CT review Flashcards
pre procedure communication
-identify patient-2 identifiers
-pregnancy status if applicable
-identify allergies
-inquire any previous diagnostic exams to determine no contrast was used that would interfere with CT exam
-explain procedure
-inquire about medications
clinical history
-obtain patient history
-document recent procedure, surgeries, symptoms, possible trauma, specific areas of pain
-screening tools may be used
-other benefits
Education
-explain procedure including
-breathing instructions
-Warm feeling, metallic taste from contrast
-post exam provide follow up care instructions
consent
1.must relate to treatment
2.must be informed
3.must be given voluntarily
4.must be obtained through misrepresentation of fraud
-risk and benefits are part of consent
-forms and signatures
-consent can be obtained by a legal guardian for a child or patients unable to consent
Level of consciousness
normal-alert, awake, able to respond
lethargic-appears drowsy but can respond or be aroused
obtunded-more depressed level, not easily aroused
stupor-state of near unresponsiveness(semicomatose)
coma-paient is completely unresponsive
Hypoxemia meaning
insufficient oxygen in the arterial blood
hypoxia meaning
insufficient oxygen levels in the tissues
hypoxia symptoms
headache
nausea
dizziness
ataxia
cyanosis
if localized may result in pain, cyanosis or cell death
oxygen administration
1.nasal cannula: rate of 1-5 LPM
2.oxygen mask: rate of 6 LPM or more
3. ventilator: used when patient does not have sufficient airway
Chest tubes
-used to drain fluid from intrapleural space or when pneumothorax is present
-do not displace or dislodge chest tube in transporting patient
-drainage system must be kept lower than chest
lab values
-acceptable values
BUN: normal range 7-25 mg/dL
creatinine: normal range 0.5-1.5 mg/dL
eGFR: normal 100ml/minute
PT(prothrombin time) normal 12-15 seconds
what happens when a eGFR is less than 30mL/min
the patient is more at risk for AKI(acute kidney injury) or CIN(contrast Induced Nephropathy)
metformin
held at time of CT exam and then withheld for 48 hours post contrast media injection
-especially important for people with a eGFR less than 30mL/min
Types of Contrast Media
-negative CM: low atomic #, air, gas granules, water
-Positive CM: barium, iodine
-barium sulphate can be used as a positive CM for opacification of GI tract
how can iodinated radiopaque CM be administered?
into bloodstream intravenously
into the intrathecal space for myelography
into the joint space during CT arthography
IV radiopaque CM
-initial opacification of blood vessels
-aids in diagnosis or aneurysm, thrombus, stenosis
-osmolality-# of particles in solution
-ionic CM: high-osmolar CM(HOCM)
-nonionic CM: does not dissociate in solution and is LOCM-safer for injections
-iso osmolar CM-same osmolality as blood-no fluid shift
enteral Radiopaque Contrast media
-administered orally or rectally to opacify GI tract
-barium or iodine
-barium is contraindicated for suspected bowel perforation
-iodine based HOCM(diatrizoate and diatrizoate sodium) used for CT for oral or rectal exams
what are the 4 H’s for receiving IV CM
history- determine risk
hydration-lower risks of adverse effects
have equipment-resuscitation and medications in case of adverse reaction
-heads up-constant assessment of patient
what can increase potential adverse reaction to IV injection or iodinated CM
-asthma
-environmental and food allergies
-renal disease
-multiple myeloma
-diabetes mellitus
-pheochromocytoma
-sickle cell disease
-hyperthyroidism
-significant cardiac disease
-anxiety
contraindications to IV iodinated CM
allergy to iodine
prior severe allergic reaction
renal insufficiency/failure
pregnancy
nursing mothers
administrative route and rates
18-23 gauge
-dose range from 50-150 ml
-can use central venous catheter just follow manufacturer specific tolerances
-flow rates are reduced(2mL/sec) for central catheters
Venipuncture
-common sites
-antecubital space
-radial aspect of wrist
- anterior surface of forearm
-posterior portion of hand
-Aseptic technique is practiced to reduce risk of infection
-handwashing between patients
-wearing gloves
-cleaning site
-do not touch tips/ends of equipment
-gentle pressure with alcohol swab after catheter removal
advantages of power injectors
consistent, reproducible flow rates
Precise volume control
Higher injection rates for better contrast enhancement
Automatic delays for proper enhancement (i.e. bolus tracking) and multiphase imaging
Ability to administer saline as a flushing agent
Injection technique
bolus injection administered by power injector
-flow rate determined by:
-clinical area of interest
-contrast volume
-venous access placement
-patient condition
-pressure capacity for IV access
-peripheral catheters(hand or wrist)-flow rate less than 1.5 ML/sec
-22 gauge catheters-flow rates up to 3mL/sec
-20 gauge or 18 gauge when flow rates exceed 3mL/sec
disadvantage of power injectors
increased risk of extravasation
what is single phase imaging
image acquisition occurs at one specific time during or after contrast injection
what is multiphase imaging
acquisition of images over timed intervals; CT images may be acquired before, during or after contrast injection (determined by clinical indication).
