Therapeutic Ultrasound Flashcards
What does therapeutic ultrasound cause?
The tissue to undergo reversible or irreversible biologically significant changes (could be structural or functional)
What is low power therapeutic ultrasound used for?
Physiotherapy (soft tissue injuries)
bone fracture healing
drug delivery
neurostimulation
What is high power therapeutic ultrasound used for?
Lithotripsy
high intensity focused ultrasound
histotripsy
What are the basic principles of high intensity focused ultrasound (HIFU)?
Absorption of acoustic energy heats the tissue, and large pressures can
cause bubble cavitation, which lead to cell death in focal region
High-intensity ultrasound beam is tightly focused on target
What is used in HIFU?
Large air-backed transducer outside the body with coupling medium between transducer and skin
high focussing gains, long pulses
How much energy is deposited in the target?
All of the heat deposited by a single-frequency plane wave (ΔT)
What makes HIFU trackless therapy?
There is no damage to overlying tissue
Large area transducer are used with a high focusing gain
Heating only sufficient to cause thermal damage in the focal region (boundary between normal and damaged tissue is sharp)
At what temperature does the tissue result in immediate cell death?
Tissue raised to 55◦C for 1 second
What is the size of a focal lesion?
It is of order ~2 mm by ~10 mm (a long gain of rice)
What needs to happen to ablate a large area of tissue?
Multiple sonications and/or spiral
patterning is used
Alternatively, the focus of the transducer can be electronically moved, in a spiral pattern for example
What happens at very high focal intensities?
Acoustic cavitation also contributes to tissue damage through mechanical effects
Cavitation enhanced heating occurs via several mechanisms:
multiple scattering which “traps” ultrasound wave as bubbles are reflected
viscous absorption: caused by high shear stress between the oscillating bubble and the surrounding medium
absorption of secondary acoustic emissions: including scattering and shock waves
What do cavitation cause?
The shape of the generated lesion to change
Sound is reflected back towards transducer causing a migration effect (caused by shielding) and a larger “tadpole” shaped lesion
What can cavitation be used for?
It create larger lesions in the same treatment time
(with same average power but different duty cycles)
Cavitation are hard to control
How are HIFU treatments conducted?
Under real-time monitoring and guidance using either MR or US
Performed under some kind of anaesthesia: general, regional and monitored to counteract the pain
What is MR guidance?
it allows precise localisation of the target on planning images
Many MR parameters are also sensitive to temperature changes
measure shifts in the proton resonance frequency using a gradient- recalled echo sequence
What is US guidance?
It is low cost and portable, and images can be obtained at high frame rates
Allows mapping of changes in backscatter and mechanical properties
Treatments try to induce boiling / cavitation to improve imaging
Temperature changes are not measured as it is sensitive to motion artefacts
What is the most common method for temperature mapping?
To measure shifts in
the proton-resonance frequency (PRF)
Resonant frequency of a nucleus is determined by local magnetic field
What is the equation for resonant frequency?
ω ∝ (1 - s)B_0
B_0 = magnetic field
s = shielding constant (depends on local enviroment)
What happens when water is heated?
Strength of hydrogen bonds is weakened→ increased electron screening of hydrogen nucleus→ω decreased
How is PRF measured?
Using gradient-recalled echo (GRE) sequences
What are phase differences proportional to?
Temperature-induced PRF change
What are the drawbacks of MR thermometry?
Gives temperature measurement relative to initial phase image (assuming that its at body temp, 37 degrees)
Insensitive to fat and bone (due to low water content)
Frame rate is low
Spatial averaging of temperature map due to voxel size gives underestimate of temperature
What does MR guidance require?
MR compatible HIFU transducers and anaesthesia equipment
What needs to be performed first, before the HIFU treatment?
Low power (sub-lethal) sonication performed first to check focal position
Corrections made for any offsets due to aberrations or misalignment
heating in actual focal region
What causes change in US backscatter?
Coagulative necrosis of tissue and formation of bubbles
What does the US backscatter create?
A hyperechoic region in the image which fades within a few minutes of the HIFU exposure
Relies on cavitation (caused by boiling) for visualisation
What happens as tissue is heated?
