CT review Flashcards
1
Q
pre procedure communication
A
-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
2
Q
clinical history
A
-obtain patient history
-document recent procedure, surgeries, symptoms, possible trauma, specific areas of pain
-screening tools may be used
-other benefits
3
Q
Education
A
-explain procedure including
-breathing instructions
-Warm feeling, metallic taste from contrast
-post exam provide follow up care instructions
4
Q
consent
A
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
5
Q
Level of consciousness
A
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
6
Q
Hypoxemia meaning
A
insufficient oxygen in the arterial blood
7
Q
hypoxia meaning
A
insufficient oxygen levels in the tissues
8
Q
hypoxia symptoms
A
headache
nausea
dizziness
ataxia
cyanosis
if localized may result in pain, cyanosis or cell death
9
Q
oxygen administration
A
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
10
Q
Chest tubes
A
-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
11
Q
lab values
A
-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
12
Q
what happens when a eGFR is less than 30mL/min
A
the patient is more at risk for AKI(acute kidney injury) or CIN(contrast Induced Nephropathy)
13
Q
metformin
A
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
14
Q
Types of Contrast Media
A
-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
15
Q
how can iodinated radiopaque CM be administered?
A
into bloodstream intravenously
into the intrathecal space for myelography
into the joint space during CT arthography
16
Q
IV radiopaque CM
A
-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
17
Q
enteral Radiopaque Contrast media
A
-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
18
Q
what are the 4 H’s for receiving IV CM
A
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
19
Q
what can increase potential adverse reaction to IV injection or iodinated CM
A
-asthma
-environmental and food allergies
-renal disease
-multiple myeloma
-diabetes mellitus
-pheochromocytoma
-sickle cell disease
-hyperthyroidism
-significant cardiac disease
-anxiety
20
Q
contraindications to IV iodinated CM
A
allergy to iodine
prior severe allergic reaction
renal insufficiency/failure
pregnancy
nursing mothers
21
Q
administrative route and rates
A
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
22
Q
Venipuncture
A
-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
23
Q
advantages of power injectors
A
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
23
Q
Injection technique
A
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
24
disadvantage of power injectors
increased risk of extravasation
25
what is single phase imaging
image acquisition occurs at one specific time during or after contrast injection
26
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).
27
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
28
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
28
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
29
in CT what may affect patient dose
1. 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
30
MDCT
overbeaming
-used to avoid effects of penumbra, this increase dose to patient
31
MDCT
overranging
-increase in dose length product(DLP) due to additional rotations at beginning and end of spiral scan
-only occurs with helical CT
32
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
33
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
33
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
34
CTDIw
was used for step and shoot scanners; takes into account the variation of dose exposures at the centre and periphery of the phantoms
35
CTDIvol
used to approximate radiation dose for each section in helical scanning
36
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
37
what are CTDIvol and MSAD used for
to approximate average radiation dose within a scan volume
38
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
39
are pitch and patient dose inversely related
YES
40
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
41
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
42
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
43
pediatric dose reduction recommendations
-do not do scans for inapprpriate indications
-reduce multiphase scanning
-reduce mA and kVp
-increase pitch
-only scan indicated area
44
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
45
Scan modes
1. scout image(topogram): used as a localizer for subsequent scan
2. helical scanning-allows for volumetric data acquisition
46
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
47
Multidetector CT(MDCT)
arrays enable CT sections of varying widths to be reconstructed at any point along the acquired volume
48
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
49
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
50
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
51
what is needed to calculate mAs
mA setting and scan time in seconds
mAs may reach 30-60 seconds
52
kVp
controls the penetrating power of the xray beam
-70-150kVp
-lower kVp decreases the patient radiation dose
53
automated tube voltage
similar to ATCM
-automatically modulates kv based on changing patient attenuation and may be available on newer model CT scanners
54
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
55
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
56
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
57
half value layer(HVL)
can measure the x-ray beam quality and determines the amount of added filtration necessary
58
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
58
in MDCT what does pre-patient collimation determine
section width and number of detector rows to be used
59
section width(slice thickness)
amount of tissue in the z axis that is shown in the 2D image
60
Detector collimation
process of determining section width in MDCT
61
section interval
spacing between adjacent CT images as measured from one centre to the next
62
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
63
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
