High Resolution Ultrasound

Facilities and Equipment

A Siemens Sonoline Elegra, running the 5.1 software, is available.  The scanner has the full array of transducers including the 13MHz 1.5D array. This transducer has a focused beam that produces a 1 mm slice thickness at the focal zone that can be moved from the near to the far field with an imaging window as small as 2 cm wide by 1.5 cm deep and a 150 micron axial resolution at the focal zone producing exquisite detail (Figures 7-9 below). The 2D frame-rate is sufficiently high to freeze respiratory and cardiac motion. Duplex and color Doppler as well as M-mode and 3D are all available on the 13MHz transducer. The detail and information content of images indicate that 13MHz imaging of mice can detect tumors in the liver or deep tissues. 

Service

Non-destructive, rapid, high spatial and contrast resolution imaging of live mice that could potentially be done in a sterile environment and without anesthesia, provides a powerful tool to monitor superficial and deep tumors. It is well known that subcutaneous tumors exhibit different vascular morphology without the influence of their native host tissue [1]. Ultrasound provides the ability to detect and monitor deep tumors analogous to physical examination of superficial tumors with minimal risk to the animal. Specific services include:

• Monitoring of in situ tumor growth
• Left ventricular volume measurement
• Imaging in a sterile environment
• Tumor viability and quantification of relative blood flow and blood volume with contrast media.

Structural In-Vivo Imaging of Live Mice

Ultrasound is a practical imaging tool that can image mice in real-time at high frame rates. It is ideally suited at imaging naturally occurring tumors and monitoring their growth (Figure 7) and because of the small size of mice can easily distinguish small intraperitoneal tumors from bowel (Figure 8). Because of the high resolution available at high frequency, it can image the liver of a mouse to detect metastases (Figure 9). Further, the availability of ultrasound contrast media allows the visualization of tumor vessels (Figure 8 & 9). While it is difficult to perform CT and MRI in a sterile environment, ultrasound imaging can be performed in a sterile fashion as is currently done with intra-operative ultrasound. The unit is draped with a sterile, transparent cover allowing access to the unit’s controls and the transducer is placed in a sterile sleeve preventing contamination of the scanning field.

The ultrasound images shown in Figures 7-9 were acquired while the animals were anesthetized. To minimize animal loss due to anesthesia, we will image mice in a sterile fashion but without anesthesia, when no intravenous administration is required. Since nude mice do not require shaving and since ultrasound acquires images in real-time, contact with skin and motion will not hinder the ability to survey the animal’s abdomen. This can be accomplished by having an assistant immobilize the animal by holding it with the dorsal skin, as is done during intraperitoneal injections, by one hand and holding the hind legs to slightly stretch the abdomen by the other. The sonographer can then quickly scan the abdomen to survey for the presence and size of tumors. A non-nude mouse can be anesthetized with Isofluorane, a quick acting anesthetic, to allow for shaving, and then it can be imaged while non-anesthetized. While it can be argued that stress could affect the animals’ physiology, anesthesia has more depressing effects on mice. Nonetheless, imaging without anesthesia is not intended to image physiology that requires contrast administration, rather, it is intended to detect and monitor the anatomy of tumors analogous to a physical exam.

Images acquired using the Matrigel angiogenesis model described were exquisite (Figure 10). They allowed the visualization of the angiogenic vascular plug before ultrasound contrast was administered because of the heterogeneous architecture where vessels developed in the bFGF-impregnated homogeneous Matrigel. When contrast was administered, dramatic enhancement of the perfused vessels occurred (arrows in Figure 10).

Sterile Imaging will be performed by first wiping down the unit with disinfecting solution and wheeling it into the treatment room. Sterile Gel will be placed between the transducer surface and the sterile sleeve covering it and its cable. The unit will be covered with a sterile plastic transparent drape to allow access to its controls. The sonographer will gown and glove according to protocol. For standard monitoring of tumor size, nude mice will be scanned without anesthesia as described above using a small amount of sterile warm gel for contact. When a tumor is detected, images will be acquired with internal landmarks to allow session to session or post mortem confirmation, and tumor dimensions will be measured and images stored digitally.

Functional In-Vivo Imaging of Live Nude Mice

Relative blood flow and fractional blood volume indices can be assessed with ultrasound contrast intermittent imaging [2 - [iii][iv]5]. Further, we also showed that fractional blood volume, determined by intermittent imaging following long interscan delay using a Vx2 tumor model, correlates with microvascular density, an index of angiogenesis. Therefore, it is possible to determine tumor vascularity and angiogenesis and possibly the effect of anti-angiogenic therapy. Since imaging sessions can be performed repeatedly, particularly if done with quick anesthesia to place an IV line, tumor response to interventions such as an anti-angiogenesis therapy can be monitored as frequently and for as long as desired for longitudinal experiments. Assessing relative blood flow using the destruction model [6] rather than the destruction reperfusion model requires only 2 or 3 seconds without animal motion as compared to 15 sec.