Speckle pattern in US image will shift due to
local changes in sound speed with temperature (produces apparent shift in medium position) and thermal expansion of propagation medium (produces actual shift in medium position)
What can the tracking the shift in speckle pattern estimate?
Temperature change
ΔT = κ δd/δx
d = displacement
κ = tissue dependent parameter
What are the drawbacks of ultrasound monitoring?
B mode:
difficult to see tumour margins on planning images
Requires cavitation to see hyperechoic regions
What are the drawbacks of US thermometry?
Very sensitive to motion artefacts (e.g. breathing)
Requires knowledge of tissue dependent parameter
Only works for low temp elevations
Not currently used clinically
What are the different types of anaesthesia used when?
General anaesthesia: Used for ablation of intra-abdominal tumours (liver, pancreas, kidney) where respiration gating is required
Regional anaesthesia (spinal-epidural or epidural): Used for ablation of prostate tumours
Monitored anaesthesia (conscious sedation): Used for less painful ablations (uterine fibroids, superficial tumours), and ablations that require patient feedback (brain, bone metastasis)
How is HIFU used to treat prostate cancer?
HIFU performed under ultrasound or MR guidance
Transducer inserted in rectum or urethra
Ultrasound frequency is ~4 MHz
How is HIFU used to treat uterine fibroids?
Non-cancerous growths in the womb (muscle wall of uterus)
HIFU typically performed under MR guidance
Ultrasound frequency ~ 1 MHz
How is HIFU used to treat transcranial
Ablation of thalamus in the deep brain
HIFU performed under MR guidance
Ultrasound frequency 0.6 MHz
How is HIFU used in cosmetic applications?
Its called ultratherapy
Ablation of superficial musculoaponeurotic system (SMAS) using HIFU
Used to initiate wound healing response
How is HIFU used in liposuction?
Low frequency causes inertial cavitation to emulsify fat cells (lipolysis), which are removed by natural physiological and metabolic processes
What is neuromodulation?
Modulation or stimulation of neurons using low-frequency (typically 250 to 500 kHz) and low-intensity (typically 10 W/cm2) ultrasound using single-element ultrasound transducer
Mechanism hypothesised to be mechanical effects altering the gating dynamics of neuronal ion channels
What is sonoporation?
Inertial cavitation, microstreaming, and jetting (all of which can be enhanced by microbubbles) alter cell membranes making them more permeable for larger molecules, which increases molecular uptake into cells
Drug filled microbubbles can act as both cavitation nuclei and allow targeted drug release
Why is delivering drugs to the brain complicated?
By the existence of the blood-brain barrier: a layer of endothelial cells that restricts large molecules, bacteria etc from entering the brain from the blood stream (sonoporation and drug filled microbubbles used to over come this)
What is sonothrombolysis?
Thrombolysis is the dissolution of blood clots (thrombosis), normally using thrombolytic drugs like tPA (tissue plasminogen activator)
Ultrasound (usually aided by microbubbles) can (a) directly erode the clot surface, and (b) can help the drug penetrate the clot
What is one way of removing large kidney stones?
Extra-corporeal shockwave lithotripsy (ESWL) : large amplitude pulse is focussed onto the region and breaks it down
a bipolar (positive and negative) pulse a few microseconds long with an amplitude of tens of MPa is focussed on the kidney stones using a parabolic reflector (thousands of pulses at rates of 1–2 Hz (treatment takes ~1 hour))
used for Kidney stones (calcium oxalate) > 6 mm too big to pass naturally
What are the mechanismS behind the effectiveness of ESWL?
Cavitation, jetting, mechanical damage due to large shear forces generated in the stones
What is histotripsy?
Using large shock wave to generate bubble which causes strong acoustic reflection and inversion of waves
Bubble impedance ≪ tissue impedance
Large peak pressure becomes large negative pressure leading to growth of bubble cloud
Can be used to fractionate tissue
How can ultrasound be used for bone fracture healing?
Ultrasound can stimulate the growth of cartilage and accelerate the ossification e.g. EXOGEN commercial device
It emits low intensity ultrasound at long pulses
What do HIFU transducers tend to be?
Large and tightly focused