64
beam pitch formula
beam pitch= table feed per rotation
----------------------------------
total collimation
65
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
66
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
67
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
68
CT detector configuration
-refers to the number, length and organization of the individual detectors: determined by manufacturer
69
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
70
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
71
image reconstruction Process
1. 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
72
a ray definition
the portion of the xray beam transmitted through the patient and incident upon a single detector
73
a ray sum definition
the measurement of transmitted radiation by an individual detector
74
interpolation definition
reconstructs a motion free image from volume set of data
75
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
76
what is the baseline CT number
water which is a CT value of 0
-Ct number range from +1000(cortical bone) to -1000(air)
77
windowing
-the process in which the CT image greyscale component of an image is manipulated via CT numbers
78
widow level
-brightness of the image
-aka window centre
-midpoint of the range of CT numbers displayed
-decrease WL= brighter image
*smaller the WL the brighter the image is*
79
window width
contrast of the image
* measure of CT numbers that an image contains
80
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
81
narrow window
-higher or shorter scale contrast
-50-500 HU
-excellent when examining areas of similar attenuation like soft tissue
-aka bone window
82
upper grey level formula
WL+(WW/2)
83
lower grey level formula
WL-(WW/2)
84
typical WW and WL values in the head and neck
1.brain: WW;130-300,WL;40
2.subdural:WW;130-300,WL50-100
3.stroke:WW;8, WL;32
4.temporal bones: WW;2800,WL600
5.soft tissues: WW;350-400,WL;20-60
85
typical WW and WL values in the chest
1.lungs: WW;1500,WL;-600
2.mediastinum: WW;350,WL;50
86
typical WW and WL value sin the abdomen
1.soft tissue: WW;400,WL;50
2.liver:WW;150,WL;30
87
typical WW and WL values of the spine
1.soft tissue: WW;250,WL 50
2.bone: WW;1800, WL;400
88
wide window width
500-2000HU
-best for imaging tissue types that vary greatly
-goal is to see all of the various tissues on one image
89
narrow window width
-tissue types with similar densities
-50-500HU
-Brian or liver
89
2 types of FOV
1.scan field of view
2.display field of view
90
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
91
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
92
multiplanar reconstruction
-reformatting images into different planes from the scan data
-only requires the image data
-Ex transverse(axial) can be displayed in coronal, sagittal or oblique planes
93
Surface rendering
-3D images of the surface of the anatomical structure
-can be used for orthopaedics, planning for craniofacial surgery, neurosurgery, radiation therapy
93
3D reformatting
-represents the entire scan volume in only one image
-include:
* surface rendering
*volume rendering
* maximum intensity projection
* minimum intensity projection
94
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
95
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
96
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
97
spatial resolution
-ability of CT system to display details separately
-a high spatial resolution is important to be able to discriminate btw structures that are located close to each other
-measured in line pairs per centimetre(lp/cm)
97
factors affecting spatial resolution
focal spot size
detector size
reconstruction algorithm
pixel size/dimension
sampling frequency
98
spatial resolution
focal spot size
-small focal spot size improves spatial resolution
-large focal spot size increase unsharpness due to penumbra
large focal spot size=decreased spatial resolution
-large focal spot size requires a high mA
99
spatial resolution
detector size
-small detectors=better spatial resolution
100
spatial resolution
reconstruction filter
-sharp filter=higher resolution but more noise
-smooth filter=less resolution but less noise
-sharp(higher resolution) kernels have better spatial resolution(bone reconstruction)
101
spatial resolution
slice thickness
-the wider the detector row the lower the resolution
-slice thickness affects volume averaging which is when different objects within a voxel are averaged to produce one less accurate pixel reading
-slice thickness is decreased which decreases volume averaging which then spatial resolution is increased
102
spatial resolution
pixel size
-the small the pixel the better the resolution
-smaller pixel=increased spatial resolution
103
spatia resolution
smapling frequency
-number of views obtained by the CT system
-increased sampling increases detail in an image
104
longitudinal spatial resolution
slice thickness
-longitudinal spatial resolution describes patient movement during volumetric acquisition in MDCT
-spatial resolution is inversely related to slice thickness
-increased slice thickness decreases spatial resolution or detail in the image
105
longitudinal spatial resolution
pitch
-increases in pitch will result in a decrease in image detail(spatial resolution)
-MDCT system pitch can be limited by selecting the slowest translation speed possible
-images can be reconstructed using the thinnest detector collimation possible
106
factors affecting contrast resolution
1.inherent subject contrast: attenuating properties of the tissue affect contrast
2.beam collimation: wide or increased beam collimation allows for more scatter to reach the IR, decreasing contrast resolution
3.algorithm selection: soft tissue algorithms reduce noise and enhance contrast resolution
4.window setting: narrow window width displays fewer shades of gray which increases contrast
5.detector collimation: increased detector collimation, without increase in mA produces more noise which decreases contrast
6.noise:technical factors with reduced noise improves contrast resolution
