DSS. This study demonstrates for the first time the renoprotective effect and the mechanism of RH and DSS combination on chronic renal injury. It could provide experimental evidence to support the rationality of the combinatorial use of TCM in clinical practices.

Contrast-enhanced voiding urosonography with intravesical administration of a second-generation ultrasound contrast agent for diagnosis of vesicoureteral reflux: prospective evaluation of contrast safety in 1,010 children

BACKGROUND AND OBJECTIVE

To evaluate the safety of intravesical administration of a second-generation ultrasound (US) contrast agent for the diagnosis of vesicoureteral reflux in children.

MATERIALS AND METHODS

One thousand and ten children (563 girls, 447 boys; mean age: 2.9 years, range: 15 days-17.6 years) with 2,043 pelvi-ureter-units underwent contrast-enhanced voiding urosonography (ce-VUS) to rule out vesicoureteral reflux. A second-generation US contrast agent (SonoVue®, Bracco, Milan) was administered intravesically through transurethral bladder catheterization at a dose of 0.5 ml/bladder filling. Possible adverse events were monitored during the examination and followed up for 7 days after ce-VUS by phone calls. Urine analysis and culture were performed 3-5 days before ce-VUS in all children and 24-48 h in any patient who reported with adverse events.

RESULTS

No case of serious adverse event was recorded. Minor events were reported in 37 children (3.66% of the study population). These included dysuria (n = 26, 2.57%), urinary retention (n = 2, 0.2%), abdominal pain (n = 2, 0.2%), anxiety (n = 1, 0.1%) and crying (n = 1, 0.1%) during micturition, blood and mucous discharge (n = 1, 0.1%), increased frequency of micturition (n = 1, 0.1%), vomiting (n = 1, 0.1%), perineal irritation (n = 1, 0.1%), and an episode of urinary tract infection 10 days after ce-VUS (n = 1, 0.1%). Of these adverse events, 91.9% were subacute in onset and 8.1% were delayed. All events were self-limited and none required hospitalization.

CONCLUSION

There were no serious adverse events. Only a few minor events were reported, most likely due to the catheterization process. Thus, ce-VUS with intravesical administration of the second-generation US contrast agent (SonoVue®) for vesicoureteral reflux detection or exclusion had a favorable safety profile in our study group.

Renal interstitial permeability changes induced by microbubble-enhanced diagnostic ultrasound

Ultrasound-targeted microbubble (MB) destruction (UTMD) has been shown to increase the glomerular permeability, providing a potential novel therapeutic approach in targeted drug release for kidney diseases. Therefore, we investigated the impact of UTMD on renal interstitial permeability using MB-mediated diagnostic ultrasound (DUS). The left kidney of Sprague-Dawley (SD) rat was insonated by UTMD with either continuous or intermittent mode for 5 min. Evans blue (EB) revealed that both modes induced renal vascular permeability increase after DUS but recovered after 24 h. Intermittent insonation caused more severe injury than continuous mode. Red blood cells leaked out of the capillaries into interstitium without glomerular capillary hemorrhage (GCH) by hematoxylin and eosin (HE) staining. Electronic microscopy revealed the disruption of focal capillary wall in interstitial tissues. Morphological results confirmed capillary wall recovered in 24 h post-treatment. Results from fluorescence-labeled MBs showed that MBs were mainly localized in the interstitial portion of the tubular region and retained at 24 h. Intriguingly, urinalysis showed no clinical proteinuria after treatment. Our results indicated that MB plus DUS specifically and reversibly enhanced the interstitial permeability without affecting glomerulus, which may be developed into a therapeutic approach for targeting drug release to individual renal compartments.

Histological observation of islet hemorrhage induced by diagnostic ultrasound with contrast agent in rat pancreas

