BIOE 498/598 DP Imaging and Therapeutic Probes(The course so far) Lecture#1: Imaging and Therapy Basics Lecture#2: Nanomedicine Lecture#3: AngiogenesisBiological Barriers Lecture#4: Targeting Strategies Lecture#5: Theranostics Lecture#6: Clinical Translation Lecture#7: Toxicity/invitro-invivoassayBioethics Lecture#8: CT Imaging Basics Lecture#9: Contrast Agents for CT Lecture#10: Contrast Agents for CT-2 Lecture#11: Contrast Agents for CT-3 and Next Generation CT Techniques Lecture#12: Energy-resolved CT Biological barriers in drug targeting Biological targeting approaches Article: Factors Affecting the Clinical Translation of Theranostic Probes
Why Ultrasound? Over half a century old technique! Arguably the most widely used imaging technologies in medicine. Portable, free of radiation risk, and relatively inexpensive compared to MRI, CT and PET. Tomographic, i.e., offering a “cross-sectional” view of anatomical structures. “Real time,”- providing visual guidance for interventional procedures. Dual use: Imaging and Therapy.
The human ear cannot hear below 20 Hz. • Elephants can use infra sound. • The human ear cannot hear above 20,000 Hz. • Bats use ultrasound to locate food. • Dolphins use it to communicate. • Ultrasound used in medical imaging operate at frequencies way above human hearing: about 2 million Hz - 20 million Hz (2-20 MHz).
Sound travels in waves. Ultrasound physics has to do with the higher frequencies of sound. Human hearing is from about 20 cycles per second or 20HZ (a low hum) to about 20,000 cycles per second or 20KHZ. A grasshopper sends out sound waves at 40KHZ. A dog can hear at about 30KHZ and bats send chirps and listens for the echoes at 100KHZ.
Dual Purpose Imaging: • B-mode imaging: Improved contrast • Doppler Ultrasound: Improved contrast and signal strength • Perfusion Imaging: Imaging where micro bubbles are deliberately collapsed to measure how rapidly the blood refills an organ or suspected tumor. • Targeted Imaging
Dual Purpose Therapy: • Thrombolysis: USCAs are collapsed to clear a blood clot • Angiogenesis: Bubbles in vasculature are popped to break open target blood vessel. • Sonoporation: Opening of cellular membrane by USCA and ultrasound exposure. • High intensity focused ultrasound (HIFU): Already an established practice for burning target tissues; use bubbles to increase heating.
Aspects of Cancer Imaging and Treatment Possible roles of US • Early detection and tumor characterization • Tumor angiogenesis imaging • Nanoparticles directed against extraluminal targets of early cancer • Multimodality imaging with photoacoustics • Molecular profiling • Monitoring treatment response • Assessment of response to anti-angiogenic therapy • Radiation therapy • Conventional chemotherapy • Image-guided delivery of therapy • Bubbles carrying chemotherapeutics • Bubbles carrying biologics • MR guided ferromagnetic bubbles for drug delivery
Dr. Karl Theo Dussik, an Austrian neurologist, was the first to apply US to image the brain. T1: ultrasonic generator, Q1: transmitter, Q2: receiver, T2: converter amplifier, W: water bath, L: light, P: photographic/ heat-sensitive paper Ultrasound in Med. & Biol., Vol. 30, No. 12, pp. 1565 - 1644, 2004
Properties of Sound Waves • Frequency • Velocity • Wavelength • Amplitude • Units to describe frequency: • Hertz= 1 cycle in one sec • kHz= 1000 Hz= 1000 cycles per sec • MHz= 1000000 Hertz • US imaging frequency range: 2-12 MHz High Frequency Wave Amplitude Crest Time Pressure Trough Period Period wavelength Low Frequency Wave Time Pressure • The number of cycles occurring in one sec of time (cycles per sec) • The high frequency wave sounds higher than the low freq wave http://www.genesis-ultrasound.com/Ultrasound-physics-2.html
Wavelength • Length of space over which one cycle occurs (distance) wavelength wavelength Distance Distance • Given a constant velocity, as frequency increases wavelength decreases (V= f) • Common US frequencies and wavelengths • -2.25MHz = 0.6 microns • -5.0 MHz = 0.31 microns • -10.0 MHz = 0.15 microns
