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MRI
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- MRI is an imaging modality that uses magnetic fields and radiofrequency pulses.
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- Protons align = parallelism
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- Protons lose their parallelism
- Protons gain synchronized precession (precession refers to the axis of spin of the protons)
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- T1 = time to return to parallelism
- T2 = time to lose precession
- TR = Repetition time between successive pulse sequences
- TE = Time to echo from RF pulse delivery to echo signal receipt
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- Spin echo pulse to create T1 and T2 weighted images
- T1-weighted - for demonstrating anatomy
- T2-weighted - for pathology
- Short Tau Inversion Recovery Sequence (STIR) = fat suppression sequence
- MARS (metal artifact reduction sequence)
- Proton Density Sequence - useful for structures of intermediate signals e.g. meniscus, cartilage
- Gradient Echo sequence - useful for fibrocartilaginous structures - TFCC, meniscus
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- Absolute - Cochlear implants (main one)
- Relative - certain pacemakers, metal implants
- Patient factors - claustrophobia (open MRI, sedation)
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- When tendon or tissue is oriented at 55° to the magnetic field, it produces bright T1 images - creating a false appearance of pathologic process
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- It has paramagnetic properties with 7 unpaired electrons
- Cleared rapidly through kidneys
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- Nephrogenic Systemic Fibrosis - rare, potentially fatal condition caused by iatrogenic gadolinium administration in patients with acute kidney injury or stage 4 or 5 chronic kidney disease (CKD)
- Also known as nephrogenic fibrosing dermopathy (NFD), a disease causing fibrosis of the skin and internal organs, similar to but distinct from scleroderma or scleromyxedema
- NSF is a specific complication of gadolinium exposure
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X-RAY/ CT SCAN/ DEXA Scans
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- A form of high energy radiation at the upper end of the electromagnetic spectrum
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- XR are rays in the electromagnetic spectrum
- 1 - Electrons are produced at the cathode through heating of tungsten to 2200°C [Keyword: Thermionic Emission]
- 2 - Electrons strike the TUNGSTEN anode at three potential sites: nucleus, outer electrons, and inner electrons. Only collisions with the nucleus and inner electrons produce "braking Radiation XR" (outer electron collisions only produce heat)
- 3 - XR are differentially absorbed by body tissues based on attenuation
- 4 - Remaining XR reach the detector to create either a digital signal or expose film (made of silver iodobromide—the more XR exposure, the darker the film becomes)
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- X-ray tube produces X-rays → rays pass through body → reach receiving drum
- At the receiving drum, X-rays are converted to light photons at the input phosphor
- Light photons are converted into electrons at the photocathode
- Electron beam travels through a vacuum tube containing focusing electrodes, then strikes the output phosphor where it converts back into visible light
- Light photons are converted into electrons and digitized to create the final image
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- Total gain occurs through minification gain and flux gain
- Due to the input phosphor having a larger area than the output phosphor, there is a minification gain (image is made smaller)
- Additionally, each photon at the input phosphor generates 1,000 photons due to acceleration through the tube, resulting in flux gain (1:1000)
- The combination of minification gain and flux gain leads to an increase in brightness of the image by several thousand times (intensified)
- As a result, the image intensifier requires less radiation to produce clear images
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- Patient should be placed nearer to the detector (Blue) and further from the emitter (Red) to reduce radiation exposure and the per-area dosage to the body
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- Transmitted through tissues
- Absorbed by tissues (attenuation)
- Scattered
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- Attenuation is the degree to which tissue absorbs and blocks X-ray transmission
- Tissues are assigned an "attenuation coefficient" measured in Hounsfield units, relative to water
- Water given a value of 0; tissues denser than water have positive values, while less dense tissues have negative values
- Water 0, Air -1000, Bone +1000
- Less dense tissues appear darker, while denser tissues appear brighter
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- "3 cardinal rules - Time, Distance, Shielding"
- Time - reduce image intensifier time
- Distance - maintain more than 90cm distance for safety
- Shielding - wear lead gowns (store properly without creasing), and goggles
- Additional measures: Position X-ray tube below operating table, keep image intensifier close to patient, avoid over-magnification, use collimation, inform team before exposure
- Monitor exposure with Geiger-Müller Counter/Dosimeter - if limits exceeded, take a radiation holiday
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- Plastic case with one quarter covered by lead and another quarter with thinned plastic
- Control test strip is placed in the lead-covered quarter - should receive no radiation
- Test strip in the thinned plastic quarter serves as the test - receives maximum radiation exposure
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- Collimated X-ray source projects a fan-shaped beam with a collimated detector opposite
- Beam passes through tissue - Tissue Attenuation based on Hounsfield Units determines tissue's X-ray absorption
- Multiple image slices combine to create 3D rectangular tissue boxes known as voxels
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- Collimation reduces the X-ray beam window size, resulting in sharper radiographs and lower radiation exposure.
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- Dual Energy X-Ray Absorptiometry
- A machine that produces two X-ray beams of different energies, which are absorbed in different proportions by bone and soft tissue.
