Medical Imaging

1. Medical Imaging • X-rays • CT or CAT scan • PET scan • MRI PHYS117B.02

2. X-rays Electrons emitted from a heated cathode, bombard the anode: Bremsstrahlung: does not depend on target material. Continuous spectrum. The free electron is attracted to the atomic nucleus in the anode. As the electron speeds past, the nucleus alters its course. The electron loses energy, which it releases as an X-ray photon. Characteristic X-rays:The free electron collides with an atom in the anode, knocking an electron out of a lower orbital. A higher orbital electron fills the empty position, releasing its excess energy as a photon. PHYS117B.02

3. How X-rays interact with the tissues in your body • The atoms that make up your body tissue absorb visible light photons very well. The energy level of the photon fits with various energy differences between electron positions. Radio waves don't have enough energy to move electrons between orbitals, so they pass through most stuff. X-rays also pass through most things, but for the opposite reason: They have too much energy. • They can, however, knock an electron away from an atom altogether. Some of the energy from the X-ray photon works to separate the electron from the atom, and the rest sends the electron flying through space. A larger atom is more likely to absorb an X-ray photon in this way, because larger atoms have greater energy differences between orbitals -- the energy levels more closely match the energy of the photon. Smaller atoms, where the electron orbitals are separated by relatively low jumps in energy, are less likely to absorb X-ray photons. • The soft tissue in your body is composed of smaller atoms, and so does not absorb X-ray photons particularly well. The Ca atoms that make up your bones are much larger, so they are better at absorbing X-ray photons. • When you pass X-rays through the body, different attenuation will be encountered from different tissues => an image occurs PHYS117B.02

4. Computed Tomography Imaging (CT Scan, CAT Scan) • Same principal in X-ray pictures and in CAT scan: • x-rays pass through the body they are absorbed or attenuated (weakened) at differing levels • The picture contains a “shadow” of the dense tissues in your body • If you want a 3D view • replace the film by a banana shaped detector which measures the x-ray profile. • Take pictures from different angles • A Computer reconstructs the image PHYS117B.02

5. The CT machine Inside view of modern CT system, the x-ray tube is on the top at the 1 o'clock position and the arc-shaped CT detector is on the bottom at the 7 o'clock position. The frame holding the x-ray tube and detector rotate around the patient as the data is gathered. Outside view of modern CT system showing the patient table and CT scanning aperture PHYS117B.02

6. CT – schematic view Diagram showing relationship of x-ray tube, patient, detector, and image reconstruction computer and display monitor PHYS117B.02

7. How CT works • Inside the covers of the CT scanner is a rotating frame which has an x-ray tube mounted on one side and the banana shaped detector mounted on the opposite side. A fan beam of x-ray is created as the rotating frame spins the x-ray tube and detector around the patient. Each time the x-ray tube and detector make a 360o rotation, an image or "slice" has been acquired. This "slice" is collimated (focused) to a thickness between 1 mm and 10 mm using lead shutters in front of the x-ray tube and x-ray detector. • As the x-ray tube and detector make this 360o rotation, the detector takes numerous snapshots (called profiles) of the attenuated x-ray beam. Typically, in one 360o lap, about 1,000 profiles are sampled. Each profile is subdivided spatially (divided into partitions) by the detectors and fed into about 700 individual channels. Each profile is then backwards reconstructed (or "back projected") by a dedicated computer into a two-dimensional image of the "slice" that was scanned. PHYS117B.02

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9. How to see soft tissues with CAT scans • In a normal X-ray picture, most soft tissue doesn't show up clearly. To focus in on organs, or to examine the blood vessels that make up the circulatory system, doctors must introduce contrast media into the body. • Contrast media are liquids that absorb X-rays more effectively than the surrounding tissue. • To bring organs in the digestive and endocrine systems into focus, a patient will swallow a contrast media mixture, typically a barium compound. • If the doctors want to examine blood vessels or other elements in the circulatory system, they will inject contrast media into the patient's bloodstream. PHYS117B.02

10. Some scan images • "CAT scanning" (Computed Axial Tomography), was developed in the early to mid 1970s and is now available at over 30,000 locations throughout the world. CT is fast, patient friendly and has the unique ability to image a combination of soft tissue, bone, and blood vessels. PHYS117B.02

