CT
moderate reactions to contrast
excessive hives tachycardia or bradycardia Hypertension or hypotension Dyspnea Bronchospasm/wheezing
Because of their millisecond acquisition times, fifth-generation CT scanners are classified as
high speed CT scanners
CT image contrast is adjusted by windowing. A narrow window width will result in
higher contrast
mild reactions to contrast
hives itching nausea/vomiting dizziness and sweating
Each pixel of information in the CT image contains numerical information in _____ units.
houndsfeld
Each pixel of information in the CT image contains numerical information in _________ units
hounsfeld
A high variation in pixel values in a homogenous phantom is a measure of _____.
image noise
Filtered back projection in CT refers to _____
image reconstruction
filtered back projection
image reconstruction
CT is able to differentiate between body tissues with similar densities better than conventional radiography due to
improved contrast resolution
CT is able to differentiate between tissues with similar densities compared with conventional radiography due to:
improved contrast resolution
Contrast is adjusted by windowing, changing window width and window level. Window width determines the:
number of grey shades displayed
The first generation of CT scanners used how many detectors
one detector
a translation is
one sweep of the source -detector assembly
Fourth-generation CT scanners use the following configuration
only rotating tube
The measured x-ray transmission values are called
raw data
The time from the end of CT imaging to image appearance is called the _____ time.
reconstruction
The time from the end of CT imaging to image appearance is called
reconstruction time
thinner slices improve
recorded detail
a rotation occurs when
source detector assembly returns to starting position and the assembly starts another translation
The concentration of scintillation detectors affects the _____ of the CT image
spatial resolution
The concentration of scintillation detectors affects the _____ of the CT image.
spatial resolution
______________ ______________ in radiography (xray) is better than in CT
spatial resolution
This is the ability to see the difference between two objects that are close together in an image.
spatial resoultion
The new development in the fourth generation CT scanner is the _____
stationary detector array
each projection Is
stored on the computer
The CT system's ability to freeze any motion of a scanned object is termed
temporal resolution
The reference material for CT is ____ and is related to the ____ of the tissue.
water, linear attenuation coefficient
3D imaging steps
1. Construction of a volume of 3D data from original 2D CT image data 2. Segmentation to crop or edit target objects from reconstructed data (Eliminates unwanted information from data) 3. Rendering or shading = provides depth perception to final image
factors that affect patient dose
-Beam Energy (kVp) - tube Current (mA) -Exposure time or rotation -Section or slice thickness (beam collimation ) -Object thickness and attenuation size -Pitch and/or section spacing -Dose reduction techniques -Distance from tube to isocenter
intravenous contrast media
-Helpful in demonstrating tumors throughout the body -Used to visualize vascular structures -Typically, non-ionic contrast used for CT studies because it has a low incidence of reaction.
rectal contrast media
-Often requested as part of an abdominal or pelvic protocol; useful in demonstrating distal colon -When used, usually mixed in the same concentration as oral contrast (low)
Eval for contrast media
-Patients over age 60 require lab measurements of renal function (BUN/creatinine levels) -All patients screened for renal function risk factors (hx kidney disease, diabetes, high blood pressure, etc.)
Factors that affect image quality include:
-spatial resoultion -contrast resolution - noise -artifacts
CT can distinguish between density differences of less than
0.5%
Factors that affect image quality in CT consist of: (1) spatial resolution (2) noise (3) contrast media
1 and 2 not contrast media!
CT scanners operate on _____ voltage generation
high frequency
the advantages of multi slice helical CT include:
1. multiphase studies 2. isotropic viewing 3. longer anatomic coverage
what are the 3 main components of CT scanner
1. operating console 2. computer 3 gantry
first CT scanners in US were installed in ________
1973 at mayo clinic in Minnesota and then later In Massachusetts (both were dedicated head scanners )
contrast resolution
Ability to differentiate between small differences in tissue densities
CT Angiograph (CTA)
An application of spiral CT using 3D imaging techniques
advantages of CTA over traditional angiography
CTA uses spiral technology to create volume data images without unnecessary radiation exposure or additional IV contrast -Overlying structures can be eliminated so only vascular anatomy is reconstructed -CTA uses an IV procedure that does not require arterial puncture
severe reactions to contrast
Cardiac or respiratory arrest Loss of consciousness Cyanosis Profound shock Convulsions
contrast media
Contrast media helps differentiate tissues within the body
CT effective dose in sievert is calculated using the formula
DLP x region specific conversion factor.
