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The Extras - Collimation, filtration, and grids Artifacts Read Fauber pg 222 - 225 These may show up on the final practical and/or exam Be able to identify cause based on appearance Scatter Radiation Results from Compton interactions (deflection) Increases with kVp Increases with volume of tissue exposed o Thickness o Area Scatter photons hit film, produce density o FOG Collimation AKA - Beam limiting device Limits the area exposed to film size or less Reduces patient exposure Old style - cones, cylinders New style - variable aperture collimator box Produces round collimation field Cylinders still used in dentistry Cumbersome to change cones for body imaging Grids Post-patient scatter reduction Increases patient dose Lead strips separated by aluminum or carbon fiber Focused (most common) or parallel How will grids affect the film? o Density? Decrease o Contrast? Increase o Artifacts? Gridlines Grid Characteristics Grid ratio (10:1, 12:1) Grid frequency (lines per inch) Grid Cutoff Read page 156 to 162 in Fauber This will be on the test. Filtration Changes beam quality and quantity o Patient exposure o Image quality Types: o Total Filtration Inherent Added o Compensating Total Filtration Total filtration required is 2.5 mm of Al or equivalent Reduces patient exposure Compensating Filters Improve image quality Evens image density for uneven thickness anatomy What else can we do to help accomplish this? Beam Hardening Weaker photons preferentially removed Beam quantity decreases Beam quality increases Half-value Layer (HVL) Defined: thickness of material required to attenuate half the photons in the beam Permits equivalency of different materials Al HVL = Pb HVL = Cu HVL First HVL removes 50% of the initial photons Second HVL removes 50% of what's left Third HVL removes 50% of what's left What percentage of beam remains after 3 HVL's? 12.5% Reading Assignment Chapter 5 in Fauber Grid cutoff, pgs 156-162 The Extras - Penumbra, Magnification, Distortion and the Inverse Square Law Geometric Properties Sharpness o Geometric unsharpness o Receptor unsharpness o Motion unsharpness Distortion o Size o Shape Vocabulary FFD (Film focal distance) o Distance from tube to film o AKA: SID - source imaged distance OFD (Object film distance) o Distance from object to film o AKA: OID - object image distance SOD (Source object distance) o Distance from tube to object Sharpness Penumbra - edge unsharpness Caused by not having a point source Different parts of focal spot produces different shadows Different shadow overlapping cause blurry edges Penumbra vs. Focal Area Larger focal spot produces more penumbra Larger filament = large focal spot Large anode angle = large focal spot Penumbra vs. FFD Longer FFD = less penumbra Penumbra vs. OFD Smaller OFD = less penumbra Receptor Unsharpness Screen Speed o Increase phosphor crystal size = increase penumbra o Increased phosphor thickness = increased penumbra Film o Increased film grain size = increased penumbra Motion Blur Magnification Increased FFD = less magnification Decreased OFD = less magnifcation Distortion Caused by unequal magnification, irregular shapes, off center anatomy and tube not perpendicular to film (tube angle) Minimized by reducing magnification, placing anatomy-of-interest in the middle of the film and avoiding tube angles when possible Does any of this affect density and contrast? FFD vs. optical density Photons spread out Less photons per unit area What will that to do to the image? Inverse Square Law Intensity = 1/d^2 Double FFD = ¼ density How do we compensate? OFD vs. optical density Scatter from object diverges and spreads out similar to primary beam What happens as OFD increases? How will that affect film density? OFD vs. Contrast Fewer scatter photons hit film as OFD increases How does this affect fogging? Decreases How does this affect contrast? Increases Reading Assignment Read Appendix D in Fauber pgs 360-362 Radiobiology ALARA (As Low As Reasonably Achievable) Guiding principle Assumes any and all radiation is harmful Only expose when necessary Must weigh benefits and risks Worst offender for overexposure is retakes Terminology – Units Roentgen (R) - measures radiation intensity Rad (rad) – measure of absorbed dose (used in class) Rem (rem) – measure of absorbed dose in humans (effect on human tissue) Tissue Sensitivity Law of Bergonie and Tribondeau Radiosensitivity decreases with cell maturity Radiosensitivity decreases with tissue age Radiosensitivity increases with metabolic rate Radiosensitivity increases with proliferation rate Sensitive Tissues High sensitivity – lymphoid tissue, bone marrow, gonads Moderate sensitivity – skin, eyes, GI lining, liver, kidney, thyroid, growing bone Low sensitivity – muscle, nerves, mature bone Tissue effects Cells/tissues damaged by direct effects (DNA hit by photon) or indirect effects (free radical formation) Early effect – occurs within minutes to days o Erythema (skin redness), death Late effect – occurs after 6 months o Cancer, death Acute