delayed adverse reactions
urticaria(hives)
pruritus(itchiness)
nausea/vomiting
drowsiness
headache
fever/chills
-contrast induced nephrotoxicity-decline in renal function after IV contrast administration
-all delayed reactions must be documented
patients at risk for development of renal impairment AKI or CIN
diabetes
renal disease or one kidney
sepsis
acute hypotension
dehydration
70+
previous chemotherapy
organ transplant
vascular disease
HIV
collagen vascular disease
First Nations peoples
how to prevent CIN or AKI
adequate hydration
use LOCM or iso osmolar CM(IOCM)
-baseline serum creatinine level should be obtained for risk patients
check for metformin
in CT what may affect patient dose
- source detector distance: increase distance from tube to detector will decrease dose
2.filtration: removes low energy photons from primary beam
-Bowtie(beam shaping) reduce overall patient dose
-ioscenter patient
3.detector efficiency-ability of detectors to absorb remnant radiation
-geometric efficiency-increase interspace material, increase patient dose
-MDCT requires more interspace which reduces geometric efficiency and decreases patient dose
4.noise reduction algorithms-built into equipment, allow for lower mAs, reducing patient dose
MDCT
overbeaming
-used to avoid effects of penumbra, this increase dose to patient
MDCT
overranging
-increase in dose length product(DLP) due to additional rotations at beginning and end of spiral scan
-only occurs with helical CT
Radiation Protection
1.limit CT exam to strict clinical indications
2.scan length only to include clinically indicated areas
3.multipahse exams only carried out when necessary
4.protocol optimization should be to keep dose as low as possible
5.decreasing kVp reduces patient dose as long as all other tech factors stay the same
6.thin slice images have more noise, which results in more mA and greater patient dose
7.Reconstruction algorithms can indirectly impact patient dose; iterative reconstruction methods can correct for higher levels of noise so lower tech factors can be tolerated
8.image noise is increased with larger patients therefor larger patients may require an increase in mA
9.greater pitch values result in decreased in patient dose
-with MDCT an increase in mA is necessary due to noise and therefore higher pitch values have no impact on dose when using MDCT
10.automatic tube current modulation reduces patient dose
11.MDCT cardiac studies that use prospective gating reduce patient dose
CT dose measurements
-exposure: ability of dryas to ionize a volume of air, C/kg
-absorbed dose: amount of Arya energy deposited in a unit of mass, Gy
-kerma: kinetic energy released in matter
-air kerma: the amount of radiation absorbed in air, can be used to measure radiation output
-effective dose: accounts for tissue types, measure of radiation risk, often used to compare radiation from different types of exams, mSv
CTDI meaning
CT dose index
-measure of the dose in a single CT section or slice
-measured using a phantom with an ionization chamber placed inside
CTDIw
was used for step and shoot scanners; takes into account the variation of dose exposures at the centre and periphery of the phantoms
CTDIvol
used to approximate radiation dose for each section in helical scanning
DLP(Dose Length Product)
-estmate of the total radiation output incident on the patient
-equal to the CTDI vol multiplied by the scan length
-the longer the scan the higher the radiation dose
-multiple scans also increase DLP
-mGy-cm
what are CTDIvol and MSAD used for
to approximate average radiation dose within a scan volume
MSAD
-accounts for image spacing or bed index on the patient dose during axial scanning
-overlapping scan increase patient dose and gaps between slices decrease patient dose
-MSAD is controlled by pitch in spiral and helical scanning
are pitch and patient dose inversely related
YES
dose notification
-describes an automated software feature that informs the technologist when the prescribed technical settings for an individual CT acquisition may result in a CTDIvol or DLP that is higher that recommended value
-designed to ensure safe radiation exposure to the patient, prior to each CT acquisition
