Medical Imaging. X-rays, CT/CAT scans, Ultrasound, Magnetic Resonance Imaging

Medical Imaging X-rays, CT/CAT scans, Ultrasound, Magnetic Resonance Imaging From: Physics for the IB Diploma Coursebook 6th Edition by Tsokos, Hoeben and Headlee And Higher Level Physics 2 nd Edition (for the IB DP) by C. Hamper

X-rays High energy electromagnetic radiation Produced in X-ray tubes Electrons accelerated through a potential difference hit a metal target and release energy via x-rays

X-rays When X-rays are incident on a surface they are either absorbed or reflected. Absorption occurs mainly through the photoelectric effect (the energy is transferred to electrons), this is dependent on the atomic number Z of atoms in the medium Bone (Z = 14) absorbs more strongly Soft tissue (Z = 7) absorbs less strongly An X-ray image will show contrast between the soft and bone tissue

X-rays - Contrast When there is no big difference between the Z s of the tissue to be imaged and surrounding area (like in the digestive tract), contrast can be improved by using a contrast medium or image intensifier, to increase absorption in the area of interest. Usually this is a barium meal (a solution of barium salts)

X-rays - Sharpness X-rays that go through the body are exposed on photographic film The image shows the shadow of high Z (bone) tissue, against low Z (soft) tissue. Sharpness depends on the source; a point-like source produces a sharper image. Collimation (using an aperture) improves the sharpness Quality is improved if the patient is very close to the source, or very far away (but then the intensity is reduced, and requires longer exposure time: not ideal)

X-rays Imaging Improvments Scattered rays blur the image, so a grid of lead strips oriented along the direction of the incoming X-rays is used to absorb scattered rays. Unwanted images of the lead strips can be minimized by moving the grid sideways back and forth during exposure so that the strip images are blurred.

X-rays Imaging Further improvements Low-energy x-rays are absorbed by the patient s skin, and are not useful for imaging, so they are filtered by an aluminum foil barrier before they reach the patient. Photographic film is more sensitive to visible light, so to reduce exposure time, intensifying screens with fluorescent crystals reemit visible light.

X-rays - Tomography CT (computer tomography ) or CAT (computer-assisted tomography) made possible better diagnosis with a less invasive procedure. a movable X-ray source emits a beam at right angles to the long axis of the patient an array of detectors reduces exposure levels and the time required for a scan The source and detectors are rotated around the patient s body and moved along the length of the body. The data can then be combined into a 3-D computer image, viewable as 2-D slices at any chosen position.

X-rays - Attenuation As light travels through a medium, there is a reduction in intensity or Attenuation. It is measured by:

X-rays - Attenuation As the photons are absorbed in the material, their number decreases, and so the number of interactions (absorption) possible also decreases. The intensity passing through an absorber of thickness x is given by: I = I 0 e μx, where I 0 = original intensity, and m = linear absorption coefficient. m is the fractional decrease in intensity per cm; it depends on the type of material (Z), and the energy of the X-ray photons.

X-rays - Attenuation The Half-Value Thickness (HVT) x1 2, is the penetration distance at which the intensity has decreased to I 0 /2. m, the absorption coefficient, depends on the type of material (Z), and the energy of the X-ray photons.

X-rays - Attenuation Since m depends on the atomic number Z and energy E, the absorption coefficient is proportional to the density, so to compare attenuation between materials with different density ρ, we use the mass absorption coefficient: μ m = μ ρ

X-rays Multiple layers The fractional decrease in intensity when X-rays go through an absorber is: If they go through a second absorber, the decrease by: The total decrease due to these layers is given by:

Examples

Ultrasound Any sound with a higher frequency than the limit of human hearing is known as ultrasound (higher than 20 khz) The ultrasound is reflected at any boundary between different types of tissue. The time taken for reflected rays to be detected, allows us to find where the boundaries must be located.

Ultrasound Generation/Detection Piezoelectric crystals: when quartz is compressed or stretched, a potential difference is induced in them. When a current flows through them, the crystals react and can resonate produce ultrasound vibrations. They also vibrate when receiving ultrasound waves A single piezoelectric can be used for generation (probe/transducer) and detection of ultrasound.

Ultrasound - frequency The signal pulse duration needs to be short enough that reflected waves don t come back while the transmitter is still sending the signal. Resolution will be affected by diffraction. In order to image a small object, small wavelengths must be used small frequencies preferred? Attenuation increases with increasing frequency low frequencies preferred? The best choice is somewhere in between ~ 200 l s of ultrasound away from the probe.