Radiation Syndrome Prodromal period – few hours to days o Symptoms present Latent period – few days to weeks o No outward symptoms Manifest illness o Rapid decline usually ending in death (hematologic, GI, nervous system) Decreasing Tissue Effects Protraction o Slower rate of exposure over longer period of time Fractionation o Same rate but broken into short session spread out over time Reduce area exposed o More dose needed for smaller area Prenatal Exposure Embryo/fetus is more sensitive Avoid exposure in non-ife-threatening cases 10 day rule o Women of child-bearing age can only be imaged between the 1st day of LMP and 10th day o No imaging between 10th and 28th day o After 28th day, must fail pregnancy check Population Effects Exposing most in a population to a little bit of radiation is bad for the population (20-30 mrad GSD) Exposing a few in a population to large amounts of radiation is bad for those few Exposing a few in a population to a little bit of radiation has a minimal impact Dose Response TQ Death Risk TQ Dose Limits (Annual) TQ Limiting Exposure Only x-ray when information gained will alter patient management Utilize shields (lead aprons) whenever possible Collimate beam as much as possible Avoid retakes (worst offenders) o Measure/proper factor selection o Proper patient positioning o Dark room protocols Typical Exposures TQ Take home message X-rays can provide important clinical information X-rays cause cellular and subcellular damage X-rays are very low risk when used sparingly and appropriately X-rays should not be used as a routine method of examination Childhood and prenatal exposure should be avoided Use the 10-day rule to avoid exposing embryo/fetus Practice ALARA always Reading Assignment Read Chapter 12 in Fauber, pgs 293-327 Digital Radiography Why digital? Greater dynamic range Greater contrast resolution Easier to store, easier to send Can be in two places at once Post-processing (clean up image, enhance edges) Manipulate image at will in real-time Why not? Digital is “slower speed” and requires higher exposure Expensive (upfront) So how does it work? Everything is the same up to the point where photons hit the receptor Two types o Computed Radiography (CR) Cassette-based o Direct Digital Radiography (DR) Cassetteless CR Uses an imaging plate instead of film/screen Utilizes phosphorescence Stores photon energy for release during “processing” Laser stimulation causes stored energy to be released as visible light Light detector determines brightness (density) DR CCD capture o Take a digital picture of a glowing phosphor plate Flat Panel capture o Indirect converstion Convert x-ray photons to light, convert light to electrons, measure electrons o Direct conversion Convert x-ray photons to electrons, measure electrons CR vs. DR TQ Displaying Digital Imaging Very high resolution display needed to show all the detail that is recorded o Typically 1500x2000 o Long scale of gray (1024 shades) o $$$ Window/Level Window o How many shades of gray to display o Essentially changes contrast Level (Center) o How bright is middle shade of gray o Essentially changes density Storing Digital Imaging TQ PACS Picture Archival and Communication System Stores, sends and displays images On-site and off-site components ensure data protection Passwords and encryption ensure data security Computed Tomography (CT) – An Introduction to Cross-sectional Imaging Review 5 Primary Radiographic Densities o Air (Black) o Fat (Dark Gray) o Water (Light Gray) o Bone (White) o Metal* (Bright White) A Brief History TQ I thought this was a CAT scan Computed Axial Tomography (CAT) was the original name Advances have to led to high resolution images (reconstructions) in non-axial planes Body Planes Axial – Divides into top and bottom o Superior/Inferior Sagittal – Divides into right and left Coronal – Divides into front and back How is this the same? Uses ionizing radiation (x-ray photons) Photons generate the same way Photons behave the same way Physics is physics So, then what’s new? Novel application of math to attenuation data Gives us new info Give us new perspective Gives us more sensitive measurements What do you mean by cross-sectional? X-ray 2D Slide 3D Slice and Dice How the heck does that work? Take lots of pictures from different viewpoints Computer takes all the data and figures out what the middle looks like Ok, now I’m confused How do we turn those seemingly useless strips of data into that sweet picture of the dude’s chest? The simplest method is back-projection How do we speed it up? Fourier analysis – crazy math that makes calculations super fast Same method is used in MR, although in a slightly different way We’ll look in more detail in the MR section Where are we at today? Super fast helical CT Electron bea, CT (heart scans) Computed Tomography – Basic of Image Interpretation What does it all mean? Each picture element (pixel) in the image represents a chunk of tissue An attenuation value is calculated for each pixel o Hounsfield unit o -1000 to +1000 A shade of gray is assigned based