Transcript Ultrasound
The Physics of Seeing Inside People A brief overview of medical imaging using MRI, PET, Ultrasound and X-ray CT Dr. S. J. Doran Department of Physics University of Surrey Structure of the Talk • Why use different methods of imaging ? To look at different things Accessibility - who can undergo a scan? Repeated exposure and safety Cost effectiveness • Basic principles of imaging Ultrasound MRI X-ray computed tomography (CT) PET Why use different methods of imaging ? 1. Different methods reveal different features • X-ray CT maps the attenuation X-rays Good for mapping bone structure • Ultrasound maps the reflection and attenuation of sound • MRI maps the distribution and “environment” of water molecules in the body • PET maps the distribution of radioactively labelled compounds Why use different methods of imaging ? 2. Accessibility and Portability MRI A sliding scale Ultrasound Why use different methods of imaging ? 3. Repeated exposure and safety The ALARA Principle - As Low As Reasonably Achievable • No medical imaging scan will be done unless the prescribing doctor is sure that it will not harm you. • Nevertheless, human biology is a complicated thing that none of us completely understands, so we try to keep all risks to an absolute minimum. • Different modalities of imaging use different types of radiation (e.g., MRI uses radio waves, PET uses gamma rays), which have different characteristics. At all times, we should pick the least invasive method. Why use different methods of imaging ? 4. Cost Effectiveness • A good guiding principle in many walks of life is … Always pick the simplest solution for your problem. • In many cases the cheapest solution is the best. You do not need to give every pregnant mother in the country and MRI scan. • The capital cost of installation varies widely: Ultrasound £20k - £100k X-ray CT £500k - £1M MRI £2M - £4M PET >£5M Basic Principles of Diagnostic Ultrasound Reflector Emitted pulse c 0 c d Transducer • • Lower amplitude reflected pulse Based on ultrasound reflection and attenuation coefficients Position calculated using equation d = ct/2 Use of Ultrasound in Obstetrics 5.5 Weeks 6 Weeks Data source : Joseph Woo Use of Ultrasound in Obstetrics Bi-parietal diameter Length of femur Measurements of foetus in utero Data source : Joseph Woo Use of Ultrasound in Obstetrics 18 Weeks 19 Weeks Data source : Joseph Woo Use of Ultrasound in Obstetrics Duplex Duplex of flow in umbilical chord Measurements of blood flow on the foetus in utero Data source : Joseph Woo Use of Ultrasound in Cardiology Standard real-time B-scan Duplex scan: colour Doppler superimposed on real-time B-scan Diagnosis: Severe mitral regurgitation due to flail posterior MV leaflet. Underlying pathology: Mitral valve prolapse with ruptured chordae tendinae. Data source : Arizona Society of Echocardiography Image Library Duplex Doppler Flow Data “Signed” velocity “Doppler Power” Flow pattern at the bifurcation of the carotid artery Data source : St. Paul’s Hospital, BC, Canada Basic Principles of MRI Precession Spin N S • In a magnetic field, spinning nuclei precess at the Larmor frequency. • The spin-up and spin-down nuclei have different energies. • Transitions between levels lead to emission of photons, which we can detect. Spin down Spin up Strong magnetic field Emitted photon Magnet for Whole-Body Imager Data source : VA Imaging Centre, University of Florida The Human Brain as seen by MRI Data sources : Left - The Whole-brain Atlas, K. A. Johnson and J. A. Becker, Harvard; Right - SMIS UK Ltd. MRI can image much more than just the brain ... Knee Knee (close up, different patient) Data source : SMIS MRI can image much more than just the brain ... Chest region Arm, wrist and hand Lumbar spine Data source : UoS, SMIS Magnetic Resonance Angiography Neck Small part of head MRI can also be used to visualise patterns of blood vessels. In these pictures, the image has been sensitised to movement, in this case the movement of blood inside veins and arteries. Data source : The Whole-brain Atlas, K. A. Johnson and J. A. Becker, Harvard Functional Imaging Functional MRI (fMRI) is the “hottest” topic in MRI for many years. If we acquire one image of the brain in a resting state, followed by another with the brain “stimulated” in some way, we can compare the two. Any regions which are different correspond to areas of brain activation. The image is a 3-D picture of a human brain, which has been “sliced” in the computer to reveal the interior at a particular level in the brain (white and grey). Superimposed on this are coloured regions, which correspond to the activated areas of the brain. Data source : UW Madison MRI Center Basic Principles of X-ray CT X-ray detector • A standard tube produces X-rays with an energy of approximately 150 keV. • The X-ray focal spot scans across and around the patient. • The technique measures the X-ray attenuation coefficient of the different tissues in the body. X-ray source 150 kV - + ~ Filament Target X-ray CT Pictures of the Head and Abdomen Bone shows up bright Air is dark Different densities of tissue give intermediate results Basic Principles of Positron Emission Tomography g-ray detector + Radioactive nucleus • • • A radioactive isotope is injected and decays, emitting a b+-particle. Within a short distance, the b+-particle bumps into an electron and the two annihilate, producing a pair of g-rays. By detecting and reconstructing where the g-rays of come from, we can measure the location and concentration of radio-isotope. A Typical PET Scanner Installation Cyclotron The PET scanner itself Radio-chemistry Lab Image Source: North Carolina Baptist Hospital/Bowman Gray School of Medicine; Children’s Hospital, University of Michigan Two Typical PET Studies The highlighted region shows which part of the brain (the parietal lobe) was active during a visual stimulation task. Data source: CVVC, Psychology Dept., Durham Univ,. “Dead” areas of brain No glucose metabolism Data source: Bowman Gray School of Medicine FDG Study of Patient with Stroke FDG “Brain Activation” Study Image Fusion — Combining Modalities Clinical Study - Lung Tumour CT Research Study MRI + PET PET Fused Location of Dopamine Receptors Data source: Bowman Gray School of Medicine Conclusion • There are many different ways of imaging the human body. • The different methods tell us different things. • It is study of basic Physics (acoustics, magnetism, nuclear and particle physics) which has discovered the principles. • It is money — the human brain is a very valuable thing — which has led to the incredible developments that we see today.