Week 3 Flashcards
What does a MRI use for imaging?
A magnetic field that is 90,000X stronger than Earth’s magnetic field and acts by aligning atoms in the body and creates a magnetic moment or spin that aligns with the created magnetic field
The image and indices created by a MRI is based on what?
Tissue specific based on hydrogen and other molecular content
True or false
There is no radiation associated with MRI imaging
True, There is no radiation associated with MRI imaging
Once the field of a MRI is created, what does a MRI image do?
It utilizes a series of radiofrequency pulses that is emitted at different points at 90 deg angles of the magnetic field, and deflects the spin of hydrogen atoms into the transverse plane
What happens when the radiofrequency pulse is turned off?
There is an assessment of how much impact/ influence there was on the atoms and how much of rotation their spin was, which then signals decay due to relaxation which is subdivided into T1 and T2 phases
What is T1 recovery?
A longitudinal assessment of the influence on the spin, which is defined as the time it takes for spins to realign with original magnetic field/ how long it takes to get back to the resting state
What is T2 decay?
A transverse measure, which is defined by the time it takes for the spin to lose energy in the transverse plane. It measures peak energy pieces
True or False
T1 is longer than T2
True, T1 is longer than T2
What are the specific factors that make up T1 and T2?
Echo Time (TE) and Repetition Time (TR)
What is Echo Time (TE)?
After original pulse, a spin echo is created by applying a refocusing RF that flips the
spin 180 deg, and spins realign and produce a signal peak at TE
What is Repetition Time (TR)?
The time between successive 90 degree angle RF pulses
What are the characteristics of a T1 MRI that can help distinguish it when looking at an image?
- Short TR and TE times
- Image created at high energy levels, which provides good anatomic detail because tissues are visualized with lower relaxation times
- High signal intensity in fat (will appear white)
What are the characteristics of a T2 MRI that can help distinguish it when looking at an image?
- Long TR and TE times
- Image created at low energy levels, so the tissues are with longer relaxation times, meaning that they take on energy a bit slower and are reluctant to give it back
- The images are grainier and with less spatial resolution compared to T1
- High signal intensity in water, making it valuable at looking for inflammation (will appear white)
What does CSF or synovial fluid look like on a T1 MRI?
Low intensity
What does CSF or synovial fluid look like on a T2 MRI?
High intensity, due to its high water content
What would an acute hemorrhage or blood look like on a T1 MRI image?
High intensity (bright white)
What would an acute hemorrhage or blood look like on a T2 MRI image?
Intermediate to low intensity
What would soft tissue tumors look like on a T1 MRI image?
Low intensity or a dark grayscale
What would soft tissue tumors look like on a T2 MRI image?
High signal intensity
What would a fracture look like on a T1 MRI image?
Darker as compared to the white and high intensity normal bone
What would a fracture look like on a T2 MRI image?
High intensity and bright as compared to the dark, low intensity bone image normally seen in T2
What would early and late avascular necrosis look like on a T1 MRI image?
Dark and low intensity
What would early and late avascular necrosis look like on a T2 MRI image?
Bright and high intensity image
What does fat look like on T1 and on T2 MRI image?
T1: White
T2: Gray
What does water look like on T1 and on T2 MRI image?
T1: Gray
T2: White
What does marrow look like on T1 and on T2 MRI image?
T1: Bright
T2: Gray
What does cortex look like on T1 and on T2 MRI image?
T1: Dark
T2: Dark
What is a T1 MRI image good for visualizing?
Anatomy
What is a T2 MRI image good for visualizing?
Pathologies
What is SE?
A spin- echo pulse sequence. Seen in T1 and T2
What is GRE?
Gradient recalled echo sequences
What is STIR?
Short T1 Inversion recovery, which is good for suppressing fat
What is FLAIR?
Fluid attenuation inversion recovery, which suppresses water and is used in imaging the brain
What is TIRM?
Turbo inversion recovery magnitude, used in assessment of osteomyelitis in bone and head and neck tumors
What is gadolinium?
A radioactive nucleotoid, which is delivered via IV injection, and is used to decrease signal on T2 weighted imaging and increases signal on T1 weighted images
What are the risks and considerations to keep in mind when performing a MRI?
- Implanted hardware or foreign bodies
- Most Orthopaedic hardware is not magnetized
- You can get an MRI if you have a Total Hip/Knee
- However, does create artifact locally
- Would not get an MRI of your low back if you had pedicle screws
- If suspicion, pre-MRI x-ray
- Pacemakers are affected
- Time (30-60 minutes), keep in mind for claustrophobia/Pain
- Obesity
What types of organs are MRIs more indicated for?
- Soft tissues
- Ligament, tendon, muscle, cartilage
- Vascular demyelinating diseases
- Neoplasm, infection, inflammation, seizure
- Post-acute CVA, TIA, dementia
What types of organs is a CT more indicated for?
• Cortical bone • Chest, abdomen, pelvis • Fracture, loose bodies • Bony stenosis • Occult fractures in patients with osteoporosis • Initial evaluation CVA, trauma, hemorrhage
How does a CT scan create an image?
A combination of multiple images generated by ionizing radiation to provide cross sectional slices, including up to 1,000 projections in a normal study which can then be digitally put together in a computer program to create 3D views
What are the types of contrast that can be done in a CT?
- CT arthrogram
* CT myelogram
What are the advantages of a CT scan?
• Evaluates osseous structures very well • Evaluates soft tissues moderately well • Evaluates cortical bone better than MRI • Modality of choice: - Subtle or complex fractures - Degenerative changes in joints - Serious trauma - Spinal stenosis - Loose bodies in joints - Measurement of osseous alignment • Lower expense and time than MRI
MRI has largely replaced a CT with contrast. What are the exceptions to this?