The most common reporting method of dose reporting on present scanners is
Dose -length product (DLP)
fifth generation scanners
Fifth generation scanners are classified as HIGH- SPEED SCANNERS because they have millisecond acquisition times. Known as electron-beam scanners, a fan-shaped electron -beam strikes stationary tungsten rings to create x-rays. Primarily used in cardiac studies, these scanners produced images with high temporal resolution.
CT first discovered by _____________ and ___________ in ________
Godfrey HOunsfield and allan Cormack in 1970 in England
scan parameters affecting noise
Matrix size Slice thickness X-ray beam energy Reconstruction algorithm Scatter radiation Patient size
Hounsfield Units
Measure of the beam attenuation capability of a specific structure. Also called pixel values, density numbers or CT numbers
the first gen of ct used _______ detectors
ONE
PET scanning
PET/CT couples a CT scanner with a positron emission tomography scanner -Uses two scanners close to one another with a single patient couch that travels between them -PET/CT is sued for attenuation correction and anatomic correlation for functional PET scans
DO NOT administer IV contrast through a
PICC line, Swan-Gantz catheter or central line.
second generation scanners
Second generation scanners were much improved as compared to first generation scanners. The x-ray tube emitted a fan-shaped beam that measured by 5-30 detectors placed closely together in a detector array. The tube and detector movement of translation/rotation remained, but the rotation was 10° between each translation, which decreased scan time to about 20 seconds per slice.
scan parameters affecting spatial resolution include
Slice thickness Focal spot size Display FOV Matrix size Reconstruction algorithm Detector aperture width
Scan parameters affecting contrast resolution include
Slice thickness Reconstruction algorithm Image display X-ray beam energy Size of patient Detector sensitivity
Radiation Treatment planning
Spiral CT used in conjunction with MPR provides a 3D approach to radiation treatment planning -Allows the dosimetrist to minimize dose to normal tissue while providing maximum radiation dose to target cancer cells
The _____ generation of CT scanners was the first to have the fan beam transect the entire patient at all times.
THIRD
Most CT scanners in use today are third-generation variations that have 4 to 320 rows of detectors in a single array ( true or false )
TRUE
Quality Control
The goal of a QC program in CT is to ensure system is producing best possible images with minimum radiation dose
first generation scanners
These scanners worked by a process known as TRANSLATION/ROTATION. The tube produced a finely collimated beam, which was also referred to as a pencil beam. There were approximately one to three detectors that were placed on the opposite side of the tube to detect radiation. The linear tube movement, also referred to as translation, was followed by a rotation of 1 degree. Scan time was about 3-5 minutes and limited to scanning the brain.
third generation scanners
This generation scanner used a ROTATE/ROTATE movement in which the x-ray tube and detector array rotated at the same time around the patient. More than 750 detectors in a curved array improved overall image quality and decreased scan time to 1 - 20 seconds per slice. Decreased image reconstruction time also REDUCED EXAM TIME for the patient.
sixth generation scanners
This scanner design has a dual-energy source and 2 SETS OF DETECTORS OFF SET BY 90°. Used for imaging moving structures such as the heart, this technology decreases patient dose because no overlapping scanning occurs.
fourth generation scanners
This scanner design used a ROTATE ONLY MOVEMENT in which the tube rotated around the patient and the detectors were in a fixed, circular position. The x-ray beam was fan-shaped and HIGHER PATIENT DOSES per scan resulted.
Multi-slice/helical CT systems (MSHCT)
This type of detector array contains many rows of elements and allows for faster scan times as compared to conventional spiral/helical scanning. -has 64 rows of elements -These types of scanners are referred to as volume CT (VCT) systems because these scanners cover entire body sections in a single breath-hold.
the cross-sectional anatomy is translated into ___________ during CT reconstruction
a matrix of values
3D imaging
a post processing technique applied to raw data to create realistic images 3 common techniques -Maximum Intensity Projection (the reconstruction of the brightest pixels from a stack of 2D or 3D image data into a 3D image) -Shaded surface display (Provides a 3D image of a particular structure's surface) -Volume Rendering (Incorporates entire volume of data into a 3D image by summing contributions of each voxel along a line from the viewer's eye to the data set)
CTDIvol
accounts for axial scan spacing
CTDI100
allows more specific dose calculations
The second-generation CT scanners had the disadvantage of increased
beam scatter
the second gen of CT machine had the disadvantage of increased
beam scatter
Computed tomography dose index (CTDI)
calculates CT dose with a normal beam width and a standard of 14 axial slices
CT =
computed tomography the process of creating a cross-sectional tomographic plane of any part of the body
CT imaging has excellent _____ because of the narrow beam collimation
contrast resolution
CT imaging has excellent _____ because of the narrow beam collimation.
