Introduction of Medical Imaging

Chun Yuan

Organization of the Course

• 8 Lectures (1.5 hours per lecture)– Introduction of medical imaging and MRI– Basic concept of image formation– Basic pulse sequences and contrast manipulation– Image Reconstruction– RF pulse and gradient pulse– Fast imaging and advanced applications– MRI hardware– Functional MRI

Text Books

• Magnetic Resonance Imaging - Physical Principles and Sequence Design– ISBN: 0-471-35128-8– Authors: E. M. Haacke, R. W. Brown, M. R.

Thompson, and R. Venkatesan– Publisher: John Wiley and Sons, 1999

• Handbook of MRI Pulse Sequences– ISBN: 0-7803-4723-4– Authors: Bernstein, King, and Zhou– Publisher: Elsevier Publishing, 2004

Credits

• Home work– One for each day– 60%

• Term project– Topics will be provided– 40%

What is Medical Imaging• Introduce some form of radiation – electromagnetic– Acoustic

• Observe its interaction with tissue– attenuation– scattering / reflection– Concentration

• Convert the observations into a clinically meaningful image– film– computer

Electromagnetic Spectrum

Imaging Considerations

• Type of information– anatomical - from head to toe– functional - cardiac, brain, etc.– quantitative vs. qualitative

• Limitations– resolution– sensitive range (e.g. view angles)– speed– cost– invasiveness

“Classical” methods

• Projection Radiography (Conventional X-ray)• Ultrasound• Conventional Nuclear Medicine

Images that are direct manifestations of the interactionbetween radiation and tissue

Projection Radiography

• Physical Principle: Variation in X-ray attenuation of different tissues

• Methodology: A beam of X-rays is directed through a patient onto a film.

• Image: An X-ray “shadow” of the patient.• History:– Roentgen’s discovery - 1895– Application to medicine – 1896– contrast materials - early 1900’s– angiography - 1927

Projection Radiography System

Projection Radiography Examples

Chest X-Ray

Mammogram

Angiogram

Ultrasound

• Physical Principle: Ultrasound waves scatter and reflect within the body

• Methodology: A pulse of ultrasonic energy is propagated into the body and backscattered echoes record the depth of objects in the body.

• Image: A “depth map” of patient organs.• History:– Concept derived from W.W.II sonar– Major clinical development - 1970’s

Ultrasound System

Ultrasound Mode

• B-mode image– Longitudinal view of digital artery– Frequency: 40MHz– Resolution: up to 50mm

• Doppler– Flow velocity in digital artery

Nuclear Medicine

• Physical Principle: Variable uptake of radioactive materials by different organs

• Methodology: Inject patient with radiolabeled substance and record time-space pattern of radiation.

• Image: A map of the radioactivity of the patient.• History:– Therapeutic administration of radiolabeled substances -

1950– Scintillation camera - 1952

Nuclear Medicine System

Nuclear Medicine Example

“Computed” methods

• Computed Tomography (CT)– X-ray CT– PET– SPECT

• Magnetic Resonance Imaging• (3D Ultrasound)

Images that are formed using mathematical methods and computers from indirect measurements of the

interaction between radiation and tissue

Computed Tomography

• Physical Principle: Projection slice theorem dictates how to reconstruct a 2-D image from multiple 1-D projections (Radon Transform).

• Methodology: Obtain multiple projection images and reconstruct images using a computer.

• Image: A 2-D slice mapping the patient’s X-ray attenuation coefficient (X-ray CT) or radioactivity (PET and SPECT).

• History:– X-ray CT proposed - mid 1960’s– Early clinical use - 1972– PET and SPECT followed X-ray CT

Computed Tomography System

X-ray CT Example

PET Example

Magnetic Resonance Imaging

• Physical Principle: Within a strong magnetic field, paramagnetic nuclei (usually hydrogen protons) will resonate in response to RF radiation

• Methodology: Place patient in a magnet, irradiate with RF field, and record spatially encoded RF echoes.

• Image: A map of proton concentration through a slice of the body.

• History:– NMR discovered - 1940’s– Imaging proposed in 1972– Current generation of machines developed in 1980’s

Nobel Prize for MRI

MRI System

MRI Example

Star Artifacts in CT

Shadow Artifacts in Ultrasound

Wrap-around Artifacts in MRI