what are possible causes of high radiation doses
1.body imaging without raising arms
2.planned or unplanned extension of the scan range
3.imaging at an additional phase
4.additional imaging due to patient motion or poor breath hold
5.additional imaging to evaluate pathologies
6.off-center positioning of the patient
7.error in recording body weight
8.exceptionally large cross-section for weight
Patient dose reduction
-smaller patient have a higher absorbed dose if no adjustment are made: therefore adjustment(mA,kVp,Pitch) should be made based on patient size
pediatric dose reduction recommendations
-do not do scans for inapprpriate indications
-reduce multiphase scanning
-reduce mA and kVp
-increase pitch
-only scan indicated area
4 steps to CT General Process go imaging
1.data acquisition-measurement of attenuation of photons passing through the patient at the detectors
2. data reconstruction-computerized processing of transmission measurements into the CT image
3.multidimensional image display-display of reconstructed grayscale image in 2D/3D formats
4.image archival and communication-mechanism of display and storage
Scan modes
- scout image(topogram): used as a localizer for subsequent scan
- helical scanning-allows for volumetric data acquisition
technical elements of helical acquisition
a.helical scan: continuous rotation of the gantry(slip ring)
b.powerful x-ray tube capable of long exposure output
c.continous movement of the patient table
d.specialized mathematical algorithms for reconstruction
Multidetector CT(MDCT)
arrays enable CT sections of varying widths to be reconstructed at any point along the acquired volume
3 qualities required for MDCT x ray tubes
1.high heat rating
2.small size, lightweight for high speed rotation
3. stable and long lasting to withstand centrifugal force of rotation and long tube life
-2 focal spot sizes available: choice depends on basetting, scan field of view; may be present if chose by operator
-smaller focal spots provide greater spatial resolution
flying focal spot technology
2 locations on the anode produce x rays(stars)
-double data samples provides oversampling that can improve temporal and spatial rosultion
mA
may be manually selected depending on patient size and required signal to noise ratio(SNR) or automatic tube current modulation(ATCM) adjusts mA throughput the scan
-ATCM provide greater radiation protection
what is needed to calculate mAs
mA setting and scan time in seconds
mAs may reach 30-60 seconds
kVp
controls the penetrating power of the xray beam
-70-150kVp
-lower kVp decreases the patient radiation dose
automated tube voltage
similar to ATCM
-automatically modulates kv based on changing patient attenuation and may be available on newer model CT scanners
Dual source CT
-use two separate x ray tubes and detector arrays
-positioned 90 degrees to each other
-the two xray tubes use different kVp values
what does dual energy allow for
1.improve resolution of soft tissue structures
2.visualization of atherosclerotic plaque in cardiac CT studies with IV contrast
3.contrast medium subtraction techniques after IV contrast administration
4.composition of urinary tract calculi can be characterized
xray tube filtration
-CT xray beam is filtered(removal of low energy photons) this decreases patient dose and reduces beam hardening artifacts
-added and inherent filtration is used
-bow tie filters reduce beam intensity toward the periphery of the patients body
half value layer(HVL)
can measure the x-ray beam quality and determines the amount of added filtration necessary
Collimation in CT
-is to restrict radiation exposure to the area of interest, which reduces patient dose and improves image quality
-2 distinct components of collimation in CT
-1.beam collimation: as the xray beam exits the tube, pre-patient collimation
-2.detector collimation: prior to the remnant beam interacting with the detectors
-beam collimation may further restrict the x-ray field to expose a smaller portion of the detector array
-In MDCT, post collimation is completed with a high-resolution comb that is placed over the detector array; this comb removes unwanted scatter and off-axis photons (similar to a grid).