- Patient to large for scanner
- Claustrophobia
- Implanted pacemaker or metal implant
What are the risk and limitations of a CT scan?
• Radiation Exposure, and is even higher with contrast
• Limitations in histological makeup, because the image is constructed based on radiodensity. EX: tumor may look the same as muscle surrounding it if there is
similar or same density
What is an image artifact?
Any feature which appears in an image, which is not present in the original image/object
Why are image artifacts a problem?
They may obscure a pathology or simulate a pathology that is not present
What are some artifacts that may be encountered in a CT scan?
- Noise
- Beam hardening
- Scatter
- Pseudoenhancement
- Motion
- Cone beam
- Helical
- Ring
- Metal
What causes a ring artifact in a CT?
A miscalibrated or defective detector element, which results in a ring centered around the center of the image. This can be fixed with a recalibration of the machine
What causes a poisson noise in a CT scan?
A statistical error of low photon counts, that results in random thin bright and dark streaks that appear preferentially along the direction of greatest attenuation. This can be reduced by combining data from multiple scans
What are the characteristics beam hardening and scatter in a CT?
Mechanisms that produce both produce dark streaks between two high attenuation objects, such as metal, bone, iodine contrast or barium. They can also produce dark streaks along the long axis of a single high attenuation object
Where are beam hardening and scatter a particular problem on a CT scan?
In the posterior cranial fossa with metal implants
What are the types of motion that can cause blurring, double images, and long range streaks in a CT?
- Voluntary by the patient
- Heartbeat
- Respiratory movement
- Bowel movement
When a motion causes a long range streak, where does it occur?
Between high contrast edges, and the xray tube position
How can motion artifacts be prevented?
Using faster scanners, which can be achieved by faster gantry(bed) rotation, or more xray sources
What causes a metal artifact?
Metal objects or fragments in patients, such as aneurysm clips, wires, dental fillings, joint replacements and etc
What are the artifacts seen in a MRI?
- Physiologic artifacts, which are caused by patient movement, which include breathing, bowel movement, and heart beats
- MRI physics, which is affected by the presence of metal and chemical shift
- MRI hardware and room shielding
- MRI software
- Interference with the MRI unit
What causes a phase- encoded motion artifact seen in a MRI?
The result of tissue or fluid moving during the scan, that manifest as ghosting in the directional phase of coding
What causes a chemical shift artifact in a MRI?
Occurs during the frequency encoding of the MRI process, resulting in a bright/whit band around the image
When are magnetic susceptibility artifacts seen?
While imaging near metallic objects
What causes a magnetic susceptibility artifacts?
A local magnetic field being induced by the metallic object near the otherwise homogenous magnetic field. The local magnetic field causes the image to have bright and dark areas near the magnetic object that are not anatomically correct
What is a zipper artifact?
Bands of electronic noise that extend perpendicular to the frequency encoding direction and will be present in all the images of the series
What are the causes of a zipper artifact?
Mostly related to hardware and software problems beyond the radiologist control. Some are caused y radiofrequencies entering the scanning room from electronic devices such as mobile devices or aircraft. Others can be caused by blinking lights or radiotransmitters
What is ionizing radiation?
Radiation that frees electrons from atoms or molecules
What are the categories of the adverse reactions seen with ionizing radiation?
- Deterministic effects (tissue reactions)
* Stochastic effects (cancer and heritable effects)
What are deterministic effects seen as an adverse reaction to ionizing radiation and their causes?
Harmful tissue reactions, which are due to the killing and malfunction of cells following large doses of ionizing radiation.
What are stochastic effects seen as an adverse reaction to ionizing radiation and their causes?
The cancer development in exposed individuals, owing to mutations of somatic cells or heritable diseases in offspring due to mutation of reproductive cells
What are the ways that radiation dose exposure is quantified?
- Air Kerma
- Dose‐area product
- Absorbed Dose
- Equivalent Dose
- Effective Dose
What is air kerma?
The dose of radiation that is absorbed in the absence of scatter, delivered to a volume of air
What is dose area product?
The integral of air kerma for the entire xray beam that is emitted from the xray tube
What is the absorbed dose/ CT dose index?
The dose from the primary beam plus scatter from the surrounding slices, which is in the units of gray
What is the equivalent dose?
The average computed measure of absorbed dose, adjusted by weighting factors, such as type of radiation damage to potential organs
What is the effective dose?
The adjustment of equivalent dose to compare the doses of radiation to different parts of the body on an equivalent basis
What is the effective dose used for?
Used to compare radiation doses on body parts on an equivalent basis. Ex: gonads and marrow are more sensitive to ionizing radiation
How is the effective dose obtained?
It is obtained when organ doses are adjusted by the international commission of radiology
What are the characteristics of an effective dose?
It takes into account the risk of nonuniform exposure to the radiation in radiological studies and is calculated by taking the whole body equivalent dose average weighted to different body tissues and adjusting it by tissue weighing factors
Why are pediatric patients more likely to develop cancer following radiation exposure as compared to geriatric patients?
Children are more susceptible to radiation due to:
- Rapid cellular growth and cell division, therefore there is increased vulnerability of damage to their DNA
- Peds exposed to radiation experience an increased chromosome aberration
- Children have a longer life expectancy than adults, hence a longer time to develop cancer risk
What are the organs that are at risk of developing cancer in a pediatric patient?
- Thyroid gland
- Breast
- Bone marrow
- Brain
- Skin
Increased risk of ___ has been identified following post natal diagnostic xrays
Increased risk of leukemia has been identified following post natal diagnostic xrays
