Introduction to Biomedical Engineering Lecture 4_ Use of Ionizing Radiation for Imaging.docx

Introduction to Biomedical Engineering Lecture 4: Use of Ionizing Radiation for Imaging Please rewatch lecture to fill in the missing notes —------------------ When an electron collides with an electron in the outer shell of another atom, it emits energy The radiative loss vs the the collisional ene...

Introduction to Biomedical Engineering Lecture 4: Use of Ionizing Radiation for Imaging Please rewatch lecture to fill in the missing notes —------------------ When an electron collides with an electron in the outer shell of another atom, it emits energy The radiative loss vs the the collisional energy loss is a function of the energy of the particle and the atomic number Z. The observed Bremsstrahlung energy spectrum shows ____ To make an image with radiation, you want to emit a higher dose of radiation so that the radiation isnot just absorbed into the body All Bremmstrahlung curves has characteristic radiation Bremmstrahlung The production of x-ray sockets with the Bremmstrahlung process is very inefficient To solve this, modern x-ray tubes have cooling oil and a rotating anode The position of the anode and the cathode determine the level of detail you see in an x-ray Rayleight Scattering Shorter wavelights indicates higher scattering —------------- For Gamma Ray Attenuation, the transmission of photons in a parallel beam through a thin medium can be given by a differential equation, whose solution is The general solution is Intensity = I_0 e ^ (- wavelength * x) For Photon Attenuation, there is a similar solution derived from a differential equation: N = n_0 e ^ (- sigma * N * x) Film based X-ray imaging Simplest form of x-ray imaging X-rays go through a patient as the patient is lying down Steps Beam Spread (divergence) Exponential attenuation of beam with depth R2 Fall off Radiation is captured from scattering and interaction of Xrays by a detector Image development through a Radiographic contrast equation C = (I1 - I2)/Il The amount of absorption from the tissues you will get is a function of mu or the What you need to do is use high enough energy to go through the patient but low enough energy to not harm the patient. The Film Screen Detection is dependent on the image that you want If you want a more sensitive image and lower energy dose, you use a thicker screen film, however, the image will be blurry unless you put in more radiation and use a thinner screen film Radiographic film is used to complete this Phosphor plates Absorb the x-rays Electrons sit in an energy trap within the phosphorous Once you shine a laser over the plate,light is emitted from the plate and an image is received CCD camera and readout of pixels You can couple a CCD to an image intensifier so that you get a clearer x-ray image but use less radiation Today, we use Flat panel Detectors This is direct detection of x-rays Array of thin film transistors are aligned on a panel This can be used indirectly, with the same scatter problem as earlier This can also be used directly, where the x-ray hits a film that has electrons move towards the transistors for image detection Digital imaging vs traditional imaging Image editing, along with injection of Sodium Iodine to create contrast in vessels, can be used to find new details in images There are also specialized systems for mammography The heel effect is used here so that the tissue is not given too much pressure or high dose of radiation There are also special devices for reducing scatter in x-ray called antiscatter grids Image reconstruction When the detector is far away from the subject or the scanner, sometimes it is important to build an image reconstruction device, scanning the intensity of the x-rays received The detector will receive the ray at points, and begin to take those points and stretch them out into stripes It does this for all x-rays that hit it. Overtime, the streaks are most visible at certain points and will create an image We can also use a CT scan, which uses Fourier Slice Theorem (using fourier transform (Spatial coordinates to frequency space)) to capture projections of slices of a patients body CT also has solid state detector arrays which oparate in a familliar manner Light hits grid Grid has light photons that amplify electrons