107
contrast resolution
-ability of the CT scanner to differentiate small differences in attenuation between closely spaced objects
-CT has low contrast resolution meaning tissue of similar attenuation value scan. be displayed
108
temporal resolution
-the ability to resolve fast moving objects and is comparable to shutter speed of a camera
-in CT it is the time needed to acquire the data to generate an image
-controlling factors include: gantry speed rotation and reconstruction method
109
noise
-contains no useful information
-appears as quantum mottle or a graininess
110
3 types of noise
1.quantum noise: insufficient quantity of X-rays reaching the voxel
2.electronic system noise: occurs in the reconstruction process by electronic components
3.artificial noise: artifacts contain no useful information and may obscure information
111
factors that affect image noise
1.xray photon flux(quantity): controlled by mA and time, an increase in either increases photon flux, SNR and patient dose
2.voxel dimension-large voxel absorbs more photons(greater photon flux) and this increases SNR
3.pitch/table speed: higher pitch exposes each voxel to less photons; unless dose is increased, high pitch results in greater noise
4.detector sensitivity and efficiency: high sensitivity and efficiency decrease noise
5.patient factors: larger patients result in an increase in noise unless technical factors are increased
6.algorithm/kernel: smooth or soft algorithms reduce noise
112
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
113
CT linearity
-describes accuracy of a CT number
-measured with a CT phantom with different materials with specific Ct numbers
-ROI measurements are taken on the CT image to determine acceptable accuracy of the CT numbers on the image
114
Artifacts
-errors during the CT scan can result in an artifact
-do not contain useful information and can obscure anatomy or pathology
115
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
116
partial volume averaging
-occurs when more than one type of tissue is contained within a voxel
-a voxel can contain values for multiple tissue types and the values are averaged together to represent an assortment of different materials
-this in unavoidable in CT
117
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
118
motion artifact
-blurring or streaking or step artifact on 3D and MPR images
-motion may be involuntary(peristalsis , tremors, cardiac) or voluntary( breathing, swallowing)
119
main methods to reduce motion artifact
1.communictaion-clear explanation
2.immobilization
* sedation may be required
* beta blockers can reduce heart rate for CT cardiac imaging
120
metalic artifacts
-cause streaking on the image
-can be in(teeth, prosthesis) or around(zippers) the patient
-often start shaped appearance
121
methods of reducing metallic artifacts
-Remove artifact whenever possible
-Decrease the section width
-Increase the kVp
-Adjust the WW and WL settings
-Reorient data acquisition (example, axial acquired slices through the sinuses)
-Use metallic artifact reduction software, manufacturer dependent
122
edge gradient
-streak artifact that occurs when a high density object is surrounded by low attenuation material
-shows up at the interface( at the boundary of the two objects)
123
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
124
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
125
step artifact
-image demonstrates the individual sections used to build the reformatted images, so MPR and 3D imaging is affected
-reformats have a lack of overall sharpness and detail or step like appearance
-isotropic data helps prevent this artifact
-overlapping scan sections helps prevent this artifact
-reformat in thinner intervals also helps reduce step artifact
126
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
127
what are the three phases of Contrast Enhancement
1.bolus
2.non-equilibrium
3.equilibrium
128
1st phase of contrast: bolus phase
* 15-22 seconds after injection
* aka arterial phase
* arterial structures are filled with contrast
* attenuation difference of 30HU or more btw the aorta and IVC
129
What phase is Ct angiography images taken
bolus phase
*hypervascular lesions of the liver are also seen during this phase
130
2nd phase of contrast: non equilibrium
* 1 minute after bolus
* aka venous phase
* difference of 10-30HU btw the aorta and IVC
*arteries are bright while venous structures become opacified
* lasts only about 1 minute
131
132
what are the 3 phases of CT
1.3 bolus
2.non-equilibrium
3.equilibrium
133
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
134
which phase are CT angiography images taken during
bolus
135
what is seen during bolus phase
* hyper vascular lesions on liver and outer cortex of kidney
136
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
137
what is seen during the non equilibrium phase
less vascular (hypo vascular) liver tumors
138
3rd phase: equilibrium
* 2 minutes after bolus phase
* attenuation difference of less than 10HU btw aorta and IVC
* phase where organ parenchyma is enhanced
139
can the Bolus and Equilibrium phases be further divided
yes
early arterial
late arterial
Hepatic arterial
Portal venous
Hepatic venous
Nephrographic
140
Early arterial phase
* 15-20 seconds post injection or immediately after bolustracking
* contrast is still in arteries and has not enhanced organs or soft tissue
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what is really arterial phase used for
detection of dissection of aorta
arterial bleeding
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late arterial phase
* 35-40 seconds post injection
* 20 seconds after bolus tracking
* All structures that get their bloodsupply from the arteries will show optimal enhancement
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what does late arterial phase enhance
hyper vascular lesions
stomach
bowel
pancreas parenchyma
spleen
kidney outer cortex
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late arterial phase is used for the detection of?