Ultrasound - Imaging Want to obtain measurable reflected waves at tissue boundaries, and good transmission through the body. The degree of reflection/transmission depends on the acoustic impedance of the media boundary. Acoustic impedance: Z = rc r is the density of the medium, c is the speed of sound in the medium. Best solution is for the media to be as close in acoustic impedance as possible (impedance matching)

Ultrasound - Imaging An air gap between the probe and patient s skin would cause most of the signal to be reflected back; this is avoided by applying a gel of similar density to skin. Very dense objects (like bone) cause multiple reflections and images, and don t transmit the signal ultrasound is not used to image bones

Ultrasound: A-Scans A-Scans present the information as a graph of signal strength (amplitude) vs. time By sweeping the transducer, information can be collected about the location and distances between different tissues.

Ultrasound: B-Scans B-Scans convert the reflected signals into dots on a 2-D plot, with an intensity proportional to the signal strength. Using an array of transducers, the dots form an image of the organ. An array of probes around the body, moved in 3-dimensions creates a 3-D image on a computer.

Ultrasound Advantages/Disadvantages Advantages Non-invasive Quick & Inexpensive No known harmful effects Great for imaging soft tissues Disadvantages Image resolution can be limited Ultrasound does not transmit through bone Cannot image the lungs and digestive system, since they contain gas which strongly reflects the signal

NMR Nuclear Magnetic Resonance / MRI Magnetic Resonance Imaging Magnetic Resonance Imaging is based on a phenomenon known as nuclear magnetic resonance A particle with an electric charge and spin (proton) behaves like a microscopic magnet In the presence of an external magnetic field, the proton will align itself either parallel ( spin-up lower energy) or anti-parallel ( spindown higher energy) to the direction of the magnetic field. A radio frequency (rf) signal with the right frequency forces the protons to resonate move to a spin-down (higher energy) state. The proton then returns to the spin-up (lower energy) state, in the process emitting a photon of the same frequency. This frequency depends on the energy difference between the states, and therefore on the magnetic field at the proton s position.

NMR Nuclear Magnetic Resonance / MRI Magnetic Resonance Imaging In a region with a different external field, a different radiofrequency will be needed to excite proton transitions. A secondary, non-uniform magnetic field is applied so that different parts of the body are exposed to different net magnetic fields. Each part of the body is then revealed by a different frequency of emitted photons. The rate of these transitions also gives information about tissue.

NMR Nuclear Magnetic Resonance / MRI Magnetic Resonance Imaging The hydrogen nuclei (in water for example) can be made to resonate in a strong magnetic field. The radio waves emitted when the nuclei lose energy can be used for imaging because: 2. The time taken for the nuclei to return to their lower energy state (relaxation time) is different for different types of tissue (600 ms for muscle, but only 180 ms for fat in a B field strength of 1 T). Consider we are imaging a body that is half fat, half bone. If only frequencies f 6, f 7, f 10 and f 11 are detected, we know where the body is. If we measure the relaxation time, we find that for f 6 and f 7 it s longer than for f 10 and f 11. This tells us that areas 6 and 7 are muscle, and 10 and 11 are fat.

NMR Nuclear Magnetic Resonance / MRI Magnetic Resonance Imaging Using a non-uniform magnetic field, and varying rf signals we can get a complete image of the different tissues in the body.

NMR Nuclear Magnetic Resonance / MRI Magnetic Resonance Imaging A patient being prepared for a MRI brain scan. The patient s head is surrounded by the large coils of the scanner s magnet. The smaller device above the patient s head is a radio frequency (rf) receiver.

Comparison of imaging techniques Method Resolution Advantages Disadvantages X-ray 0.5 mm High quality image. Quick and inexpensive. CT Scan 0.5 mm Can distinguish between different types of tissue MRI 1 mm No radiation danger Non-invasive Superior high quality images Can distinguish between different types of tissue (good contrast) Bones don t get in the way, so great for viewing the brain Ultrasound 2 mm No radiation danger Can be used to view unborn babies Inexpensive and versatile Radiation Danger: X-rays are ionizing, so overexposure is dangerous. Can t be used on pregnant women. Not so good for soft tissue (images are obscured). Some organs are not accessible. Radiation Danger. Expensive and bulky equipment. Difficult for patients who are claustrophobic or children Long exposures (about 45 min) Exposure to magnetic fields difficult for patients metallic implants or pacemakers. Not all organs can be viewed Not very high resolution