CT

Anode

positively charged electrode that attracts electrons; in CT this is where the accelerated electrons from the cathode will strike to produce x-rays; large amounts of heat are generated from this process.

Beam Current

stream of electrons traveling from the cathode to the anode; measure in mA.

Beam Hardening Artifacts

Artifacts such as streaks, blurs and shadows which result from the lower energy photons that are preferentially filtered by the intervening tissues being absorbed more than high energy photons as the beam passes through the patient; effect is dependent on the composition of the tissue and the path of the beam thus these artifacts are commonly observed in larger patients.

Bowtie Filter

Pre-patient filter that reduces the dynamic range of the data acquisition system and improves noise. Reduces x-ray beam intensity, improve homogeneity, reduces skin dose.

Bremsstrahlung

x-ray emitted when a high energy electron is slowed down and/or deflected by a positively charged nucleus; major source of x-rays in CT.

Cathode

negatively charged electrode that acts as a source of electrons; in CT this heated filament boils off electrons and accelerates them towards the anode.

Characteristic X-Ray

Monoenergetic x-rays that are produced by electrons dropping to a lower energy shell in the atom. These x-rays may result from ionization of the atom by a process that removes an inner shell electron such as a photoelectric interaction. High speed electrons from cathode interact with an inner shell electron of tungsten of anode. Inner electron is ejected, outer electron moves in, releasing energy.

Cone Beam

Used in 3rd generation CT machines. Increases the speed of the exam by increasing the area imaged by being able to reach multiple detectors in the fan beam array and multiple rings while constantly moving.

CTDI

Computed tomography dose index. The average dose imparted by a single axial acquisition to a standard 100mm pencil chamber dosimeter inside a PMMA phantom over the width of 14 CT slices.

Dose Length Product (DLP)

Represents the integrated dose in terms of the total scan length. This is an indicator of biologic risk.

Flat Filter

Pre-patient filter that removes low energy x-rays that would otherwise be absorbed by the patient.

Fluence

The total number of photons integrated over time. It is defined by the current and exposure time.

Focal Spot

The area on the anode of an x-ray tube or the target that is struck by electrons and from which the resulting x-rays are emitted.

Geometry

center-to-center spacing of detector elements decreases as the distance from the ring center increases; geometric corrections make the spacing uniform across the field of view.

Helical CT

Continuously rotating x-ray tube and detector.

High Contrast (Spatial) Resolution

The ability of the system to resolve high contrast objects of increasingly smaller size. Can be influenced by pixel size, reconstruction filters, and geometric resolution limits.

Hounsfield Unit

A unit of measured value placed into the pixels of a CT image. The value represents a relative density to water, which has a value of zero on the Hounsfield scale, with densities lower than water having a negative number (air=-1000) and densities higher than water having a positive number (bone 100-3000)

Kernel

The image filter used in the CT reconstruction algorithm to calculate the sharpness of a reconstructed image.

Kilovolt Potential (kVp)

The peak voltage that can be applied between the cathode and anode of an x-ray tube; the potential difference between the cathode and anode.

Low Contrast Resolution

The ability of a system to resolve objects having small difference from background.

Low Dose CT

x-ray beam energy and beam current are set low to minimize radiation exposure to the patient.

Milliampere (mA)

A unit of electric current that describes the flow of charge per second. Used to measure the number of photons generated by the x-ray tube in CT imaging.

Partial Volume Effect

When the size of an object begins to approach the resolution limits of an imaging system, resulting in a voxel containing more than one type of tissue.

Pitch

The ratio of table movement per revolution over the collimated slice thickness of one row of a multi-slice CT detector.

Postpatient Collimator

It removes scattered x-rays that would lead to image degradation.

Prepatient Collimator

It reduces the x-ray flux in the z-direction, and in single slice CT scanners it defines the slice thicknes

Scout Scan

An x-ray examination performed with the CT gantry in a fixed position to generate a planar image. Used to define the body region to be imaged during the CT scan. This is sometimes referred to as a topogram.