*liver: HCC-FNH adenomas( liver tumors?)
*pancreas: adenocarcinoma-insulinoma
*bowel ischemia
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hepatic phase
* aka portal venous phase
* 70-80 seconds post injection
* 50-60 seconds after bolus tracking
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what does the hepatic phase enhance?
hepatic parenchyma
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the hepatic phase helps in detection of
hypo vascular liver lesions: cysts, abscess, most metastases
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what does the nephrogenic phase enhance
all renal parenchyma including medulla
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nephrogenic phase
* 100 seconds post injection
* 80 seconds after bolus tracking
* renal parenchyma including the medulla enhances
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the nephrogenic phase helps with detection of
small renal cell carcinoma
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delayed phase
* aka washout phase or equilibrium phase
* 6 minutes post injection
* 6 minutes after bolus tracking
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what does delayed phase enhance
* fibrotic lesion
* kidney and urinary collecting system
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what does delayed phase help in the detection of
* liver: cholangiocarcinoma
-fibrotic metastases
* kidney: transitional cell carcinoma
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test bolus
* a small test quantity of contrast is injected and sequential axial slices at a set region of interest are acquired to calculate the time of peak contrast enhancement and determine optimum scan delay
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bolus tracking
sequential axial slices at a set ROI are conducted during the contrast injection util a threshold enhancement is met
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the liver
* has dual blood supply
* gets blood from portal vein and hepatic artery
* enhancement of liver occurs at different times due to dual blood supply
* hepatic arterial phase occurs about 20-40 seconds after injection
* portal venous phase occurs about 60-90 seconds after Strat of injection
* due to this most scans of the liver are completed in multiple phases
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when is multiphasic imaging used
pancreas
liver
kidneys
abdominal CTA protocols
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CT of the liver
-normal CT attenuation of unenhanced liver is 38-70HU
-in healthy patients the liver is 10HU greater than the spleen
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Pancreas in CT
* typically located btw T12 and L2
* water or low attenuation contrast is preferred so it does not obscure small stones
* if the margins pancreas is not able to be differentiated from the duodenum the patient may be given oral CM and and more slices can be obtained with the patient in right decubitus position
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Kidney in CT
* unenhanced CT is generally used to demonstrate calcification and calculi
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enhancement phases of the urinary tract
* corticomedullary phase: 30-70 seconds after CM bolus
* nephrogram phase: 100-120 seconds after CM bolus
* excretory phase: 3 minutes after CM bolus
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CT urography(CTU)
* provides comprehensive evaluation of the upper and lower urinary tract
* some protocols use split bolus injection where there is 2 and 15 minutes btw injections
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CT of adrenal glands
* determining if a adrenal mass is benign or malignant is done by assessing its attenuation values and by evaluating the degree of CM washed out on delayed imaging
* goal is to reduce the amount of biopsies and the number of follow up studies
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CT in the diagnosis of Acute Appendicitis
* The variable position of the appendix is important because it contributes to the diverse clinical presentation of acute appendicitis
* the variation in position also make sit hard to ,coat on cross sectional images
* combination of CM can be used oral, rectal, Iv or no CM
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CT for Urinary Tract Calculi
* kidney stones, renal stones, renal calculi, nephrolithiasis, urolithiasis are interchangeably
* stones can be seen on non contrast helical CT(NCHCT)
* protocols use a helical scan mode from the top of the kidneys to the base of the bladder and a slice thickness of 3mm or less
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common Abdomen or pelvis protocols
* routine abdomen pelvis
* abdomen pelvis aorta( post stent, non gated)
* arterial venous liver
* arterial venous pancreas
* mesenteric
* enterography
* appendicitis or diverticulitis
* colonography
* adrenal mass
* renal mass
* renal stone
* CT urogram
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how is CT used for MSK
* provides specific information about bones and other mineralized tissue
* is useful for evaluating bone and soft tissue tumors
* adds detail to information obtained with conventional radiography in cases of multiple fractures
* I used to evaluate joints, especially after air or iodinated CM is injected into the joint
* lower extremity is usually done supine feet first
* upper extremity is usually done supine head first
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General MSK CT imaging methods
* AP and lateral scout images are taken to localize area of interest
* when doing long bones the plane of the CT section should be perpendicular to the long axis
* most MSK protocols include multiplayer reformations
* if fracture is seen 3D reformations are often performed
