3/13/2009ib physics hl 21 ultrasound medical imaging physics – ib objectives i.2.7describe the...
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3/13/2009 IB Physics HL 21
Ultrasound Medical Imaging Physics –IB Objectives
I.2.7 Describe the principles of the generation and the detection of ultrasound using piezoelectric crystals.
I.2.8 Define acoustic impedance as the product of the density of a substance and the speed of sound in that substance.
I.2.9 Solve problems involving acoustic impedance.
I.2.10 Outline the differences between A-scans and B-scans.
I.2.11 Identify factors that affect the choice of diagnostic frequency.
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Ultrasound Productionand Detection
Based on piezoelectric effect
From http://en.wikipedia.org/wiki/Medical_ultrasonography
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Piezoelectric Effect in Crystals Applied electric field produces mechanical vibration Also, mechanical vibration produces electrical signal
Single crystal can be both ultrasound source and detector
Not at exactly same time Mechanical vibration moves at same frequency
as electrical vibration (1 MHz to 20 MHz)
+ + + + + +
- - - - - - -Piezoelectric crystal (e.g., quartz)
Electrodes
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Wave Motion in Solids After piezoelectric crystal starts sound wave
(ultrasound wave?), wave travels through tissue Wave may reflect, refract, or be transmitted
between two different materials (organs, tissue types, etc.)
Acoustic impedance (~index of refraction) Acoustic impedance (Z) is product of
Density of medium and Speed of wave:Z = v [units of kg m-2 s-1] [Rayl]
Ex: waterSpeed is 1,480 m/s; = 998 kg/m3
Zwater = 1.48 x 106 kg m-2 s-1
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Reflection and Transmission ofWaves with Ultrasound
Reflection and transmission:When wave goes from medium with impedance Z1 to a medium with impedance Z2
Reflection fraction: (Z2 – Z1)2/(Z2+Z1)2
Transmission fraction: (2Z2)2 / (Z2 + Z1)2
Limiting cases: If Z1 = Z2, no reflection, and transmission = 1 Reflection fraction + transmission fraction = 1
Note: acoustic impedance is frequency-dependent
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Reflection and Transmission ofWaves with Ultrasound - Examples
What is fraction of sound reflected and transmitted when Sound travels from water to muscle (Z muscle =
1.7 x 106 kg m-2 s-1) Sound travels from water to air (Z air = 400 kg m-2
s-1)
Note: acoustic impedance is frequency-dependent
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Scan Modes with Ultrasound A Mode (Amplitude modulation) B Mode (Brightness mode) M Mode (Moving mode) Doppler (Doppler imaging)
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Scan Modes with Ultrasound A Mode: Amplitude modulation
Single transducer generates ultrasound, receives ultrasound
Information is available in only one-dimensional scan
Height of returning pulseproportional to strengthof returning pulse
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Scan Modes with Ultrasound A Mode: Amplitude modulation Assuming speed of sound in muscle / soft tissue is
1,540 m/s How far under the skin does the organ start? How long is the organ?
0.18 ms35 s
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Scan Modes with Ultrasound B Mode (Brightness mode)
Several transducers on handgrip record travel time simultaneously
Can build up 2-D picture of reflections Brightness of image on screen is proportional
to strength of reflection
Transducers
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Scan Modes with Ultrasound B Mode (Brightness mode)
Several transducers on handgrip record travel time simultaneously
Can build up 2-D picture of reflections
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Ultrasound Frequency Choice High frequency - high resolution
Get more detail with a higher frequency scan than a lower frequency scan
High frequency – high attenuation Higher frequencies are attenuated faster than
lower frequencies Get more penetrating images using lower
frequencies
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Ultrasound Imaging - Cautions Difficult to get imaging from brain High-intensity scans can transfer energy to object
being scanned Potential warming / damage to imaged object
Fetus
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Ultrasound Imaging – Other uses Doppler shift scans
Determine blood flow speed High speed – indication of blockage
Moving ultrasound (M Scan) Real-time image of moving objects E.g., heart beating
Fetus
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Ultrasound - Key Ideas
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Magnetic Resonance Imaging (MRI) Also called Nuclear Magnetic Resonance (NMR)
scan
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NMR Scans –IB Objectives
I.2.12 Outline the basic principles of nuclear magnetic resonance (NMR) imaging
I.2.13 Describe examples of the use of lasers in clinical diagnosis and therapy.
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Fundamental Concept - Magnetic Energy
Atoms in imaged object, especially hydrogen atoms, have a magnetic moment (~a compass) Magnetic moment is a consequence of the spin of
the proton No classical analog Charge in motion produces magnetic field Like a compass, the magnetic moments of the
hydrogen atoms want to line up in the direction of the applied magnetic field
The stronger the field, the more the atoms line up with it
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Fundamental Concept - Magnetic Energy
N
N
N
N
N Appliedmagneticfield
Most of the atoms in the sampleare oriented in the direction ofthe magnetic field
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Fundamental Concept - Magnetic Energy
N
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N
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Appliedmagneticfield
When atoms shift theirmagnetic fields to beopposite the external field,they gain energy (photon).
When atoms shift their magneticfields to be along the externalfield, they give up energy (photon).
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MRI / NMR Scanner
NMR scanners send in a radio signal in to the sample, with just the right amount of energy to flip the nuclear magnetic moments back an forth, from opposite to along the magnetic field. Resonance effect
Frequency is called the Larmor frequency Able to localize the resonant area with slightly
deformed magnetic fields Gradient fields
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MRI Scanner - Operation
Scanner detects where large numbers of hydrogen atoms are ~Water
Builds up 2-D image of object / body, which can be converted into a 3-D image
Resonance of hydrogen nuclei also sensitive to nearby atoms (electrons) Distinguish compounds that hydrogen is in
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MRI Scanner - Details
Useful for imaging skull and brainWhole-body diagnosis
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MRI Simulation - Questions
What is the relationship between the applied external magnetic field, and the frequency of the radio-wave energy that flips the spins? Direct, inverse, or no relation
How do the fringe fields help localize the RF signal from the body?
TUMOR HUNT: Uncheck “Show atomic nuclei” Click “Add tumor” Look for evidence of tumor in RF signal
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MRI Scanner - Cautions
Non-ionizing radiation Intense magnetic field
No magnetized objects or metal
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MRI - Key Ideas
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MRI - Homework Write a 1-2 paragraph summary of NMR scans. Include:
Hydrogen magnetic moment External magnetic field Energy of 50 MHz radio photon Gradient fields (optional)
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Scanning Techniques Excellent table and discussion, p. 502
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Diagnostic and Therapeutic Lasers Excellent table and discussion, p. 501