Ultrasound Wavelength and Frequency • High frequency US waves High axial resolution More attenuation Superficial structure • Low frequency US waves Lower resolution Less degree attenuation Deeper penetration • High frequency transducers (10-15 MHz) to image superficial structures (e.g. stellate ganglion blocks) • Low frequency transducers (2-5 MHz) to image the lumbar neuraxial structure Higher frequency waves are more highly attenuated than lower frequency waves at a given distance
Velocity • Average speed of US in the human body is 1540 m/sec • Directly related to the stiffness of media • Inversely related to the density of media • Slowest in air/gasses • fastest in solids Medium Velocity (m/sec) -------------------------------------------- Air 330 Fat 1450 Water 1480 Soft tissue 1540 Blood 1570 Muscle 1580 Bone 4080 c = × f = c / f
Amplitude • The strength/intensity of the sound wave at any given • point in time • Represented by the height of the wave • Amplitude/intensity decreases with increasing depth • Magnitude of the pressure changes along the sound • wave • Power: rate at which energy is transferred from a sound beam- • proportional to the amplitude squared • Intensity (Watts/cm2) is the concentration of energy in a sound beam
Attenuation Coefficient 8 MHz 10MHz 12MHz The ultrasound amplitude decreases in certain media as a function of ultrasound frequency (attenuation coefficient) ScN-Sciatic nerve, PA - Popliteal artery. Practical consequence of attenuation: the penetration decreases as frequency increases
8 MHz 10MHz 12MHz A 0.5-mm-diameter object • Ultrasound frequency affects the resolution of the imaged object. • Resolution can be improved by increasing frequency and reducing the beam width by focusing. For a constant acoustic velocity, higher frequency US can detect smaller objects and provide a better resolution image.
Spatial Resolution 10 MHz Carotid, thyroid, breast and muscle scans. 20-45 MHz intravascular ultrasound (IVUS).100 MHz : 15 micron resolution (~40 times magnification in optical microscopy). GHz range ultrasound enables imaging of a single cell. Axial resolution is the minimum separation of above-below planes along the beam axis. It is determined by spatial pulse length, which is equal to the product of wavelength and the number of cycles within a pulse. Axial resolution = wavelength (λ) × number of cycle per pulse (n) ÷ 2
Common Frequencies for Clinical US Dystrophic calcification of the choroids Portal Vein Ultrasound Color Doppler imaging shows a thrombus in upper PV moderately dilated (14.5 mm) with splenomegaly: Cirrhosis with PV thrombosis. Ablative therapy MRI of a large tumor in the left kidney (L) and 12 days following HIFU treatment (R).
Wavelength and Frequency • Wavelength and frequency are inversely related • The unit frequency is Hertz (Hz) = 1 cycle in one sec Cardiac US imaging frequency range TTE 2-3 MHz TEE 3.5-7 MHz IVUS 10-40 MHz IVUS TTE TEE
Interaction Between Ultrasound and Tissue • Attenuation • Reflection • Refraction • Scattering Tissue absorbs the ultrasound energy, making the waves disappear. These waves don't return to the probe and are therefore "wasted". The more the body tissues that the ultrasound waves have to cross, the more attenuation the waves suffer. That is one reason why it is more difficult to image deeper structures. True reflection r=i
Reflection Reflection occurs at the boundary/interface between two adjacent tissues The difference in acoustic impedance (z) between two tissues causes reflection of the sound wave z= density x velocity Reflection from a smooth tissue interface (specular) causes the soundwave to return to the scan head US image is formed from the reflected echoes
Transmission Not all of the sound wave is reflected, therefore some of the wave continues deeper into the body These waves will reflect from deeper tissue structures Scattering Redirection of the sound wave in several directions Caused by interaction with a very small reflector or a very rough interface Only a portion of the sound wave returns to the scan head
When a voltage is applied to an piezo electric crystal (shown in red below), it expands. When the voltage is removed, it contracts back into its original thickness. If the voltage is rapidly applied and removed repeatedly, the piezo electric crystal rapidly expands and relaxes, creating ultrasound waves.