Contrast enhanced diagnostic ultrasound CEDUS has been shown to induce capillary hemorrhage in heart and kidney. This study characterized the capillary hemorrhage induced in rat pancreas. The pancreata of anesthetized hairless rats were accessed by laparotomy. A 1.5 MHz diagnostic ultrasound probe with 2.3 MPa peak rarefactional pressure amplitude and 1 s intermittent trigger was used to scan the pancreas, located at the focus (3.8 cm), through saline coupling. The probe was swept to expose the entire organ in 5 min during infusion of Definity® contrast agent at 10 µL/kg/min, and this was repeated in a reverse sweep. The entire pancreas was removed, spread flat for fixation and histological slides were prepared from the mid-plane. Slides were scored blind for islet hemorrhage over the entire area of the organ. Intra-islet microlesions were evident and hemorrhage surrounded many islets. The hemorrhage often impacted nearby acini, and expanded into inter-lobular septa. In CEDUS pancreata removed soon after scanning, 76.2±11.8% (n = 6) of islets had evidence of hemorrhage and/or islet microlesions compared to 1.1±2.5% (n = 5) for sham CEDUS (P<0.001). In pancreata removed after 4 hr, fibrin formation was detected by immunohistology in the hemorrhage and intra-islet microlesions. Diagnostic ultrasound with contrast agent induced substantial capillary hemorrhage in rat pancreas, concentrated particularly in the islets.

Targeted renal therapies through microbubbles and ultrasound

Microbubbles and ultrasound enhance the cellular uptake of drugs (including gene constructs) into the kidney. Microbubble induced modifications to the size selectivity of the filtration capacity of the kidney may enable drugs to enter previously inaccessible compartments of the kidney. So far, negative renal side-effects such as capillary bleeding have been reported only in rats, with no apparent damage in larger models such as pigs and rabbits. Although local delivery is accomplished by applying ultrasound only to the target area, efficient delivery using conventional microbubbles has depended on the combined injection of both drugs and microbubbles directly into the renal artery. Conjugation of antibodies to the shell of microbubbles allows for the specific accumulation of microbubbles in the target tissue after intravenous injection. This exciting approach opens new possibilities for both drug delivery and diagnostic ultrasound imaging in the kidney.

Contrast-enhanced diagnostic ultrasound causes renal tissue damage in a porcine model

OBJECTIVE

Glomerular capillary hemorrhage (GCH) has been reported and confirmed as a consequence of contrast-enhanced diagnostic ultrasound (CEDUS) imaging of rat kidney. This study assessed renal tissue injury in the larger porcine model.

METHODS

The right kidneys of anesthetized pigs were imaged in 8 groups of 4 pigs. A Vingmed System Five ultrasound machine (General Electric Co, Cincinnati, OH) was used at 1.5 MHz in the B-mode to intermittently scan the kidney at 4-second intervals. An Acuson Sequoia 512 machine (Siemens Medical Solutions, Mountain View, CA) was used in the 1.5-MHz Cadence contrast pulse sequencing mode with intermittent agent clearance bursts at 4-second intervals. Kidneys were scanned transabdominally or after laparotomy through a saline standoff. The second machine's probe was placed in contact with the kidney for 1 group. A perflutren lipid microsphere contrast agent (Definity; Lantheus Medical Imaging, Inc, North Billerica, MA) was infused at 4 μL/kg/min (diluted 33:1 in saline) for 4 minutes during scanning.

RESULTS

Blood-filled urinary tubules were evident on the kidney surface for all groups except the group with the probe in contact with the kidney. Glomerular capillary hemorrhage was found by histologic processing in 31.7% ± 9.8% (mean ± SD) of glomeruli in the center of the scan plane for 1.7-MPa transabdominal scanning and 1.5% ± 2.9% of glomeruli in sham samples (P < .05). In addition, hematuria was detected after scanning, and tubular obstruction occurred in some nephrons.

CONCLUSIONS

Renal tissue damage was induced by CEDUS in the porcine model. This result, together with previous studies in rats, support a hypothesis that GCH would occur in humans from similar CEDUS exposure.

Voiding urosonography with US contrast agent for the diagnosis of vesicoureteric reflux in children: an update

Voiding urosonography (VUS) entails the intravesical administration of US contrast agent (USCA) for the diagnosis of vesicoureteric reflux (VUR). VUS is now recognized as a practical, safe, radiation-free modality with comparable or higher sensitivity than direct radionuclide cystography (DRNC) and voiding cystourethrography (VCUG), respectively. An extensive review of the literature regarding both the procedural aspects and comparative diagnostic values of VUS has been published (Darge Pediatr Radiol 38:40-63, 2008a, b). The aim of this review is to provide an update on various facets of VUS that have taken place since the publication of the above-mentioned two reviews.

