Non-invasive Medical Imaging how it works

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Non-invasive Medical Imaginghow it works

• Krishna S. Nayak
• Departments of Electrical Engineering and
  Medicine
• University of Southern California
• June 20, 2006

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Medical Imaging

• Projection Radiography (X-ray)
• Computerized Tomography
• Nuclear Medicine
• Ultrasound
• Magnetic Resonance Imaging

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Projection Radiography
? (x-ray attenuation)
X-ray source
OPAQUE Bone, Iodinated contrast agents
Shadow
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Computerized Tomography
??x,y) (x-ray attenuation)
Collect projections at all angles Digital
Reconstruction
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Nuclear medicine

• SPECT Single Photon Emission Computerized
  Tomography
• PET Positron Emission Tomography

s(x,y,z) source inside body
Collimator
Measures uptake of radio-labeled materials
(biochemistry). Images reflect function not
anatomy.
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Ultrasound
r(x,y,z) acoustic reflectivity
Propagate pulse and receive reflection Compute
position and velocity (Doppler shift)
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2D
3D
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Magnetic Resonance Imaging

• (second half)

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Magnetic Resonance Imaging

• Non-invasive medical imaging method, like
  ultrasound and X-ray.
• Clinically used in a wide variety of specialties.

Abdomen
Spine
Heart / Coronary
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Magnetic Resonance Imaging

• Advantages
• Excellent / flexible contrast
• Non-invasive
• No ionizing radiation
• Arbitrary scan plane
• Research
• New contrast mechanisms
• Fast imaging

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MRI Systems

• 2-3 million

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Nuclear Magnetic Resonance
1H
NMR atoms act like tiny bar-magnets 1H, 13C,
23Na, 31P, etc...
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Static Magnetic Field
Longitudinal
z
x, y
Transverse
1-3 Tesla gt 10,000 times the earths field
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Polarization
No Applied Field
Applied Field
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Resonance!
Fast Effect. 64-128 MHz!
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Relaxation
Precession
Decay
Recovery
Slow Effect. 10 ms to 4 s
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NMR Atoms Tuning Forks ?
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Excitation
Rotating Frame
Lab Frame
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Signal Reception
z
B0
y
Signal
x
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MR Image Formation

• Gradient coils provide a linear variation in Bz
  with position.
• Result is a resonant frequency variation with
  position.

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Selective Excitation
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Slope
g
G
Position
Frequency
(a)
(b)
Magnitude
RF Amplitude
Frequency
Time
(d)
(c)
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Image Acquisition
Frequency
Position
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Image Reconstruction

• Received signal is a sum of tones.
• The tones of the signal are the image.
• This also applies to 2D and 3D images.

Fourier Transform
Image
Received Signal
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Frequency-domain acquisition
FFT
k-space (frequency space)
object space
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k-Space Trajectories
2D Fourier Transform
Echo-Planar
Spiral
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Resolution

• Image resolution increases as higher spatial
  frequencies are acquired.

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Image Noise and SNR
Low Signal-to-Noise Ratio
High Signal-to-Noise Ratio
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Contrast in MRI

• Hydrogen (water) density results in contrast
  between tissues.
• Many other mechanisms, some based on relaxation.

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T2 Contrast
Long Echo-Time
Short Echo-Time
CSF
Signal
White/Gray Matter
Time
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T1 Contrast
Short Repetition
Long Repetition
White/Gray Matter
Signal
Signal
Time
Time
CSF
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MRI Physics

• B0 polarizes atomic nuclei
• Spins precess and relax to align with B0.
• B1 allows manipulation of magnetization.
• Excitation sequences provide image contrast.
• Image Data is acquired in k-space

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Acknowledgements... http//en.wikipedia.org http/
/www.radiologyinfo.org/ Manbir Singh, Richard
Leahy
Cardiac MRI _at_ USC http//mrel.usc.edu/
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