Piezoelectric crystal is compressed to generate a voltage Listen Striking
Transducer G E L Air is the biggest enemy; medium viscosity gel helps
Sound wave, Medium and Tissue Interaction Irregular surfaced objects such as nerves scatter the ultrasound waves in all directions. A small portion of the waves are reflected back to the probe ("scattered reflection“). Part of the ultrasound waves continues into the second substance, but becomes slightly bent away from their original direction (pink arrow). (Refraction) If an object is large and smooth like a nerve blocking needle, all the ultrasound wave is reflected back. This is very useful since it helps us to clearly see needles when performing ultrasound guided nerve blocks("specular reflection“)
A Mode Scanning Simplest form of US -wave that comes out of the probe travels in a narrow pencil like straight path. Ophthalmologists can use it to measure the diameter of the eye ball.
The time difference between the first bump (amplitude) and the second bump represents how long the US wave took to travel between the two walls. • Longer the length, longer is the time difference. • The speed of US in the eye is ~1500 m/sec
Doppler Effect US readout • The dopplereffect causes the frequency of waves reflected from a moving object to be different from the frequency of the wave sent out of the probe. • If the object is moving towards the probe, the reflected frequency is increased. • If the object is moving away, the reflected frequency is decreased.
Doppler Effects: Mathematical Calculations Doppler shift is given by the following formula: FD = (V X FO ) / C where FD: Doppler shift FO: original frequency V : velocity of blood C : speed of sound in tissues Therefore V = (FD X C) / FO A further refinement of this formula is: V = (FD X C) / (2FO X Cosθ) Factor 2: Doppler shift occurs twice – when the original wave is incident on the moving RBC and when the moving RBC reflects it back. Cosθ : a correction for the angle between the US beam and the direction of blood flow. Cosθ = 1: beam is parallel to the direction of blood flow (maximum velocity is measured). Cosθ = 0: beam is perpendicular to the direction of blood flow (zero velocity is measured). BART
Example of Color Doppler Color Flow Imaging of a mitral regurgitation jet
Linking it all : Frequency, Wavelength, Resolution, and Depth Correct depth = reasonable high frequency = reasonably short wave length = reasonably good resolution. Too much depth = low frequency = long wavelength = poor resolution Too little depth = Wont see structures of interest !
Why do You Need a Contrast Probe? US image of poplitealarea 1) Sciatic nerve (hyperechoic with stippled "honeycomb" structure) 2) Adipose tissue (hypoechoic) 3) Muscles (note the striations and hyperechoicfascial lines on muscle surfaces) 4) Vein (anechoic - partially collapsed under pressure to US transducer) 5) Popliteal artery (anechoic - pulsating) 6) Bone (hyperechoic rim with hypoechoic shadow below it)
Why do You Need a Contrast Probe? US tissue imaging applications Contrast between anatomical structures is low due to the small differences in acoustic impedance of different tissue types (typically <10%). US Doppler applications Echogenicity(a material's ability to produce echoes) is even worse, red blood cells are 2-3 orders of magnitude less echogenic then tissues
Solution? US tissue imaging applications Tumors and organs, require more blood flow than others. US Doppler applications In blood, the low reflection is due to the minute difference in impedance between plasma and red blood cells. Blood itself is the enemy and friend?? so why not change its acoustic properties?
What happens at a gas-tissue interface? Specific acoustic impedance z (also called shock impedance) is the ratio of sound pressure p to particle velocity v at a single frequency • 99% of the incident energy is reflected at a gas-tissue interface. • The result demonstrates that gas contrast agents could help image blood flow and structures based on their blood use.
Ideal Characteristics of an Ultrasound Probe • High echogenicity • Low attenuation • Low blood solubility • Low diffusivity • Ability to traverse pulmonary system • Lack of biological effects in repeat exposures
Ultrasonic Molecular Imaging of Tumor Angiogenesis in Breast Cancer Orthotopicallyimplanted human breast adenocarcinomaxenograft (MDA-MB-231 cells) in nude mouse imaged following intravenous administration of 5×107 VEGFR2-targeted probe. Scale bar =3 mm Bubbles coalescing in the extravascular space become detectable by US due to their micron sized collections
A slow motion capture of interialcavitations of micro bubbles Contrast harmonic image of liver in vivo Right frame:Image using both tissue and contrast agent fundamental frequency backscatter Left frame:the fundamental tissue signal has been filtered leaving the higher harmonic signal corresponding only to the contrast agent and, subsequently, the blood flow of the liver.
Contrast Agents and Their Distribution Relative to the Vascular Endothelial Cells