-MDCT collimation directly affects the volume of tissue exposed
-wider collimation results in greater anatomic coverage
in MDCT what does pre-patient collimation determine
section width and number of detector rows to be used
section width(slice thickness)
amount of tissue in the z axis that is shown in the 2D image
Detector collimation
process of determining section width in MDCT
section interval
spacing between adjacent CT images as measured from one centre to the next
Slice thickness in CT
-contiguous images: equal section thickness and interval
-noncontiguous images: greater interval than the section width
-overlapping images: section interval that is less than the section width; this improves multiplanar(MPR) reformatting and 3D reconstructions
pitch
-the relationship between collimation and table movement during scanning
-for helical MDCT the term beam pitch is used
-a pitch setting less than 1 reduces the tables speed for each rotation and this increases the acquired data and improves image quality
-low pitch also increases patient dose
-increasing pitch(greater than 1) moves the patient through the gantry faster, exposing tissue for a shorter time and reduces dose
beam pitch formula
beam pitch= table feed per rotation
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total collimation
CT detectors
-Ct detectors measure the transmitted radiation and converts it to an electronic signal for image reconstruction
-all modern MDCT systems use solid-state detectors that emit light that is converted to an electronic signal
3 qualities for a CT detector
1.high efficiency- efficient at absorbing photons and converting the photons into a signal
2. rapid signal decay-quick response and recovery time
3.high dynamic range-capable of measure a broad range of xray transmission data
CT detector Array
-MDCT system has a curvilinear detector array: it has multiple parallel rows of individual detectors running along the longitudinal axis(z axis)
-allows for large coverage of an area in the scan time
-images of varying thickness can be reconstructed at any z-axis position
-thin section images provide maximum resolution and thicker images for ease of display
CT detector configuration
-refers to the number, length and organization of the individual detectors: determined by manufacturer
3 general formats of MDCT system configuration
1.uniform matrix array-multiple detectors that are the same length
2.adaptive array-thinnest detectors in the centre, with increasing widths moving out from centre
3.hybrid array-consist of two detector sizes, narrower detectors in the middle, with wider detectors on either side
CT Computer system
-must have fast and efficient data processing capability and large storage capacity
-prepocessing software processes the signal data from the detectors
-reconstruction software uses mathematical algorithms so that raw data can be processed into image data
-postprocessing software can manipulate the image data for review and interpretation: windowing,3D and MPR reformats, ROI, distance
image reconstruction Process
- scans occur and xray beam is attenuated
2.transmitted radiation is measured and the detector emits an electronic signal
3.this is the raw data and algorithms are applied to reconstruct this information
a ray definition
the portion of the xray beam transmitted through the patient and incident upon a single detector
a ray sum definition
the measurement of transmitted radiation by an individual detector
interpolation definition
reconstructs a motion free image from volume set of data
Image display
-each voxel in the tissue is assigned a number based on the degree of attenuation
-the 3D tissue slice is displayed as a 2D image on the monitor comprised of pixels in a matrix
-pixels displayed are CT numbers or hounsfield units
what is the baseline CT number
water which is a CT value of 0
-Ct number range from +1000(cortical bone) to -1000(air)
windowing
-the process in which the CT image greyscale component of an image is manipulated via CT numbers
wide window
-lower or longer scale contrast(more shades of grey)
-defiend as 500-200 HU
-best used in areas of acute differing attenuation values
-lungs
-aka lung window
narrow window
-higher or shorter scale contrast
-50-500 HU
-excellent when examining areas of similar attenuation like soft tissue
-aka bone window