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- It measures vertebral bodies L2 to L4 and calculates their mean density.
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- False Negative (bone density appears better than it is) - Conditions that cause sclerosis or increased localized bone deposition
- Degenerative changes in spine or hip, healed fractures, avascular necrosis, benign or metastatic bone-forming lesions
- Metal implants increase "bone density"
- False positive (bone density appears lower than it is) - Conditions that cause removal of local bone
- Laminectomy defects, lytic lesions, spina bifida
- This is why images are provided - to identify possible false positives or negatives.
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- DEXA measures apparent density because it is a 2D measurement, quantified in g/cm²
- Quantitative CT gives a 3D volumetric representation of bone density
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- Due to high variability both within and between patients, effects are difficult to determine in the short term
- It is recommended to repeat the scan 3 years after starting treatment
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ULTRASOUND (US)
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- It is a high frequency sound wave.
- Waves are either reflected or refracted at tissue interfaces.
- Fluid-filled structures appear low echogenic, while fat-filled structures appear high echogenic.
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- Reflected - High acoustic mismatch results in high reflection.
- Absorbed - Sound waves create friction between oscillating particles, producing heat.
- Refracted - Beam bends when crossing an interface at an oblique angle.
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- It consists of three components: transducer, monitor, and computer unit.
- 1 - The transducer contains piezoelectric CERAMIC crystals. When voltage is applied, they produce sound waves.
- 2 - These sound waves travel to tissues where they are reflected, absorbed, or refracted.
- The amount of wave reflection at interfaces depends on the difference in acoustic impedence. Greater differences in acoustic impedance result in more reflected energy.
- 3 - Returning sound waves (echoes) are detected by the transducer to generate signals.
- 4 - The computer unit converts these signals into images.
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- Acoustic impedance is determined by tissue density, increasing from air to soft tissue to bone.
- The larger the difference in acoustic impedance, the more energy is reflected, creating a brighter image.
- At interfaces between soft tissue and air or bone, almost all energy is reflected due to large impedance differences, preventing deeper imaging.
- This explains why coupling gel is needed between probe and skin to eliminate air.
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- Therapeutic - Guiding procedures, biopsies, and injections
- Diagnostic - Assessing tendons and masses
- Screening - DDH
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- (+) No radiation, compact, cost-effective, portable, enables dynamic screening
- (-) Operator-dependent, limited views, poor penetration of deep structures
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- The Doppler effect is a change in sound wave frequency caused by motion, similar to how a siren's pitch changes as it passes by.
- For blood flow assessment, differences between echo and transduced pulse frequencies create the Doppler signal.
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NUCLEAR IMAGING
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- Imaging modality that uses a radioisotope or Positron emitting isotopes coupled with a carrier substance that binds to target tissue
- Gamma rays are produced by the radioisotope as it decays
- Positrons are produced by positron emitting isotopes
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- Technetium 99m
- Indium 111
- Gallium-67
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- 6 hours
- 70% excreted in 24 hours
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- Radioisotope Technetium 99m is coupled with Methylene Diphosphate compounds (99TcMDP), which have affinity for osteoblasts
- Tracer is injected into body
- Gamma camera records photoemissions in 3 phases:
- 1 - Flow (immediate - shows arterial flow)
- 2 - Blood pool (5 min post-injection - shows vascularity)
- 3 - Static aka delayed phase (3 hours post-injection - shows bone activity)
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- Degenerative spine with compression fractures
- Bone infarcts secondary to steroid use, sickle cell disease
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- Hydration - need to hydrate and urinate frequently as radioisotopes are excreted by kidneys. Wash hands well after toileting
- Breast feeding - express milk before procedure. No breastfeeding for at least 4 hours, preferably 24 hours after procedure
- Avoid children - after scan
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- Cold = osteoclastic activity >> osteoblastic - RCC, MM
- Aggressive cancers with high osteoclastic activity can also be cold, e.g., breast cancer
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- Inflammation shows increased flow and pool phase but is negative in the static/delayed phase
- Osteomyelitis shows increased activity in all 3 phases
- Medial Tibial Stress Syndrome (not yet a stress fracture): Shows normal arterial and blood pool phases but longitudinal uptake on delayed images
- Stress fractures are "three phase positive"
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- Paradoxical increase in uptake and size of bone scan following chemotherapy for metastatic lesions. May indicate bone repair after successful chemotherapy
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- PET stands for Positron Emission Tomography
- Cyclotron produces positron-emitting isotopes
- Isotopes are tagged to glucose (e.g., Fluorodeoxyglucose - FDG)
- After injection, high-grade malignancies show greater FDG uptake than low-grade/benign lesions due to higher glycolysis rates
- Camera detects positron emission decay
- Computer collects data - integrated with PET CT or MRI
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- Indium-111 - autologous leukocytes are obtained, labeled with indium-111, then reinjected
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The Lord by wisdom founded the earth; by understanding he established the heavens Proverbs 3:19