An ex vivo study of the correlation between acoustic emission and microvascular damage

The objective of this study was to conduct an ex vivo examination of correlation between acoustic emission and tissue damage. Intravital microscopy was employed in conjunction with ultrasound exposure in cremaster muscle of male Wistar rats. Definity microbubbles were administered intravenously through the tail vein (80microL.kg(-1).min(-1)infusion rate) with the aid of a syringe pump. For the pulse repetition frequency (PRF) study, exposures were performed at four locations of the cremaster at a PRF of 1000, 500, 100 and 10Hz (one location per PRF per rat). The 100-pulse exposures were implemented at a peak rarefactional pressure (P(r)) of 2MPa, frequency of 2.25MHz with 46 cycle pulses. For the pressure amplitude threshold study, 100-pulse exposures (46 cycle pulses) were conducted at various peak rarefactional pressures from 0.5MPa to 2MPa at a frequency of 2.25MHz and PRF of 100Hz. Photomicrographs were captured before and 2-min postexposure. On a pulse-to-pulse basis, the 10Hz acoustic emission was considerably higher and more sustained than those at other PRFs (1000, 500, and 100Hz) (p<0.05). Damage, measured as area of extravasation of red blood cells (RBCs), was also significantly higher at 10Hz PRF than at 1000, 500 and 100Hz (p<0.01). The correlation of acoustic emission to tissue damage showed a trend of increasing damage with increasing cumulative function of the relative integrated power spectrum (CRIPS; R(2)=0.75). No visible damage was present at P(r)< or =0.85MPa. Damage, however, was observed at P(r)> or =1.0MPa and it increased with increasing acoustic pressure.

In vivo gas body efficacy for glomerular capillary hemorrhage induced by diagnostic ultrasound in rats

Glomerular capillary hemorrhage (GCH) in rat kidney provided a model for assessing in vivo gas body efficacy in diagnostic or therapeutic applications of ultrasound. Two diagnostic ultrasound machines were utilized: one monitored the harmonic B-mode contrast enhancement of the left kidney and the other exposed the right kidney for GCH production. Definity contrast agent was infused at 1, 2, 5, or 10 microL/(kg x min) and infusion durations were 30, 60, 120, or 300 s. Exposure of the right kidney was at a peak rarefactional pressure amplitude of 2.3 MPa at 1.5 MHz. The circulating dose was estimated with a simple model of agent dilution and gas body loss. For 300 s infusion at 5 microL/(kg x min), the left kidney image brightness increased to a plateau with an estimated 6.4 +/- 1.3 microL/kg circulating dose with no GCH in histological sections. Exposure of the right kidney with a 1-s image interval reduced the estimated circulating dose to 1.3 +/- 0.3 microL/kg and induced 68.4% GCH. Dose and duration increases gave rapidly diminishing treatment effectiveness per gas body. The effective in vivo agent dose in rats can be reduced greatly due to high gas body destruction in the small animal, complicating predictions for similar conditions of human treatment.

Ultrasound bioeffects and safety

The main mechanisms by which ultrasound can induce biological effects as it passes through the body are thermal and mechanical in nature. The mechanical effects are primarily related to the presence of gas, whether drawn out of solution by the negative going ultrasound pressure wave (acoustic cavitation), a naturally occurring gas body (such as lung alveoli), or deliberately introduced into the blood stream to increase imaging contrast (microbubble contrast agents). Observed biological effects are discussed in the context of these mechanisms and their relevance to ultrasound safety is discussed.

Glomerular capillary hemorrhage induced in rats by diagnostic ultrasound with gas-body contrast agent produces intratubular obstruction

Glomerular capillary hemorrhage (GCH) induced by ultrasonic cavitation during diagnostic imaging represents a unique contrast agent-related nephron injury. Consequences of GCH during 1.5-MHz diagnostic ultrasound with contrast agent were examined by histologic methods in rats. Definity was infused at 10 microl/kg/min for 5 min at the start of 8 min of intermittent image-exposure, with 2.3 MPa in situ peak rarefactional pressure amplitude. Kidney samples were taken for histology at 5 min, 30 min, 4 h, 2 d, 1 week and 4 weeks post exposure. In addition, samples were taken at 4 h from groups treated with heparin or aminocaproic acid. GCH was found in 61% of glomeruli in the center of the scan plane 5 min after exposure, which declined (p < 0.05) to 36.3% after 4 h. The width of Bowman's space was significantly increased for glomeruli with GCH relative to glomeruli without GCH (p < 0.05), consistent with tubular obstruction. Antibody staining revealed fibrin clotting in Bowman's space in 4-h samples and this persisted in the 2-d samples. Heparin reduced and aminocaproic acid increased the GCH seen in 4-h samples. Tubular dilation was evident with injury to the epithelium after 2 d. After one week, areas of inflammatory cell infiltration were present. After four weeks, areas of interstitial fibrosis were revealed by Masson's trichrome stain. The consequences of GCH induced by diagnostic ultrasound with contrast agents include rupture of glomerular capillaries, procoagulant activity resulting in intratubular obstruction, and the potential for progression of the resulting tubular injury toward interstitial fibrosis.