on the window/level and the attenuation value The Physics of MRI – An Introduction Key Terms Field strength (T) Field direction (B0) Dipole Precession RF pulse Lamor frequency Resonance Signal Noise Signal-to noise ratio (SNR) T1 T2 TR TE Echo Spin echo (SE) T1-weighted image (T1WI) T2-weighted image (T2WI) Proton density image (PDWI) Magnetism Types of magnets o Permanent Kitchen magnets, bar magnets o Electromagnet Electric motors, solenoids o Superconducting Special type of electromagnet Field strength o Varies with location within the field o Measured in tesla (T) Field direction o Varies with location within field o Typically referred to as B0 o Usually aligned with longitudinal axis of patient Paramagnetic properties T1 T2 (T2*) Relaxation time is defined as time required to relax 63% of the way 5x T1 or T2 is effectivelt 100% relaxed T1 relaxation Relaxation back towards the magnetic field direction B0 Signal decays exponentially Fat relaxes faster than water On the order of 200 to 2000 ms T2 relaxation Dephasing of precessing atoms Decays exponentially Much faster than T1 relaxation Affected by local field inhomogeneities (T2*) Water dephases slower than fat On the order of 20 to 200 ms It’s all about the HYDROGEN Why hydrogen? o Dipole – acts like a mini-magnet o Plentiful – very important Precession Atomic wobble Occurs about the main magnetic field Can be perturbed How do we perturb the precession? Knock it over using a radio frequency (RF) pulse o RF pulse – short burst radio wave Radio frequency needed to knock over termed the Lamor frequency (resonance) Precession Length of RF pulse determines how far the atoms gets knocked over Flip angle – how far the atom is knocked over o 90’, 180’ (RF pulses) Why does it get knocked over? RESONANCE! Resonance Natural frequency of a system Input into a system at the response frequency causes unbounded perturbations Signal THIS is what it’s all about! No image without a signal Listen for signal after pulsing It is the currency of MRI Noise This is signal what we don’t want o Comes from the background (lights, electronics, etc.) o Adjacent tissue Reduces image quality Signal-to-Noise Ratio (SNR) Measure of signal quality Want to maximize this o Increase signal o Minimize noise What causes the signal? Rotation in transverse plane cause RF signal In reality, signal strength decays to quickly to be usuable due to dephasing (T2*) Signal So how do we cause tissue to produce usable signal o Use RF pulses in a specific sequence to cause an echo o Echo – RF pulse given off by tissue due to rephrasing of processing nuclei o The echo is the signal we’re looking for The Pulse Sequence Spin echo technique o 90’ pulse cause precession in transverse plane o 180’ pulse causes rephrasing of the precessing nuclei o Echo produced The Echo 90’ RF pulse starts the nuclei processing in the transverse plane but they quickly diphase and signal is not generated 180’ RF pulse flips the spins causing them to temporarily rephrase and give off an RF pulse (the echo) Tissue contrast Contrast created by differences in T1 and T2 between different tissue types The greater the difference, the greater the contrast Pulse timing is modified to weight the image for T1 or T2 or a mix (proton density) Imaging Parameters TR- repetition time (time between 90’ pulses) TE- echo time (time between 90’ & 180’ pulses) Parameters vs. Image contrast Short TR Long TR Short TE T1-weighted Proton density weighted T2-weighted Long TE X Tissue Appearance T1-weighted image (T1WI) o Fluid is low signal o Fat is high signal T2-weighted image (T2WI) o Fluid is high signal o Fat is variable Proton density image (PDWI) o Fluid is generally intermediate signal, fat is generally higher signal o Generally used for anatomical detail Bone (cortical) and ligaments low on all sequences Marrow depends on red vs. yellow and T1 vs. T2 Muscle intermediates on all sequences Constructing an Image – The Crazy Math Key Terms Voxel Gradient Frequency Phase Slice selection gradient Spatial encoding Frequency encoding Phase encoding Fourier transform Fat suppression STIR Gadolineum Fast spine echo (FSE) Echo train Gradient echo (GE) Vocabulary Voxel – 3D extenstion of a pixel; represents a volume of tissue being imaged Gradient – variable field strength (B0) over distance Frequency – rate of repetition Phase – offset How do we select a slice? Remember precession and Larmor frequency o Precessing nuclei only knock over if RF pulse frequency matches the Larmor frequency, which varies with field strength Apply a gradient to the magnetic field o Slice selection gradient o Only nuclei within that slice will be pushed to precess in the transverse plane so only they will be able to generate signal Slice Selection TQ How do we get an image? Must have a way of determining where echoes are coming from We use spatial encoding to encode within the echo information which can be used to figure out where it came from o Uses gradient to encode the information into the frequency and the phase of the echo Phase encoding Apply a