11. PET scans PHYS117B.02

12. PET scans • PET imaging relies on the nature of the positron and b+ decay. The positron was first conceived by Paul Dirac in the late 1920s, in his theory combining quantum mechanics and special relativity. It was experimentally discovered in 1932, the same year as the neutron. The positron is the antimatter counterpart to the electron, and therefore has the same mass as the electron but the opposite charge. • Beta+DecayWhen a nucleus undergoes positron decay, the result is a new nuclide with 1 fewer proton and 1 more neutron, as well as the emission of a positron and a neutrino: • The radionuclides that decay via positron emission are proton-rich and move closer to the line of stability while giving off a positive charge. The neutrino is very light, and interacts only very weakly with other particles. Thus it escapes the body undetected and is not directly relevant to nuclear medicine. However, its presence in the positron decay makes the energy of the positron variable, as opposed to gamma emissions, which are of a fixed energy for a given radionuclide. PHYS117B.02

13. Positron Annihilation • As positrons pass through matter and lose energy through ionization and excitation of nearby atoms and molecules. • After losing enough energy, and having traveled a distance in the neighborhood of 1 mm (depending on the initial positron energy), the positron will annihilate with a nearby electron • Conserve energy and momentum in this reaction: • Initial energy comes from the mass of the electron and positron • Final energy : kinetic energy of 2 photons (511 KeV each) • Why 2 photons ? • Well we need to conserve momentum. In the initial state – electron and positron at rest Ptot =0 . Since a photon can not exist at rest ( it moves with the speed of light), you need 2 photons back-to-back to get Ptot = 0 in the final state PHYS117B.02

14. A Coincidence Event • The simultaneous emission of the 2 photons in opposite directions is the basis of coincidence detection and coincidence imaging. The line along which the photons are emitted can be pointed in any direction, but if you measure enough lines, they will cross in some region of the body where most of the emissions happened. • A ring of radiation detectors surrounds the patient in whom a positron emission and subsequent annihilation has occurred. • The simultaneous detection of 2 photons is referred to as a "coincidence". This meaning is very different from the common usage of the term "coincidence" to mean that 2 events happened without common cause. More coincidence events along a line means more radiation from this part of the body PHYS117B.02

15. Some commonly used nuclides (b+emitters) PHYS117B.02

16. How to trace the location of the positron emission • PET scanner shows in fine detail the metabolism of glucose, by tracing the positron emission from 18F. • Cancerous tissue uses more glucose, so it produces stronger signals. Abnormal lymph nodes (cancer) from a PET scan image PHYS117B.02

17. MRI ( used to be NMR), but people are afraid of the word nuclear …. PHYS117B.02

18. The MRI machine • The basic design used in most MRI scanners is a giant cube. The cube in a typical system might be 7 feet tall by 7 feet wide by 10 feet long (2 m by 2 m by 3 m), although new models are rapidly shrinking: • There is horizontal tube running through the magnet from front to back. • The patient, lying on his or her back, slides into the magnet on a special table. Whether or not the patient goes in head first or feet first, as well as how far in the magnet they will go, is determined by the type of exam to be performed. • MRI scanners vary in size and shape, and newer models have some degree of openness around the sides, but the basic design is the same. PHYS117B.02

19. Nuclei have spin! MRI is tracing the water in your body ( i.e. – the location of hydrogen …or protons). • Aligning the spins in external magnetic field. • Now the z-axis is defined. • In addition, the levels with different magnetic quantum number are split in energy (the degeneracy is lifted). Thus then proton can absorb radiation ( it’s in the RF range) and move between energy levels ( flip its spin. • Without the external magnetic field the energy levels with different projections of j are degenerate. • The protons can “spin” around any axis PHYS117B.02

20. Why do we need the magnetic field ? • A very uniform, or homogeneous, magnetic field of incredible strength and stability is critical for high-quality imaging. You can form this field using a solenoidal coil. It forms the main magnetic field. It is needed to split the energy levels in the nuclei. PHYS117B.02