In situ kidney insonation with microbubble contrast agents does not cause renal tissue damage in a porcine model

OBJECTIVE

Knowledge and quantification of the microcirculation are very important for estimating the status of an organ. Real-time contrast-enhanced sonography assesses microvascular tissue perfusion. This technique has been proposed as innocuous; however, data from experimental animals (rats) have shown renal interstitial microhemorrhage after the procedure. Therefore, we developed a porcine model to explore potential renal damage that in situ exposure might cause.

METHODS

Kidneys from 8 anesthetized pigs were surgically exposed. An ultrasound contrast agent (sulfur hexafluoride) was infused through the femoral vein in a continuous perfusion. Destructive ultrasonic flashes were applied with a high mechanical index over only 1 kidney (the contralateral kidney was used as a control). Blinded histologic and laboratory analyses were performed to reveal any lesions.

RESULTS

Histologic analysis of the kidney samples showed no evidence of renal damage. Biochemical parameters that could represent renal tissue damage and hemoglobin levels did not change after the microbubble-ultrasound interaction.

CONCLUSIONS

The ultrasound contrast agent-ultrasound interaction in anesthetized pig kidneys under the output level for the imaging visualization and microbubble destruction used did not cause tissue damage. Our results suggest that this procedure could be used in humans for regular analysis of the kidney microcirculation with minimal risk of tissue damage.

Frequency dependence of kidney injury induced by contrast-aided diagnostic ultrasound in rats

This study was performed to examine the frequency dependence of glomerular capillary hemorrhage (GCH) induced by contrast-aided diagnostic ultrasound (DUS) in rats. Diagnostic ultrasound scanners were used for exposure at 3.2, 5.0 and 7.4 MHz, and previously published data at 1.5 and 2.5 MHz was also included. A laboratory exposure system was used to simulate DUS exposure at 1.0, 1.5, 2.25, 3.5, 5.0 and 7.5 MHz, with higher peak rarefactional pressure amplitudes (PRPAs) than were available from our DUS systems. The right kidneys of rats mounted in a water bath were exposed to intermittent image pulse sequences at 1 s intervals during infusion of diluted ultrasound contrast agent. The percentage of GCH was zero for low PRPAs, and then rapidly increased with increasing PRPAs above an apparent threshold, p(t). The values of p(t) were approximately proportional to the ultrasound frequency, f, such that p(t) /f was approximately 0.5 MPa/MHz for DUS and 0.6 MPa/MHz for laboratory system exposures. The increasing thresholds with increasing frequency limited the GCH effect for contrast-aided DUS, and no GCH was seen for DUS at 5.0 or 7.4 MHz for the highest available PRPAs.

Needle size and injection rate impact microbubble contrast agent population

The most common type of ultrasound contrast agents are encapsulated microbubbles, typically 1 to 5 microns in diameter. These microbubbles are injected into the bloodstream to provide image enhancement during an ultrasound examination. Because of their compressibility, these microbubbles are inherently sensitive to changes in pressure. For imaging, this is beneficial in that these microbubbles oscillate in an acoustic field and allow imaging systems to detect their response uniquely from tissue. However, this sensitivity also means that microbubbles can be readily destroyed by significant hydrostatic pressure. Injection of these microbubbles through a small-gauge catheter, such as is sometimes performed in small animal imaging studies, can result in microbubble destruction. In this manuscript, the effects of microbubble injection through catheters of varying diameter are examined. Our results indicate that the concentration and size distribution of microbubbles can be substantially altered in cases of rapid injection through small-gauge needles.

