Chapter 17: Therapeutic applications of ultrasound Flashcards
The goal of therapeutic applications of ultrasound is to modify the function or structure of the tissue, rather than produce an anatomical image. This is possible through two mechanisms?
The mechanical vibrations caused by the ultrasound waves can affect tissue can cause the tissue to heat up or generate internal radiation forces that can agitate the cells or tissue scaffolding.
What are the applications of ultrasound therapy?
Treat diseases such as cancer, to improve drug delivery while minimising side-effects, and to treat a wide spectrum of neurological and psychiatric conditions.
What are the two categories of ultrasound therapies?
- High intensity focused ultrasound - both thermal and mechanical bio-effects. 2. Low intensity focused ultrasound - bio-effects are mechanical.
How does HIFU work? And why is it sometimes referred to as a trackless therapy?
Ultrasound waves non-invasively destroy cells inside the human body by sending a high-energy beam of ultrasound into the tissue using a focused transducer. It is referred to as a trackless therapy because it only causes cell death at the focus.
Why does HIFU usually use transducers with large diameters?
This means the transducers have a high focusing gain, which results in a very high pressure only within a small region.
A sustained increase in tissue temperature then causes cell death via ____ ____ (i.e., the tissue is ‘cooked’).
A sustained increase in tissue temperature then causes cell death via coagulative necrosis (i.e., the tissue is ‘cooked’).
at very high focal intensities, acoustic cavitation also contributes to tissue damage through mechanical effects via several mechanisms
- Multiple scattering - The bubbles reflect the ultrasound waves, and essentially trap them in the focal region, giving more time for the waves to be absorbed. 2. Viscous absorption - the high shear stress between the oscillating bubble and the surrounding medium leads to enhanced absorption due to viscous relative motion 3. Absorption of secondary acoustic emissions - an oscillating bubble may induce nonlinear scattering, and a bubble undergoing inertial collapse can emit a shock wave. Both of these effects will generate high-frequency ultrasound waves, which are absorbed more strongly.
If a sufficiently large cloud of bubbles develops, this may cause ______, which is _________. This leads to _______
- shielding which is where the ultrasound waves are no longer able to reach the intended target. - migration effect and a larger, tadpole-shaped lesion
What is “sonication”?
A destroyed region of tissue or focal lesion which is typically very small.
HIFU is usually very painful, what kinds of anaesthesia is used?
General anaesthesia - used for the ablation of intra-abdominal tumours where control of the patient’s respiration rate is required Regional anaesthesia - ablation of prostate tumours Monitored anaesthesia - less painful ablations and that require patient feedback
What are some applications of HIFU?
Uterine fibroids Prostate diseases haemostasis (stopping bleeding) vascular occlusion
HIFU treatments are performed under real time monitoring and guidance using either ___?
mri or ultrasound
What are the advantages of MR guidance?
- precise localisation (as MR has good soft tissue contrast) - Temperature elevation can be calculated by proton resonance frequency using a gradient-recalled echo sequence
What are the limitations of MR thermometry?
- it provides a temperature measurement relative to an initial phase image which is assumed to be homogeneous and at body temperature. To avoid time drift - requires cooing between sonications, so it takes long. 2. MR is also insensitive to tissues with low water content, so cant be used to reliably estimate temp in fat or bone. 3. Frame rates are low 4. spatial averaging of the temp maps due to the voxel size can also cause the peak temp to be underestimated 5. Compatibility of all equipment to MR
Ultrasound guidance for HIFU advantages?
- low cost 2. portable 3. high frame rates
Ultrasound guidance for HIFU disadvantage?
- you cant see tumours well so hard to plan the surgeries
how is Ultrasound guidance for HIFU done?
changes in backscatter caused by coagulative necrosis (burning) and the formation of bubbles
How do you avoid interference between the ultrasound image and therapy?
performed sequentially or in an interleaved fashion
In principle, it is also possible to measure temperature changes using ultrasound how?
heating the tissue will change the speed of sound, as the formation of a b-mode image assumes a constant speed of sound, this manifests as shifts in the speckle pattern.
What is Neuromodulation?
modulation or stimulation of neurons using low-frequency and low intensity ultrasound
Mechanism is related to mechanical effects altering the gating dynamics of neuronal ion channels.
(So you can make neurons more likely or less likely to fire. Or actually start an action potential.)
What is Sonoporation?
Ultrasound and bubbles can be used to get drugs into things.
Inertial cavitation, microstreaming and jetting alter cell membranes making them more permeable which increases molecular uptake into cells.
Drug filled microbubbles can act as both cavitation nuclei and allow target drug release
What is sonophoresis
the application of ultrasound to the skin to increase permeation of drug uptake.
Mechanism:
may be due to cavitation or local heating
What is sonothrombolysis?
is the dissolution (breakdown) of blood clots normally using thrombolytic drugs
Ultrasound directly erodes the clot surface and can help the drug penetrate the clot
Extra-corporeal shockwave lithotripsy
- using a steep shock wave (nonlinear)
- breaks up stones in the kidney for example
mechanism: induces strong shear forces, cavitation etc.
Histotripsy
Super intense HIFU
Big peak negative pressure - causes cavitation
causes strong acoustic reflection and reflection of waves
large peak pressure becomes large negative pressure growth of bubble cloud
Bone fracture healing?
ultrasound can stimulate the growth of cartilage and accelerate fusion
mechano-sensitive parts of cells and surface proteins are changed (mechanotransduction) triggers certain pathways and helps bone healing
