IGRD110: CT, MT, and US Imaging

Computed Tomography: Basic Principles
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Computed Tomography: Basic Principles
based on the same physical principles as conventional radiographs (x-rays).
X-rays
penetrating EM radiation that travels in a straight line at the speed of light. Paths are not changed by electrical or magnetic fields. The degree of penetration depends on their energy and the characteristics of the material through which they pass. Possesses enough energy to ionize matter and can therefore damage living cells.
Conventional radiographs
x-rays are generated and emitted from a stream of high-velocity electrons. X-rays are then directed as a beam through a body area. All tissues differ in their ability to absorb x-rays. X-ray films record the x-ray radiation that reaches them.
CT: Rapid scanning minimizes motion artifact and allows acquisition of images...
in multiple phases of contrast enhancement (ex. arterial and portal venous phases in the liver)
CT: Specific organs can be highlighted by variations in the gray tone scale in the processes known as...
windowing and leveling
Image: CT: Specific organs can be highlighted by variations in the gray tone scale in the processes known as...
CT: Magnified views, accurate measurements, and 3D reconstructions are possible. Measurements of relative tissue density can be calculated...
for each pixel and numerically represented as Hounsfield units for comparison with reference tissues.
CT: Scans are often performed after...
administration of gastrointestinal contrast or with timed doses of intravascular contrast media to enhance the differences between adjacent structures.
CT: can also be a valuable therapeutic adjunct because it allows for...
precise 3D targeting for invasive procedures such as percutaneous drainage, tube thoracostomy, and percutaneous biopsies.
CT: Detector
Portion of CT scanner at gantry that detects radiation that has penetrated the object being scanned
CT: Collimation
Process of controlling the size and shape of the x-ray beam that emerges from the x-ray tube or source
CT: GantryDonut-shaped portion of CT scanner that contains both the x-ray source and detector(s)CT: Helical CT/Spiral CTthe rotating x-ray beam traces a helical or spiral path around the patient as the CT table moves continuously through the gantry during scanning, in distinction to earlier scanners, in which the table stepped incrementally as each slice was obtained.CT: Hounsfield Unit (HU)arbitrary unit used to represent the x-ray attenuation capacity of tissue depicted by each pixel. water = 0; bone = 300CT: Multidetector CT/multirow detector CT/multislice CT.Machine making simultaneous use of multiple rows of detectors that allow acquisition of a volume of data in a single gantry rotation such that multiple slices can be reconstructed from the data acquired in that single rotation.CT: Multiplanar reconstructionProcess of reformatting CT images in multiple planes based on data analysis of a volume of CT data obtained helically. This process is practical only with multislice scanners that have the necessary longitudinal (z-axis) resolution to allow distortion-free reconstruction. Reconstruction is possible in any anatomic plane that might be useful to visualize the anatomy of interest, although standard coronal and sagittal planes are most commonly used.CT: PhaseA set of CT images obtained during a single passage of the patient through the scanner, usually after the administration of contrast mediaCT: Pitchdescribes the distance that the patient table travels during one 360 rotation of the gantry divided by the collimated width of the x-ray beam. Pitch =1 when the entire patient surface area is scanned without overlap/gap in the x-ray beam.CT: SliceSingle tomographic reconstruction of an anatomic cross-sectionCT: WindowingProcess of determining the contrast and brightness levels assigned to CT image data. The optimal window level and width vary according to the tissue of interest (ex. lung vs soft tissue).CT: X-ray source/x-ray tubePortion of CT scanner located within the gantry that generates penetrating electromagnetic radiationCT: More than 90% of the collective population dose of radiation from diagnostic radiographs comes from a few high-dose procedures, including...CT scans, interventional radiology studies, and barium enemasCT: The risk of malignancy from a given radiation exposure is greater in...young children and womenCT: Radiation exposure risks decrease with...increasing age in both sexesCT: Of all CT scans, those focused on the abdomen and pelvis impart...the greatest radiation exposure, particularly when multiple acquisition phases are usedCT: Methods to reduce the radiation dose delivered to a patient include *reducing/minimizing*...milliampere-seconds value (according to patient size), beam energy, the # of phases per study, an area being imagedCT: Methods to reduce the radiation dose delivered to a patient include *increasing*...pitch, width of x-ray beam collimation (for multirow detector scanners),CT: To reduce the radiation dose delivered to a patient, reconstructions using existing accessible data...should be generated in lieu of additional scans/phasesCT: Contrast-induced nephropathy (CIN)the term used to describe nephrotoxicity associated with the use of intravascular