gradient during precession that causes nuclei within a particular row to precess at slight offset position By working back from that offset, we can determine which row a particular echo came from Frequency encoding Apply a gradient during the echo that causes all the nuclei within a particular column to echo at a particular column to echo at a particular frequency o Remember Larmor frequency varies with field strength (B0) By working back from the frequency of an echo we can determine which column a particular echo came from How do we work back? Fourier transform o Convert a wave function from the time domain to the frequency domain Making an image When we apply the Fourier process to the signals received, we are left with essentially 3 pieces of information for each voxel in the image o Its signal intensity o Its ‘x’ coordinate (frequency) o Its ‘y’ coordinate (phase) Pulling it all together TQ Typical Spin Echo Sequence TQ Advanced techniques Signal suppression o Fat suppression STIR Chemical suppression o Fluid suppression FLAIR Signal enhancement o Gadolinium Other echo techniques o Fast spin echo o Gradient echo Fat suppression STIR – Short tau inversion recovery o Takes advantage of the short T1 of fat o Add 180’ pulse to begin sequence o Remaining pulses timed to catch fat when it can’t produce a signal o Results in uniform suppression of fat o Only works in T1 and T2 o Can’t be used in Gadolinium Fat Suppression Chemical suppression (fat saturation) o Takes advantage of the slightly different resonance frequency of lipid molecules o Uses extra pulses and gradients to ‘spoil’ the fat signal o Can be used with any sequence type to suppress fat signal Fluid Suppression FLAIR TQ Same principle as STIR but pulsed timed to suppress water signal instead Provides enhanced contrast on T1WI Especially important in fast spin echo and ultrahigh field imaging where contrast is normally decreased Used mostly in brain and spine imaging Signal Enhancement Gadolinium o Paramagnetic compound o Reduces the T1 of the surrounding tissues causing hyper-intense signal o Distributed in the blood and rapidly enters the interstitial space o Does not cross the intact blood-brain barrier o Pattern of signal changes tells us something about the lesion o Often used with fat-saturation technique o Always compared to unenhanced T1WI Fast Spin Echo (FSE) TQ Gradient echo (GRE) TQ The Future TQ Nuclear Imaging – Scintigraphy and SPECT Radioactive Isotopes Atomic isotopes – same number of protons, different number of neutrons Some isotopes are radiotive and give off photons (14C example) These atoms can be included into complex molecules o Termed radiolabeled molecule Radiotracers Radiation (photons) given off by radioactive molecules can be detected Detecting the radiation allows us to track where the molecule is Tracking radiolabeled metabolically active molecules (radiotracers) can tell us physiological information What can it tell us? Where certain biochemical reactions are occurring Where certain type of cells are at or are going Where certain molecules are being stored Where blood is flowing Whether certain molecules are being absorbed by the body Whether certain biological barriers are intact The list could go on How does it work? As radioisotopes decay, they give of gamma rays Gamma ray photons hit a scintillator and is recorded by the computer There is usually an anterior and a posterior detector Resulting image shows where gamma photons were detected Photons generally detected in close proximity to where they were generated Multi-phase scans TQ What isotopes are used? Technetium-99m – Bone How about 3D? SPECT – Single photon emission computed tomography o Similar concept to CT, take pictures from different angles o Detector rotates around the patient o Computer calculate where photons where coming from in 3D space o Maybe combined with a CT machine CT for anatomy SPECT for physiology o Images can be fused together o SPECT data can be fused with separately acquired MR Image Fusion Images are registered and then overlaid o Automatic for SPECT/CT o Manual for SPECT/MR Shows anatomy and physiology simultaneously Nuclear Imaging – PET Positron Emission Tomography AKA – PET imaging Provides similar information to SPECT Uses fluorodeoxyglucose (18F) or 18F-FDG 18F gives off a positron as it decays FDG is a glucose analog and its transported in the body just like glucose More glucose = more metabolically active tissue What is a positron? It’s anti-matter Just like an electron but with a positive charge Matter and anti-matter don’t mix Causes an annihilation event that gives off 2 gamma photons 2 photons travel in opposite directions How it works? The computer looks for gamma photon pairs that strike 180’apart at nearly the same time Information is processed to produced slice data displaying local tissue uptake Image Fusion This data can fused with CT or MR data to provide anatomical info Together we can learn something about anatomy and physiology Currently dual PET/CT scanners are in use and PET/MR are being developed