Intro to BME_ Medical Imaging (Scott Davis).docx

Medical Imaging: Introduction to Biomedical Engineering Common medical imaging modalities MRI Radio frequency waves Computed Tomography X-rays Advanced X-ray machins Simple X-ray Positron Emission Tomography PET Scan Radioactive drug is injected Imaging Gamma rays emitted from the body Ultrasound Vi...

Medical Imaging: Introduction to Biomedical Engineering Common medical imaging modalities MRI Radio frequency waves Computed Tomography X-rays Advanced X-ray machins Simple X-ray Positron Emission Tomography PET Scan Radioactive drug is injected Imaging Gamma rays emitted from the body Ultrasound Vibrations or Mechanical Waves Looking for Echos or reflections (Interfaces of tissue) SPECT scan Similar to PET scan X-rays Not good for soft tissue, as the readings will not be as clear X-rays: Probe with X-rays ad shadow imaging Looking for Density and atomic umber thickness Ultrasound: Probe tissue with mechanical waves Look for changes in density MRI Probe tissue with magnetic fields and radio waves Contrast with variations in relaxation times of hydrogen nuclei PET Probe with radioactive decay or light emitted from tracer drugs Contrast with the accumulation of Signal in specific areas of the body Note: Fluoroscopy is real-time x-ray imaging; History In 1895, Roentgen discovered the use of X-rays to his own detriment (Died from radiation exposure) Was studying cathode rays Discovered these images first on 8 November 1895 Weeks of validation work before publishing on 28 Dec Within a year, 1000 new papers were published on X-rays 1896: A doctor first uses an X-ray to treat a young Eddie Maccarthy after a skating accident This occurred 3 weeks after the discovery at Dartmouth X-rays produce a sort of shadow with gradiency X-rays can either be absorbed or scattered by tissue 10^4 - 10^5 MeV X-ray system There are four groups X-ray tube Cathode and anode Generate electrons To make this work, heat a coil up to around 2000 degrees This will boil electrons off of the coil. Electrons will bounce off of the cathode (fillament) and slam at a high speed into an anode The speed at which electrons strike the anode depends on the voltage applied When electrons hit the anode, 1% of electrons are converted to X-rays in a scientific phenomenon called Bremsstrahlung radiation The rest of the energy is converted into heat, which is why these components have to be kept in a cool environment AND the anode must spin to dissipate and tolerate the heat it recievs Detector/Sensor Tissue interaction center The amount of x-rays a tissue will absorb or scatter depends on the density and chemical composition of the tissue The intensity of the X-ray beam through the tissue can be calculated using the equation Beers Law I = Io*e^-(mu)*x The X-ray will have electrons on the atoms of the tissue knocked off and emit light (Photoelectric effect) and a phenomenon called Compton scatter, where the X-ray just becomes off center after knocking electrons off of the lowest energy shell Higher energy results in more scatter in a single direction, whereas lower energy results in more averaged scatter Highest resolution photos are obtained through the photoelectric effect Tissue contrast is dependent on the chemical composition, density, photon energy and thickness, and lighter regions indicate high attenuation of signal (more photons through) Bone is most dense, thus, it will attenuate the most signal Soft tissue however is more grayish Iodine can sometimes be imaged as well to image blood vessels with X-rays Detection was originally done with film screens Detectors detect what passes through the body (X-rays specifically) Digital Flat Pannel Arays that have a phosphore screen on them receive X-rays and convert them to photons to convert from xray to photographic images As X-rays are not normally detected at the position from which they entered the body, anti scatter grids are placed under the detector to align the x-ray Mammograms specifically is one of the highest resolution imaging procedures involving x-rays You dont have to use as many X-rays or as much power to get a clear reading Spreading the tissue out by compressing it reduces the amount of scatter and enables easy visualization of Example Image: An image of pneumothorax, a hole between the lung and the pleural system is difficult to see clearly for untrained radiologists Mammograms are equally challenging to interpret The images are viewed side by side in two views MLO and CC view Mammography works better for older women, as inclusion of adipose tissue makes it easier to visualize irregularities. For instance, microcalcifications occur when the tissues micro environment changes and the fibrous tissue begins to deposit calcium. Another example, a palpable mass in a Breast, cannot be easily viewed in a mammogram. Only ultrasound or another imaging technique can be used to view what would be a amorphous black node in the organ Fluoroscopy is real time x-rays but it requires amplification of the signal in order to function properly Continuation from yesterday’s notes: 4/4 Fluroscopy Ablation: This catheter can examine where examine electrical signal is coming from in tissues and destroy the tissue receiving that signal CT: X-ray scans in parallel (Computed Tomography) We remember that X-rays gave us planar imaging CT scans are projected from a single xray source that also gives us several x ray detectors, little squares wrapped around the xray source and the patient. These x-ray detection sources rotate around the patient and every fraction of a second, the detectors detect the signal. There are many small projections that you collect during this process and all of it can be used to create slices of the inside of the body In 1955, Alan Cormack devised the mathematics behind this X-ray method. Then in 1960, a engineer from a record company created in secret a CT scanner. EMI, previously a record company, became a CT company and dominated the CT industry. His name was Hounsfield. Later, Hounsfeld and Cormack received the nobel prize. Heres how this works: We have an xray tube and a detector lined up facing each other in parallel. You move this pair by slight increments to capture the body in vertical slices We now use one single x-ray tube to scan each of the vertical slides at once We now have this apparatus spin around the patient at a large speed so that we can track movement of fluids, contrast agents, or any movement How do we get an image from this