iodinated contrast media. The pathophysiology involves contrast-induced reductions in renal perfusion and renal tubular flow resulting in an acute reduction in glomerular filtration rate (GFR). Risk factors include preexisting chronic kidney disease (CKD) and diabetes.CT: Strategies to prevent CIN include...use of intravenous fluid hydration before and after a CT scan with contrast and ordering alternative imaging tests that do not use radiographic iodinated contrastMagnetic Resonance Imaging: Basic Principlesbased on the magnetic properties of hydrogen nuclei in human tissue.MRI: When an outside source of strong magnetic energy is applied, the magnetic fields of the hydrogen atoms align themselves parallel to...the direction of the external magnetic field, along an axis parallel to the length of the patient called the longitudinal axis or z-axisMRI: A radiofrequency (RF) pulse is applied that is specifically targeted to hydrogen atoms. The pulse frequency is chosen such that it is equal to...the frequency of precession of the tiny magnetic field of each hydrogen atom.MRI: The RF pulse results in the efficient absorption of energy by the hydrogen atoms known as...resonance, causing the hydrogen atoms to tip and assume a new orientation (ex. longitudinal to transverse axis) of precession.MRI: When the RF pulse is terminated, the precessing hydrogen atoms return to their baseline alignment, releasing RF energy that can be...detected and converted into an image. This emitted RF energy is the echo measured in so-called spin-echo MRI pulse sequences.MRI: The frequency of the spinning protons/hydrogen atoms (ie. the frequency of the RF pulse needed for resonance) is determined by...the Larmor equationMRI: Tissue contrast in MRI is related to...differences in rates of magnetization decay and repolarization within specific tissues (so-called T1 and T2 constants, which vary in different tissues) as well as the proton content of different tissues.MRI: The differences in T1 and T2 constants can be exploited by...selecting scan parameters that highlight these characteristics. T1-weighted images use short intervals between application of RF pulses (short TR) and short intervals between pulse application and signal acquisition (short TE). T2-weighted images emphasize differences in the T2 time constant of various tissues by using long TR and long TE.MRI: Diffusion MRIMRI that measures the diffusion of water molecules in different tissues. This is a highly sensitive study for the diagnosis of acute ischemic stroke.MRI: GadoliniumElement with paramagnetic properties that is used as an intravenous contrast agent in MRIMRI: Gradient magnetsThree paired coils each oriented along one of the orthogonal axes, designed to produce gradients within the main magnetic field. These gradients allow spatial encoding of points within the image.MRI: Larmor equationEquation describing the relationship between an externally applied magnetic field and the frequency of precession of nuclei in that fieldMRI: Pulse sequencesA preselected set of defined RF pulses and magnetic field gradient applications used to produce MR imagesMRI: RF pulseElectromagnetic signal emitted to cause resonance of hydrogen nuclei in the magnetic field. This process leads to the eventual release of RF energy, which is measured and used to create the MR image.MRI: RelaxationThe process that occurs after termination of the RF pulse. The system of precessing nuclei relax, going from a higher energy state to a lower energy state and releasing RF energy, which is detected and converted to the MR image.MRI: T1 relaxation (spin-lattice relaxation)regrowth of longitudinal magnetization as the spinning nuclei realign with the external magnetic field. The rate of this regrowth is tissue specific, and described by the T1 time constant of various tissues.MRI: T1-weighted imagesImages obtained using short intervals/repetition times (TR), between application of RF pulses. The time to echo (TE), or the time at which the emitted RF signal caused by relaxation is measured, is also short. Using short TR and TE produces images in which tissue contrast is produced primarily by differences in the T1 time constant of various tissues.MRI: T2 relaxation (spin-spin relaxation)Decay of magnetization in the transverse plane after the RF pulse is terminated. The rate of this decay is tissue specific, and described by the T2 time constant of various tissues.MRI: T2-weighted imagesImages obtained using long intervals, or TR, between application of RF pulses. The TE, or the time at which the emitted RF signal caused by relaxation is measured, is also long. Using long TR and TE produces images in which tissue contrast is produced primarily by differences in the T2 time constant of different tissues.MRI: Contrast media containing ______ can be administered intravenously to highlight differences seen in tissues on MR images.gadoliniumMRI: diffusion MRI has been developed to measure the diffusion of water molecules in certain tissues. This study is highly sensitive for the diagnosis of...acute ischemic strokesMRI: The advantages of MRI include...the absence of ionizing radiation and the infrequent need for contrast mediaMRI has been especially useful for the diagnosis of...multiple sclerosis, brain neoplasms, soft-tissue masses, bone masses, spinal injuries, joint injuries, and