Radiology: Anatomy - Medical imaging techniques Flashcards
Outline the most to least radiopaque substances found in the body
Compact bone
Spongy bone
Water and most tissues
Fat
Air
How does conventional radiography work?
Highly penetrating beam of X-rays transilluminates the patient, showing tissues of differing densities of mass within the body as images of differing intensities on the film or monitor
Tissue or organ that is relatively dense (e.g. compact bone) absorbs or reflects more X-rays than less dense tissue (e.g. spongy bone), and so produces a bright area because fewer X-rays reach the film or detector
How does CT work?
Beam of X-rays passes through the body as the X-ray tube and detector rotate around the axis of the body
Multiple overlapping radial energy absorptions are measured, recorded, and compared by a computer to determine the radiodensity of each volumetric pixel (voxel) of the chosen body plane
The computer maps the voxels into a planar image (slice)
How does ultrasonography work?
Visualises superficial or deep structures in the body by recording pulses of ultrasonic waves reflecting off tissues
Transducer in contact with skin generates high-frequency sound waves that pass through the body and reflect off tissue interfaces between tissues of differing characteristics (e.g. soft tissue and bone)
Echoes from the body reflect into the transducer and convert to electrical energy
Electrical signals are recorded and displayed on the monitor as a cross-sectional image
Why does ultrasonography of the pelvic viscera require a full bladder?
Full bladder serves as an “acoustical window”, transmitting sound waves to and from the posteriorly placed pelvic viscera with minimal attenuation
Distended bladder also displaces gaseous intestinal loops out of the pelvis
Why does ultrasonography of the pelvic viscera require a full bladder?
Full bladder serves as an “acoustical window”, transmitting sound waves to and from the posteriorly placed pelvic viscera with minimal attenuation
Distended bladder also displaces gaseous intestinal loops out of the pelvis
What is the difference between bone and air in terms of their response to ultrasound waves?
Bone reflects nearly all ultrasound waves, whereas air conducts them poorly
How does MRI work?
Appearance of tissues on generated images can be varied by controlling how radiofrequency pulses are sent and received
Free protons in the tissues that become aligned by the surrounding magnetic field are excited (flipped) with a radio wave pulses
As the protons flip back, minute but measurable energy signals are emitted
Tissues high in proton density, such as fat and water, emit more signals than tissues low in proton density
Tissue signal is based primarily on three properties of protons in a particular region of the body (T1 and T2 relaxation, and proton density)
What do T1 and T2 mean?
T1 (longitudinal relaxation time) is the time constant which determines the rate at which excited protons return to equilibrium. It is a measure of the time taken for spinning protons to realign with the external magnetic field.
T2 (transverse relaxation time) is the time constant which determines the rate at which excited protons reach equilibrium or go out of phase with each other. It is a measure of the time taken for spinning protons to lose phase coherence among the nuclei spinning perpendicular to the main field.
Are moving fluids light or dark on T1-weighted MRI?
Moving fluids (including CSF) are dark on T1-weighted MRI
This is because although they have a high density of free protons, the excited free protons tend to move out of field before they flip and give off their signal
What are some of the advantages of MRI compared with other imaging modalities?
Better for tissue differentiation
Tissues can be reconstructed in any plane from the data acquired, including the generation of a three-dimensional reconstruction
How does nuclear medicine imaging work? Give an example
Provides information about the distribution or concentration of trace amounts of radioactive substances introduced into the body
Shows images of specific organs after IV injection of a small dose of radioactive material
Radionucleotide is tagged to a compound that is selectively taken up by the target organ (e.g. technetium-99m methylene diphosphonate, 99mTc-MDP, for bone scanning)
How does PET work and what is it used for?
Positron emission tomography (PET) scanning uses cyclotron-produced isotopes of extremely short half-life that emit positrons
Used to evaluate the physiologic function of organs on a dynamic basis
What is the difference between PET and SPECT?
Single-photon emission computed tomography (SPECT) scans are similar to PET but use longer-lasting tracers
Less costly but require more time and have lower resolution