* reconstruction algorithms is based on clinical application
* standard algorithm I used if soft tissue or muscle is of primary interest
* if bone is needed, it can be reconstructed in a bone algorithm
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wrist positions
* arm over head
* patient stand or sit on far side of scanner and extend the arm into scanner
* wrist rests on abdomen
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MSK protocols
shoulder or scapula
wrist
elbow
hip or proximal femur
knee or tibial plateau
ankle or distal tibia
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Thoracic Imaging
*short scan times help reduce artifacts created by motion
* when possible scan of Chets should be done in one breath hold
* airway imaging is done on inspiration and expiration
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High Resolution CT(HRCT)
* used for assessment of lung parenchyma in patients with diffuse lung disease
* some HRCT only scan a representative portion of the lung parenchyma
* volumetric HRCT use helical mode to acquire images of the entire lung
* most HRCT include more than one series of scans
* prone images can help differentiate actual disease from densities related to the effects of gravity that mimic disease
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CTA diagnosis of PE
* done caudal to cranial direction to help minimize respiratory artifact
* due to radiation dose the decision to use CTA for pregnant or young I taken seriously
* dose and timing is crucial
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Cardiac CT
* understanding the structural anatomy and the path and timing of circulation is essential to the creation of high quality cardiac CT images
* to reduce motion artifact on CTA images a patients heart rate can be temporarily lowered with B-blockers
* cardiac gating attempts to use only those images acquired during periods of the lower cardiac motion
* nitroglycerin can be given sublingually before coronary CT exams to dilate vessels to improve visualization
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B-blockers purpose
* used to lower heart rate to less than 65 to 70bpm
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prospective ECG Triggering
* aka sequential or cine-mode scanning
* acquires images only in the positions of the cardiac cycle with the lowest cardiac motion
* minimizes radiation dose
* very sensitive to cardiac motion artifacts and image misregistration
* can be problematic in patients with irregular heartbeats
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Retrospective ECG Gating
* Helical data are acquired throughout the cardiac cycle, and images are then reconstructed in specified portions of the cardiac cycle
* disadvantage is high radiation dose
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ECG-pulsed tube modulation
* developed to address concerns about radiation dose
* automatically decreases the tube current during the systolic phase of the ECG tracing
* Cannot be used for patients with arrhythmia
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CT coronary Calcium screening
* 4 detector row CT scanner with 5 second gantry rotation time is the minimal requirement for a coronary calcium measurement
* the amount of calcification seen on cardiac CT is expressed as a calcium score
* b/c the measurement is done mainly as a screening exam on asymptomatic patients dose is a concern
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thoracic protocols
* routine chest
* lung nodule
* high resolution CT
* tracheobrachial
* chest/abdomen
* chest for pulmonary embolism
* cardiac calcium scoring
* Chets aorta
* CTA-heart(general)
* CTA-coronary arteries
* CTA- pulmonary veins
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head imaging methods
* immobilize the head
* program the slices so that they are parallel to the supraorbital metal line to reduce radiation exposure to the eyes
* due to dense bone beam hardening artifacts are often seen in images of the posterior fossa
* most often done using step and shoot method
* narrow window widths are used to demonstrate the brain due to the small differences in attenuation
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Intracranial Hemorrhage(ICH)
* CT is most often initial exam for ICH
* ICH appearance changes with time
* ICH appears hyper dense to normal brain tissue for first 3 days
* hyperdens centre surrounded by concentric areas of hyper dense and hypotenuse tissue for 4-10 days
* isodense center surrounded by areas of hypotenuse for 11days to 6 months
* hypotenuse to normal tissue after 6 months
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CTA advantages
* less invasive
* widely available
* less expensive
* time saving
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neck imaging
* supine with neck slightly extended
* most often done in helical mode
* IV contrast is used unless contraindicated
* want mucosa, lymph nodes, pathologic tissue to enhance while vasculature remains opacified
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CTA of head and neck
*Rapid, high-resolution scans are taken while contrast is in the arterial enhancement phase
* goal is to accurately measure stenosis of the carotid and vertebral arteries and their branches
* goal is to evaluate the COW for completeness using 3D reformation
* goal is to detect other vascular lesions
184
CT venography(CTV)
modification of CTA used for the depiction of venous anatomy
185
Spine protocols
* MRI is usually modality of choice
* when evaluating bone abnormalities CT is superior
* AP and lateral scout
* CM helps visual intramural structures
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intrathecal administration of CM
* CT examinations are performed after myelography to enhance or clarify findings.