infections of the brain, joints, spine, and soft tissues. It also remains the most accurate imaging modality for the early diagnosis of ischemic stroke.MRI: Disadvantages include...high cost, limited availability in certain locations, inability to scan patients with some implants (ex. pacemakers), long scan times, susceptibility to metallic and motion-related artifacts, and need for sedation in claustrophobic patients.MRI: TeslaInternational unit of magnetic field strength/degree of magnetic inductionMRI: Time-of-flightTechnique using flow-related enhancement to generate images of blood vessels without the use of contrast mediaMRI is contraindicated in patients with...permanent cardiac pacemakers, metallic fragments within the eyes, and ferromagnetic intracranial aneurysm clipsUltrasound: Basic Principlesbased on the pulse-echo principle in which sound waves are transmitted through a medium and reflected backUS machines consist of a transducer...that both emits the ultrasonic sound waves and detects the echo.US: The piezoelectric crystals within the transducer...both convert electrical energy into acoustic waves and reverse the process when returning sound waves are receivedUS: For B-mode US, the brightness of each pixel...corresponds to the amplitude of the reflected sound waveUS: Hyperechoic structures appear...brighter than surrounding tissuesUS: Hypoechoic tissues appear...darker than surrounding tissues.US: The strength of an echo is determined by...the differential acoustic impedance between adjacent structures.US: Images can be made clearer by changing the...angles of the transducer, increasing piezoelectric crystal size, changing the US frequency, or focusing the sound beam.US: Acoustic impedanceProperty of tissue defined as the product of the tissue density multiplied by the velocity of sound waves traveling through the tissue. The strength of an echo is determined by the difference in impedance between adjacent tissues.US: Acoustic intensityPower of a US pulse per unit of cross-sectional areaUS: AttenuationLoss of US signal strength as it passes through tissue as a result of absorption and scatteringUS: B-modeBrightness mode US. Brightness of each pixel corresponds to amplitude of the reflected sound wave.US: Color-flow DopplerUS image in which flow (typically blood flow) is color coded for motion toward or away from the transducer. The image produced is a hybrid combining anatomic information from a B-mode image as well as pulsed Doppler analysis.US: Continuous-wave DopplerDevice with separate transmitting and receiving probes that allows for continuous transmission of sound waves. Used to detect flow within blood vessels.US: Doppler USDevice that uses Doppler shift effect of sound reflected off a moving object. The magnitude of the frequency shift can be used to calculate the velocity of moving objects such as red blood cells.US: Duplex USUS using both B-mode imaging and Doppler waveform analysisUS: EchogenicCapable of producing echoes. Gallstones are very capable of producing echos and thus have higher _______, while fluids are less ______.US: GainAmplification of the returning US signalUS: HyperechoicImage echoes are brighter than surrounding tissuesUS: HypoechoicImage echoes are darker than surrounding tissuesUS: IntensityMagnitude or strength of US signalUS: Piezoelectric propertyMaterial property by which mechanical motion/deformation is converted into electrical current, and vice versa.US: Pulsed-wave DopplerTransmission of sound waves in pulses. Transmission and reception are performed by the same transducer. The depth of a tissue echo can be measured and therefore localized in 3 dimensions.US: Doppler spectrum/Doppler waveformThe range of frequencies/frequency shifts measured over time, which is converted by the US unit to velocity measurements. When velocity (y-axis) is plotted versus time (x-axis), the result is the characteristic waveform seen when interrogating a blood vessel. Flow velocity is related to vessel stenosis.US: TransducerDevice from which US waves are emitted and echoes are detectedUS: As US frequency is increased to improve axial resolution, the depth of tissue penetration...is decreased. This property makes US especially well suited for evaluating superficial structures such as the thyroid, breast, and carotid artery.US: Machine gain can be adjusted...to increase the volume of echoes.US: Doppler US is a useful adjunct for interrogating...moving objects, such as red blood cells within blood vessels.US: Using the Doppler shift effect of sound reflecting off a moving object, the velocity of the object...can be calculated by analysis of the frequency changes. Increases in velocity are then related to stenoses in the involved vessel.US: Spectral analysis of the reflected frequencies is used to...calculate the flow velocity within a blood vessel.US: Color flow Doppler US equipment color codes for...the direction of flow within a vesselUS: Ideal as an adjunct for numerous bedside or operating-room procedures due to...real-time capability and mobility of the equipmentUS: Specialized probes have enhanced use in...transvaginal, transrectal, and endoscopic settings.US: Although US has no ionizing radiation, US energy has the potential for...inducing temperature increases and cavitation of tissues as well as triggering an inflammatory response.