mechanism: These detectors plot the intensity of each xray from where it passes through the body These detectors ALWAYS use Beers law to calculate this intensity Through air, the intensity of the Xray will be high for example, but since bone is dense, if you project Xray through the bone, the intensity drops To get an image, we transform the plot of intensity to a plot of attenuation and take the log of both sides, which is essentially a plot of mu * t (____ coefficient * time) There is no attenuation in air To get an image from this data, what we do is that we take that attenuation plot and we take the highest values from that plott Then, we correlate high attenuation with heavy or darker spots on a line. What we do is that we take those heavy darker spots and we smear it across a canvas We smear this attenuation signal from many different angles and the points that are darker resemble the most dense regions of the body through which the xrays went through CT scans usually use much more radiation than normal X-rays (Dental Xrays are usually the lowsest dose) Nuclear Emission Imaging is different, however, in which here we are more concerned with where a contrast agent goes. Planar Nuclear Imaging SPECT: Single Photon Emission Tomography Involves rotating around the patinet and using back projection PET scans The Radiopharmaceutical is a radioactive tracer that is injected into the patient. This tracer emits energy that the PET scan can detect. The idea is to scan the patient after injecting the tracer into the body and accumulate in regions of interest, such as a tumor Note that many scanners are built into each other. For example, it is really convenient to combine CT and PET scans, so many companies build them both as one machine Tracer production utilizes Cyclotrons, a particle accelerator. The cost for these productions is upwards $6 thousand dollars per PET scan or tracer injection and the half life of the tracer (decay) is incredibly fast. They must be used the same day they are made in order to be useful. Cyclotrons accelerate protons and bombard target atoms with them to create radionuclides. The radioactive atoms must then be bonded to other molecules incredibly quickly Another way to produce radionuclides are radionuclide generator One of the most used radionuclides is Flourodeoxyglucose, which acts as glucose and is taken up by highly metabolic cells (tumors) Note that, though PET is more sensitive, SPECT is often used instead, as it is much less expensive In a PET scan, GAMMA rays are used to excite photons on a single atom in both directions perpendicular to the angle at which Gamma rays entered the body. There are detectors surrounding the patient that analyze the time/speed at which the photons reach each detector on both sides Spatial, Contrast resolution is a tradeoff for each imaging device we look at. Continuation 4/5 MRI MRI is founded on the principles of Nuclear Magnetic Resonance (NMR) It localizes NMR signals to provide actual images The MRI works by aligning all the protons in your body along a very powerful magnetic field. These protons in your body align into essentially tiny radio antenna that alow Essentially, we can generate RF waves by alternating current through a wireloop, and Faraday’s law tells us that if we change a magnetic field or emit a magnetic field through a wire, it will begin producing current. The Nuclei are similar to the coil. Nuclei can be thought of as little bar magnets. Specifically the Hydrogen atoms surrounding each Nuclei. The hydrogen atoms surrounding the nuclei, in the absence of an external magnetic field, will all align randomly, but when you put the hydrogen atoms in a magnetic field, most of them line up with the magnetic field. Some line up against the magnetic field but most line up with the field in warm conditions. Moreover, the hydrogen atoms will begin to precess, much like a top, and produces torque in a perpendicular direction. The percession occurs at a specific frequency called the Larmor frequency, which is described mathematically as w0 = gamma * Bo (strength of magnetic field) The magnetic field will give us a net magnitization in the z direction At the Larmor frequency, Radio frequency energy is absorbed by the body. However, when RF is shot at the atoms, the spin returns to the xy plane briefly. This is why MRI pulses have to be short to be able to continuously read data from the body’s cell. Now note that we havent actually measured anything yet. Tipping the atoms downward into the xy plane creates an atom rotating in the transverse plane that is rotating at the lamor frequency. NOW, these atoms remember, are like tiny bar magnets, and if we rotate bar magnets around a coil of wire, it produces a magnetic field and voltage signal. So, we can tip these atoms down, turn them into bar magnets, put them next to a coil of wire, and get voltage/signal Now, there are different ways to return the atoms back to their original z orientation after shooting RF at them. One of them is the T1 process, whcih is cossed by thermal motion or tumbling of neighboring atoms at the same Larmour frequency. If the other atoms around one atom are changing their magnetic field, that one atoms magnetic field will do the same thing. T2 is another form of relaxation (return atoms to position) where we keep the position of the atoms at xy but we allow their spins to become more random By definition, T2 is actually a subset of T1 To obtain an image we need to first localize the radiofrequency signal so that it is stronger in one area than another. This allows some atoms to rotate at the Lamour frequency more tahn others. At any specific subsection of the body, you will get a voltage signal that is a combination of all the frequencies of all the atoms in the wave. To decouple each of these waves, we use a Fourier transform. Each of these frequencies encodes the position and type of cell in a specific location, which can tell us information about the inside of the body A common method used is slice select pulses, where we only manipulate the atoms in one position of the body or one slice. We apply the gradient again so that each of the atoms emit frequencies at different time. Separating each of the received frequencies gives us The magnetic fields can be made of permanent, electromagnetic, or superconducting magnets Ligher and darker areas correspond to how long the ato in that tissue or medium take to reach relaxation Note: All of these methods occur after you turn off the