contrast resolution
the characteristic of CT imaging that allows it to differentiate between between tissues with similar density
contrast resolution
The most significant geometric factor that contributes to spatial resolution is
detector aperture width
Contrast is adjusted by windowing, changing window width and window level. Window level determines the:
determines the midpoint of the gray range
in 1974 - first _______ _________ scanner was developed by
full body scanner by Robert Ledley
Spiral CT or Helical CT
he gantry is rotating continuously while the table moves through the gantry aperture simultaneously. The continuous gantry rotation combined with continuous table movement forms a spiral path. -A volume of tissues rather than a group of individual slices are scanned. This method is very useful in detecting small lesions. -A volume of tissue can be scanned in a single breath, which reduces respiratory motion. These scans are useful for non-cooperative/combative patients, patients having difficulty lying flat, trauma patients and pediatric patients. -This scanning method decreases the amount of contrast material necessary to visualize structures, making the exam safer and more cost effective.
first CT scanners only scanned
heads
Many CT x-ray tubes have the capacity for millions of _____ with each exam.
heat units
Many CT x-ray tubes have the capacity for millions of ____________ with each exam
heat units
effective dose
is a radiation quantity that takes into account the amount of radiation absorbed by the body, as well as the type of tissue exposed.
CTDIw
is a weighted average dose measurement system
Multiples scan average dose (MSAD)
is the average dose resulting from scans over an interval length on the patient
dose length product (DLP)
is the most common reporting method of dose reporting on present scanners It is reported in mGy/cm
to reduce patient dose
keep kVp, mA , exposure time , to a minimum
CTDIvol x
length of the scan in cm
Oral contrast for CT should be mixed so it is
less concentrated than for general examinations
CT Numbers or Hounsfield Units relate to the ________ _______________ of the tissue within each voxel
linear coefficient
The main drawback of the first-generation CT scanners was
long scanning times
each CT projection records variations in
mass density and effective atomic number
An array of numbers arranged in a grid of rows and columns is called a
matrix
The pixel is reduced when the _____ size is increased and the _____ size is fixed.
matrix , FOV (field of view)
projections or scan profiles are
measured x-ray transmission values (amount of radiation detected). Once collected, electrical signals are digitized. Based on strength, each signal is assigned a whole number value.
Positive CT numbers indicate tissues that are
more dense than water
CT eliminates superimposition by
passing a tightly collimated beam through a part at many different angles
The degree of spatial resolution in any CT imaging system is limited to the size of the
pixel
The degree of spatial resolution in any CT imaging system is limited to the size of the _____.
pixel
image noise in CT scanning depends on:
pixel size / matrix size xray beam energy detector efficiency slice thickness reconstruction algorithm ( scatter radiation and patient size )
All of the following are methods of 3D reconstruction techniques in CT, except: a.volume rendering b.maximum intensity projection c.shaded surface display d.pixel-voxel reregistration
pixel-voxel reregistration
The patient dose in CT is determined by the _____ collimator
prepatient
the patient dose in CT is determined by the __________ collimator
prepatient
oral contrast media
required for imaging the abdomen - helps distinguish between bowel loops and other abdominal structures -currently used is Omnipaque and mixed at a low concentration to prevent artifacts
What is the principal disadvantage of third generation CT scanners
ring artifacts
What is the principle disadvantage of third generation CT scanners
ring artifacts
dynamic scanning
scanner is programmed to scan through a body area rapidly and enable image reconstruction. also provides images with different structures enhancing at different rates after contrast administration -Rapid, sequential scanning at the same level to observe contrast filling a structure -Rapid serial scanning at consecutive levels during the injection of contrast
The third generation of CT scanners allowed for further reduction in
scanning time and scattered radiation
the third gen of CT scanners allowed for further reduction in
scanning time and scattered radiation
CT detectors are most commonly
scintillation (solid-state) detectors
This generation of CT scanner has two sets of detectors that are offset by 90 degrees
sixth gen
Dose in CT is considered to be:
size dependent
window width is used to adjust:
the number of grey shades displayed
spatial resolution refers to
the sharpness of an image
What generation of CT scanners was the first to have the fan beam transect the entire patient at all times
third get
CT has improved contrast resolution due
to reduction in scatter radiation
Each sweep of the source-detector around the body during CT is called a
translation
each sweep of the source-detector around the body during CT is called a
translation
scanners are classified by
tube and detector movement (used to be categorized into generations
The CT scanner must be calibrated so that _____ is at CT number zero
water
The gantry is the device that houses the
xray tube detector array