MRI/CT/US Flashcards
How MRI works
A cross-sectional imaging modality where a STRONG MAGNETIC FIELD forces protons in the body to align with that field to produce images in multiple planes
A radiofrequency current causes the protons to be stimulated, and spin out of equilibrium, straining them against the pull of the magnetic field.
When the radiofrequency field is turned off, the MRI sensors detect the energy released as the protons realign with the magnetic field.
Contrast agents (e.g., GADOLINIUM) may be given to increase the speed at which protons realign with the magnetic field. the faster the protons realign, the brighter the image.
Radiofrequency (RF) Coils
RF coils are an essential MRI component used for transmission of the rf field to excite nuclear spins and for reception of the MRI signal.
RF coils play an important role in image quality in terms of
- SIGNAL-TO-NOISE RATIO
- SIGNAL UNIFORMITY
- IMAGE RESOLUTION
RF coils are associated with potential image artifacts and rf heating that may lead to
patient burns.
Proper selection and use of rf coils are therefore essential to:
- ACHIEVE PTIMAL IMAGE QUALITY
- TO MAXIMIZE DIAGNOSTIC CAPABILITY
- PREVENT IMAGE ARTIFACTS
- REDUCE THE RISK OF RF HEATING AND PATIENT BURNS
Quality control of rf coils is important to ensure proper functioning of the coils to provide
consistent image quality and avoid coil problems that may affect image evaluation or
interrupt patient imaging.
Contraindications of MRI Scans
Absolute contraindications:
Cardiac pacemaker
Cochlear implants
Anuerysm clips
Prgenant/ Possible pregnancy
Internal pacing wires
Metal /Piercings
Possible Contraindications:
Body piercing
Dentures
Joint replacement
Any metal fragments
Which contrast agent is used in MRI
Gadolinium
What does the acronym RALP stand for?
a) Right
b) Anterior
c) Left
d) Posterior
How CT works
A computerized x-ray imaging procedure where a narrow beam of x-rays is aimed at a patient and rotated around the
body, producing signals that are processed by the computer to generate cross-sectional images, or “slices.”
The collected slices are digitally “stacked” together to form a 3d image of the patient, allowing easier identification of basic structures and providing more detailed information about possible tumours or abnormalities.
- CT is based on the fundamental principle that the density of the tissue passed by the x-ray beam can be measured from the calculation of the attenuation coefficient
- CT allows the reconstruction of the density of the body, by two-dimensional section perpendicular to the axis of the acquisition system
How US works
- US can be portable
a) True - US can detect muscle and tendon tears?
True - On US, fluid appears
a) Black - US uses sound waves
True - If a patient presents to you with a suspected rotator cuff tear, which imaging modality are you
likely to refer the patient to?
US
US probes (aka transducers) use ceramic crystal materials (piezoelectric) to send and receive inaudible, HIGH FREQUENCY SOUND WAVES above the threshold frequencies of human hearing (above 20khz) to assess anatomy and patholgy
USED FOR REAL TIME VISUALIZATION OF STRUCTURES
PIEZOELECTRIC CRYSTALS create sound waves when an electric field is applied and vice versa. this allows the transducer to detect waves that bounce off boundaries between different structures (e.g., between tissue and bone).
The speed and timing of each reflected sound wave is measured to determine the distance between the transducer and the boundary which generates images.
During an ultrasound exam, gel is applied to the skin to prevent air pockets from forming between the transducer and the skin, blocking ultrasound waves from passing into the body.
How CT works: Windowing
CT windowing (aka grey-level mapping) - an image processing task that helps highlight the key anatomy so that the images can be analysed easily.
The most common window settings include:
1. bone window - for viewing the bones
2. soft tissue window – to evaluate soft tissues e.g., solid organs
and vasculature
3. lung window - for evaluating the lungs
4. abdomen window - to evaluate the abdominal cavity and its
contents
5. brain window - to evaluate the brain parenchyma (typically
on a non-contrast scan)
- sensitive at picking up intracranial
haemorrhage with a narrow window
Ultrasound - Interpretation
Fluid = BLACK
■ Tissue = GREY
■ The DENSER the tissue, the brighter white it will
appear
■ Bone will always be the brightest
■ Usually described by a tissue’s echogenicity (ability
to bounce soundwaves)
■ HYPERECHOGENIC = brighter as they have higher
echogenicity
■ HYPOECHOGENIC = darker as tissues have lower
echogenicity
■ ANECHOGENIC = areas that lack any echogenicity
appear totally dark
■ Contrast enhancement can be used in the form of
microbubbles
Which modality measures bone density and composition by passing low- and high-energy x-rays through the body?
DEXA
Which modality is used to diagnose osteopenia or osteoporosis?
DEXA
Which of these modalities can be used for musculoskeletal imaging?
a) MRI
b) US
c) CT
d) XRAY
- Which modality uses magnetic field to generate images?
MRI
CT is more expensive than MRI?
False
Tissues with high attenuation coefficient
Show up white