21. Magnets: most important component in MRI machine • The magnets in use today in MRI are in the 0.5 Tesla to 2.0 Tesla range. Magnetic fields greater than 2 Tesla have not been approved for use in medical imaging, though much more powerful magnets -- up to 60 Tesla -- are used in research. Compared with the Earth's about 30 microTesla magnetic field, you can see how incredibly powerful these magnets are. • The MRI suite can be a very dangerous place if strict precautions are not observed. Metal objects can become dangerous projectiles if they are taken into the scan room. For example, paperclips, pens, keys, scissors, hemostats, stethoscopes and any other small objects can be pulled out of pockets and off the body without warning, at which point they fly toward the opening of the magnet (where the patient is placed) at very high speeds, posing a threat to everyone in the room. Credit cards, bank cards and anything else with magnetic encoding will be erased by most MRI systems. PHYS117B.02

22. Magnets can be dangerous In this photograph, you can see a fully loaded pallet jack that has been sucked into the bore of an MRI system. PHYS117B.02

23. The resonance • The MRI machine applies an RF (radio frequency) pulse that is specific only to hydrogen. The system directs the pulse toward the area of the body we want to examine. The pulse causes the protons in that area to absorb the energy required to make them go to an energy level with different magnetic quantum number. This is the "resonance" part of MRI. The specific frequency of resonance is different for different types of tissue. • These RF pulses are usually applied through a coil. MRI machines come with many different coils designed for different parts of the body: knees, shoulders, wrists, heads, necks and so on. These coils usually conform to the contour of the body part being imaged, or at least reside very close to it during the exam. PHYS117B.02

24. Spin relaxation: the signal is recorded • Next step: spin relaxation back to the ground state. The signal is recorded by a detector of RF radiation. But how do we know where the signal is coming from ? PHYS117B.02

25. MRI position information • Applying a gradient field ( position dependent field strength) which alters the energy level splitting in nuclei which are in different parts of the body. • When you record the signal every part of the body “plays a different note” PHYS117B.02

26. The gradient magnets • Every MRI system has a gradient magnet in addition to the main magnet. There are three gradient magnets inside the MRI machine. These magnets are very, very low strength compared to the main magnetic field; they may range in strength from 18 to 27 milliTesla. These magnetic fields are needed to provide position information in the image. • At approximately the same time, the three gradient magnets jump into the act. They are arranged in such a manner inside the main magnet that when they are turned on and off very rapidly in a specific manner, they alter the main magnetic field on a very local level. What this means is that we can pick exactly which area we want a picture of. Thus we take a picture of a "slice." PHYS117B.02

27. MRI images MRI provides an unparalleled view inside the human body. The level of detail we can see is extraordinary compared with any other imaging modality. MRI is the method of choice for the diagnosis of many types of injuries and conditions because of the incredible ability to tailor the exam to the particular medical question being asked. By changing exam parameters, the MRI system can cause tissues in the body to take on different appearances. This is very helpful to the radiologist (who reads the MRI) in determining if something seen is normal or not. We know that when we do "A," normal tissue will look like "B" -- if it doesn't, there might be an abnormality. PHYS117B.02

28. Visualization • In X-ray and CT scan you would use injectable contrast, or dyes to alter the X-ray intensity from different regions of the body. • The contrast used in MRI is fundamentally different. • MRI contrast works by altering the local magnetic field in the tissue being examined. Normal and abnormal tissue will respond differently to this slight alteration, giving us differing signals. These varied signals are transferred to the images, allowing us to visualize many different types of tissue abnormalities and disease processes better than we could without the contrast material. This MRI scan shows the upper torso in side view so that the bones of the spine are evident PHYS117B.02

29. Summary: How MRI works • The nuclei of a patient's hydrogen atoms align with the scanner's magnetic field. • Pulses of radio waves are sent into the scanner that make the hydrogen nuclei flip their spin and jump into a higher energy level (precess around a different axis) • After the radio wave pulsing stops, the nuclei realign their spin with the external magnetic field • During the realignment process, the nuclei emit photons of radio frequency. These signals are captured by the computer system that analyzes and translates them into an image of the body part being scanned. • A gradient in the magnetic field makes the energy level splitting different at different locations in the body – thus analyzing to frequency of the emitted radio waves, we get position information. Different tissues have different resonance frequency – thus you get contrast in the image. • The image appears on a viewing monitor and then is sent to a camera to be developed on several large sheets of film. • Radiologists interpret the images on film or directly from a viewing station. They dictate a report of the findings which is sent to the patients referring physician. PHYS117B.02