Small Vessel Detection Diameter (mm) Number vpeak (cm/s) Doppler shift (Hz) Aorta 10 1 100 5,620 Large Arteries 3 40 18 1,010 Main branches 600 4.5 250 Terminal branches 0.6 1,800 1.5 84 Arterioles 0.02 40,000,000 0.18 12 Capillaries 0.008 1,200,000,000 0.08

Microvascular Assessment: Challenges Low Velocities (velocity resolution and ‘clutter’ issues) Low Volume (small signals) Low Flow rate Small sizes (spatial resolution) (optional) Intravital microscopy of rat cremaster muscle C Ellis U of Western Ontario 50 microns

Microvascular Challenges: Low Velocities clutter removal issues velocity resolution issues

Low Velocities: Clutter removal • Kidney tissue velocity: ~ 3 cm/s (30 x capillary velocity) • Myocardium velocity: ~ 15 cm/s (150 x capillary velocity)

Microvascular Challenges: Small signals hematocrit decreases with vessel size, results in decreased signal strength

Microvascular Challenges: Spatial Resolution

Microvascular Assessment: Higher Frequencies - Improves blood signals - Improves velocity resolution - Improves spatial resolution

Microvascular Assessment: Higher Frequencies Problem: Attenuation Clutter

Microbubble Contrast Agents Encapsulated gas microbubbles (e.g. lipids, albumin, polymers) ~ 2-8 m in diameter Injected intraveneously into the bloodstream The illustration at the lower right corner of the scrreen is a cross section of a thin slice of the myocardium. It is composed of a complex network of tiny capillaries which carry the blood supply to the myocardium. They are so small that they are beyond the limits of Doppler to resolve. The same complex network of capillaries is also seen in organs such as the renal cortex and various lesions. Since contrast agent microbubbles are the same size as red blood cells (approx. 3-4 microns) they can easily pass through the circulation system and allow us to image them with advanced techniques such as harmonic imaging. To understand how contrast agents work, imagine Ella Fitzgerald singing and a glass cracking because of the pitch and frequency of her voice. Microbubbles have similar resonancy properties. They are used as reflectors and intensify acoustic properties during ultrasound. Bubbles are sized smaller than a blood cell so they can travel with the blood cells when injected intravenously. As they enter the site of interest where the ultrasound beam is directed, their affect is to increase the echo level from blood. In fact, a single bubble gives an echo about a million times stronger than a red blood cell. You only need a small number of bubbles to get an increase in signal. These bubbles are not toxic, and have a high level of patient acceptance.

Scattering from Bubbles Bubbles: highly compressible, low density relative to plasma

Microbubble Contrast Agents Wavelength of 3 MHz = 0.5 mm ( = v/f, v= 1500 m/s ) Bubble size is 0.003 mm Bubble Radius Time Pressure The illustration at the lower right corner of the scrreen is a cross section of a thin slice of the myocardium. It is composed of a complex network of tiny capillaries which carry the blood supply to the myocardium. They are so small that they are beyond the limits of Doppler to resolve. The same complex network of capillaries is also seen in organs such as the renal cortex and various lesions. Since contrast agent microbubbles are the same size as red blood cells (approx. 3-4 microns) they can easily pass through the circulation system and allow us to image them with advanced techniques such as harmonic imaging. To understand how contrast agents work, imagine Ella Fitzgerald singing and a glass cracking because of the pitch and frequency of her voice. Microbubbles have similar resonancy properties. They are used as reflectors and intensify acoustic properties during ultrasound. Bubbles are sized smaller than a blood cell so they can travel with the blood cells when injected intravenously. As they enter the site of interest where the ultrasound beam is directed, their affect is to increase the echo level from blood. In fact, a single bubble gives an echo about a million times stronger than a red blood cell. You only need a small number of bubbles to get an increase in signal. These bubbles are not toxic, and have a high level of patient acceptance.

Mass - Spring System A mass on a spring has a resonant frequency determined by its spring constant k and the mass m. The resonant oscillating frequency (natural) is:

Free oscillating bubble: Analogy With a bubble, the effective mass is provided by the surrounding liquid, and the spring is due to the gas compressibility. For a ‘free’ bubble the resonant frequency is… (the ‘Minnaert’ frequency) - Assuming adiabatic condition - Surface tension neglected For an air bubble in water

Acoustically Driven Bubbles Three regimes can be considered -Linear (lower pressure) -Non-linear (intermediate pressure) -Destruction (higher pressure) Bubble Radius Time

Linear regime Soft-shelled agent 4 micron bubble US: f=1 MHz, 70 kPa

Linear regime US: f=1 MHz, MI=0.05 Diameter vs Time Frequency content 2 4 6 8 10 12 6.7 6.8 6.9 7 7.1 7.2 7.3 7.4 7.5 7.6 7.7 1 2 3 4 -50 -40 -30 -20 -10 Diameter [mm] dB Time Frequency [MHz]

Nonlinear regime US: f=1 MHz, 200 kPa Soft-shelled agent 4 micron bubble

Nonlinear regime US: f=1 MHz, 200 kPa Diameter vs Time Frequency content 2.5 3 3.5 4 4.5 5 -10 -20 Diameter [mm] dB -30 -40 -50 10 20 30 40 50 60 70 80 1 2 3 Time Frequency [MHz]

Bubble Destruction US: f=1.7MHz, 1.3 MPa hard-shelled agent 3 micron bubble Hard-shelled agent

Microbubble Imaging Methods Examine the kinetics of Bscan enhancement (earliest approach) Detect ‘nonlinear’ signals (bubble specific) - energy: harmonic, subharmonic, differences in transmit band) - methods: e.g.filtering; phase and/or amplitude modulation Employ ‘destruction-reperfusion’ approaches (destroy agent in beam and images kinetics of inflow- ‘negative bolus’)