Simulation of diagnostic ultrasound image pulse sequences in cavitation bioeffects research

Research on cavitational bioeffects of diagnostic ultrasound (DUS) typically involves a diagnostic scanner as the exposure source. However, this can limit the ranges of exposure parameters for experimentation. Anesthetized hairless rats were mounted in a water bath and their right kidneys were exposed to ultrasound. Amplitude modulation with Gaussian envelopes simulated the image pulse sequences (IPSs) produced by diagnostic scanning. A 10 mulkgmin IV dose of Definity((R)) contrast agent was given during 1-5 min exposures. Glomerular capillary hemorrhage was assessed by histology. A stationary exposure approximated the bioeffects induced by DUS within the beam area. However, the use of five closely spaced exposures more faithfully reproduced the total effect produced within a DUS scan plane. Single pulses delivered at 1 s intervals induced the same effect as the simulated DUS. Use of 100 ms triangle-wave modulations for ramp-up or ramp-down of the IPS gave no effect or a large effect, respectively. Finally, an air-backed transducer simulating DUS without contrast agent showed a zero effect even operating at twice the present DUS guideline upper limit. Relatively simple single-element laboratory exposure systems can simulate diagnostic ultrasound exposure and allow exploration of parameter ranges beyond those available on present clinical systems.

Doppler mode pulse sequences mitigate glomerular capillary hemorrhage in contrast-aided diagnostic ultrasound of rat kidney

Glomerular capillary hemorrhage (GCH) induced in rat kidney by diagnostic ultrasound involving contrast agent destruction was characterized for different modes to explore possible mitigation strategies. Anesthetized hairless rats were scanned at 2.5 MHz in a water bath with contrast agent infused at 10 microl/kg/minute via tail vein. B mode flash echo imaging (FEI), color Doppler (CD) FEI and realtime Doppler imaging at 1 frame per second were tested, which had image pulse sequences of approximately 0.53 ms, 15.8 ms, and 83.5 ms duration, respectively. Bioeffects endpoints included grossly observed blood-filled tubules, histological evaluation of GCH, and detection of hematuria. B mode FEI for 1 minute induced GCH in 38.6+/-17.1% of glomeruli in histology from the scan plane for a peak rarefactional pressure amplitude (RPA) of 2.6 MPa. The threshold for GCH was approximately 1.5 MPa, confirmed by 10-minute exposure with agent infusion. Paradoxically, CD mode FEI delivered many more pulses but produced less GCH (P < 0.02), and real-time Doppler mode induced only 5.3 +/- 3.8% (P < 0.005). Hematuria results followed the GCH trends. These findings indicate a promising strategy, which is to use relatively slow ramp-up of pulse RPAs in agent-destroying image pulse sequences, for mitigating potential bioeffects in contrastaided diagnostic ultrasound.

Nephron injury induced by diagnostic ultrasound imaging at high mechanical index with gas body contrast agent

The right kidney of anesthetized rats was imaged with intermittent diagnostic ultrasound (1.5 MHz; 1-s trigger interval) under exposure conditions simulating those encountered in human perfusion imaging. The rats were infused intravenously with 10 microL/kg/min Definity (Bristol-Myers Squibb Medical Imaging, Inc., N. Billerica, MA, USA) while being exposed to mechanical index (MI) values of up to 1.5 for 1 min. Suprathreshold MI values ruptured glomerular capillaries, resulting in blood filling Bowman's space and proximal convoluted tubules of many nephrons. The re-establishment of a pressure gradient after hemostasis caused the uninjured portions of the glomerular capillaries to resume the production of urinary filtrate, which washed some or all of the erythrocytes out of Bowman's space and cleared blood cells from some nephrons into urine within six hours. However, many of the injured nephrons remained plugged with tightly packed red cell casts 24 h after imaging and also showed degeneration of tubular epithelium, indicative of acute tubular necrosis. The additional damage caused by the extravasated blood amplified that caused by the original cavitating gas body. Human nephrons are virtually identical to those of the rat and so it is probable that similar glomerular capillary rupture followed by transient blockage and/or epithelial degeneration will occur after clinical exposures using similar high MI intermittent imaging with gas body contrast agents. The detection of blood in postimaging urine samples using standard hematuria tests would confirm whether or not clinical protocols need to be developed to avoid this potential for iatrogenic injury.