scan field of view(SFOV)
-determines the area within the gantry from which the raw data are acquired
-determined by the size of the xray beam
-determines the max possible size of the reconstructed image
-should be large enough to complete,y cover the widest dimension go cross section of the patient
-usually a small SFOV for head
-usually a large SFOV for chest, abdomen, pelvis
DFOV
-determines how much of the raw data is used to create an image
*example: an abdomen is scanned but the operator decodes to zoom or target the spine
Surface rendering
-3D images of the surface of the anatomical structure
-can be used for orthopaedics, planning for craniofacial surgery, neurosurgery, radiation therapy
Volume rendering
-3D representation of the imaged structure
-all voxels contribute to the image, allows the image to display multiple tissues and show their relationship
Minimum Intensity Projection(MinIP)
-data visualization method that enables detection of low density structures in a given volume
-most hyper dense structures of the volume are represented
-most often used for airways, vessels, ducts, trapped air
Maximum Intensity Projection(MIP)
-data visualization method that enables detection of highly dense structures
-can distinguish structures that are hyper dense with respect to surrounding tissues
-often used in CT angiography or in detecting small lung nodules
spatial resolution
pixel size
-the small the pixel the better the resolution
-smaller pixel=increased spatial resolution
spatia resolution
smapling frequency
-number of views obtained by the CT system
-increased sampling increases detail in an image
CT uniformity
-describes how uniform the image of a homogenous material appears
-a uniform phantom or material should demonstrate consistent CT values regardless of the pixels position on the matrix
-evaluated by placing several ROI measurements in different locations on a uniform testing tool or phantom
-values should not differ by more than 2 HU
beam hardening
-show as areas of light and dark streaking bands across the image
-caused by the more complete attenuation of low energy photons by radio dense structures and the remaining photon energy that strikes the detectors has an increased average energy
-high kVp may help with this
partial averaging artifact
-occurs when a structure is only partly positioned within a voxel
-the attenuation for the object is not accurately represented in the pixel
-appears as unsharpness or haziness of the borders of the objects
-thin sections can reduce this artifact
motion artifact
-blurring or streaking or step artifact on 3D and MPR images
-motion may be involuntary(peristalsis , tremors, cardiac) or voluntary( breathing, swallowing)
metalic artifacts
-cause streaking on the image
-can be in(teeth, prosthesis) or around(zippers) the patient
-often start shaped appearance
common sites for edge gradient to occur
1.between dense bone and surrounding tissue
2.bowel loops with contrast or distended with air
3.blood vessels with IV contrast
4.areas surrounding biopsy needles during a biopsy
patient positioning or out of field artifacts
-cause by anatomy outside the scan field of view(SFOV) is not in the reconstructed image but does attenuate the beam
-streaking artifacts occur, most often in the shoulders, abdomen or lower pelvis
-most common example is having patients arms down while doing a chest or abdomen
equipment induced artifacts
1.rings: usually caused by faulty detectors
2.streaks: detector malfunction or misalignment of the tube and detectors
3.aliasing: streaks caused by insufficient data
4.tube arcing; serve streaks caused by an electrical short circuit
5.cone beam artifacts: like partial volume artifacts
6.windill: from increased pitch
1st phase: bolus
- aka arterial phase
- 15-22 seconds post injection
- arterial structures are filled with contrast and brightly displayed
- 30 or more HU difference btw aorta and IVC
2nd phase: non equilibrium
*aka venous phase
* 1 minute after bolus phase
* difference of 10-30HU btw aorta and IVC
* arteries are still bright but venous structures are becoming opacified
* most body images are acquired in this phase
* lasts only 1 minute
3rd phase: equilibrium
- 2 minutes after bolus phase
- attenuation difference of less than 10HU btw aorta and IVC
- phase where organ parenchyma is enhanced