* A delay of 1 to 3 hours between the intrathecal injection and scanning is recommended to allow the CM to dilute.
* CM that is too dense may mask intradural structures
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suspicion of stroke
* t-PA(issue plasminogen activator) is treatment for acute stroke that must be administered within 3 hours of first signs
* ICH contraindicates t-PA therapy
* non contrast CT of brain is performed to differentiate ischemic stroke from hemorrhagic stroke
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Neurologic Protocols
Routine head
Skull base (posterior fossa)
Temporal bones
Sinus screen
Trauma facial bones
Orbits
Sella turcica
Soft tissue neck
Brain perfusion
CTA—circle of Willis
CTA—circle of Willis/carotid
CTV—cranial venography
Cervical spine
Thoracic spine
Lumbar spine
CTA—spine
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advantages of CT guidance of percutaneous procedures
* provides precise 3D localization of lesions
* permits clinicians to plan an access route by showing relationship of surrounding structures
* b/c the tip of the needle can be seen, interventions can be performed on small structures
* information can be gathered on materials ranging from low to high density
* CM can be injected when needed
* patient can be placed in a variety of positions
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Techniques fro CT guided interventions
* improves accuracy of the procedure and diminishes the associated risks
* two different imaging techniques are used
1.sequential CT
2.CT fluoroscopy(CTF)
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Sequential CT
* Process of (a) scan acquisition, (b) adjustment of the needle, (c) another scan acquisition, until needle is confirmed to be in the correct location
* disadvantages
*procedure can be lengthy
* Intermittent visualization prohibits the clinician from making the rapid adjustments that are possible with conventional fluoroscopy
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CTF
* must be purchased in addition to a helical scanner
* scanner should be large bore type(60cm min)
* allows for near ray time capabilities of traditional fluoroscopy
* concerns about exposure to operator
191
techniques to reduce radiation dose
* when sequential CT is used set mAs as low as possible
* when CTF is used keep exposure time and mA as low as possible
* with CTF use only intermittent fluoroscopy
* when using CTF personnel must not enter CT beam with their hands
192
common indications fro CT guided procedures
*Drainage of abscesses and pneumothoraces
*Puncture of abdominal and thoracic lesions
*Percutaneous diskectomy of herniated disks
*Lumbar spine or pelvic interventions
*Percutaneous administration of chemotherapeutic agents
*Thermoablative procedures
*Radiofrequency ablative procedures
*Percutaneous vertebroplasty
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basic Steps for CT Guided Biopsy
1. explain the procedure to the patient, obtain written consent
2.obtain appropriate laboratory values
3.plot the scan using patients previous imaging
4.scan the selected area
5.best location for needle entry is selected: place metallic marker on the skin
6.repeat scan to confirm suitability of selected location
7.measure distance from marker on patient skin to lesions
8.patienst skin is prepared according to aseptic guidelines
9.repeat scan at the needle location and one slice above and below until tip of needle is visualized
10.if Ct confirms correct location a tissue sample is taken and prepared according to lab protocols
11. postpocedure scan is taken to identify complications
