CT

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

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

each projection Is

stored on the computer

The CT system's ability to freeze any motion of a scanned object is termed

temporal resolution

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!

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

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

moderate reactions to contrast

excessive hives tachycardia or bradycardia Hypertension or hypotension Dyspnea Bronchospasm/wheezing

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

CT scanners operate on _____ voltage generation

high frequency

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

The time from the end of CT imaging to image appearance is called

reconstruction time

thinner slices improve

recorded detail

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

______________ ______________ 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

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 reference material for CT is ____ and is related to the ____ of the tissue.

water, linear attenuation coefficient

The gantry is the device that houses the

xray tube detector array