Segmentation

replace tissue values in transmission or CT scans with known tissue values in order to reduce noise in the transmission attenuation correction maps

Sinogram

A collection of projections for one slice that are arranged by radial distance and angle.

Slip Ring Technology

Allows for continuous rotation of x-ray tube and eliminates the need for cables

Thermoionic Emission

A boiling off of electrons at the filament"; the process by which charge carriers, such as electrons or ions, move over a surface or some sort of energy barrier by the induction of heat.

Window/Level

represents the central Hounsfield unit of all the numbers within the window width.

Window Width

Range of Hounsfield Units displayed on an image

Explain the process of x-ray production that occurs in the x-ray tube of a CT scanner.

Within the x-ray tube, is the cathode which includes a filament or coil of wire. When a current is applied to the filament, it is heated, electrons are boiled off and are then accelerated toward the anode assembly which includes a tungsten target. The electrons interact with the target and produced via Bremsstrahlung and characteristic x-rays. The x-ray beam then exits the tube from a window. There is a tremendous amount of heat that is produced from this process and only about 1% of the energy applied to the system results in the production of x-rays that exit the tube window. The remaining 99% of the energy creates heat.

Compare the purpose of the filter to that of the pre-patient collimator

The purpose of the filter is to absorb most of the low-energy Bremsstrahlung x-rays that do not contribute to the quality of the image and therefore only increase radiation exposure to the patient. The filter is made of a low atomic material such as aluminum. A bowtie filter may also be used to ensure a more uniform energy distribution and thus improves noise homogeneity. The pre-patient collimator is used to shape the beam and reduce scatter radiation, therefore reducing unnecessary dose to patients as well as improving image contrast resolution.

Pitch increases

noise

Pitch <1

indicates overlap from one rotation to the next

Pitch = 1

table movement per rotation equals the slice width, such that tissues exposed to radiation are contiguous

Pitch >1

gaps are present between rotations; not all tissues are exposed to radiation

kVp and image quality

pertains to the maximum potential energy of the x-rays being emitted during a CT scan; x-ray energy needs to be high enough to pass through the patient and be detected as opposed to being absorbed within the patient; quality of x-rays

mA and image quality

pertains the number of x-rays being emitted during a CT scan; quantity of x-rays

Compare high contrast resolution and low contrast resolution.

High contrast resolution affords the ability to differentiate objects or organs with a significant density difference such as a calcified lesion in the lung.

Low contrast resolution refers to the ability to see small objects with little density differences such as a liver mass.

beam hardening artifact

Lower energy x-rays are more easily and readily absorbed than higher energy x-rays. In a CT scan, this means that the x-ray beam beyond an absorbing material (such as a metal implant) is harder and therefore has different absorption properties than other parts of the x-ray beam not being attenuated in this manner.

CT voxel values are given in terms of

Hounsfield units

CT voxel values are given in terms of Hounsfield Units, which express the attenuation of a given tissue relative to that of

water

F18DG decays by

positron emission

Explain positron emission

proton loses an e- and becomes a neutron and gives off a positron

FDG gamma energy

511 keV

FDG half life

110 mins

Producing FDG

one of the hydroxyl groups in a glucose molecule gets replaced with a fluorine atom through a series of chemical reactions. The resulting FDG (fluorodeoxyglucose) molecule is capable of undergoing glycolysis in cells throughout the body, mimicking a glucose molecule.

F18 FDG MOL

passive transport (High to low conc.), facilitated diffusion via GLUT protein glucose transporters

How is FDG trapped in cells

phosphorylates in first step of glycolysis.

What is F18 FDG ideal to image?

cancer/inflammatory cells due to high metabolic rate due to glycolysis.

FDG Indications

Oncology, neurology, infection/inflammation

Oncologic FDG indications

Lymphoma, Breast Cancer, Head and Neck Cancer, Melanoma, Myeloma, Sarcoma, Leukemia

Neurological FDG indications

Alzheimer’s, Frontotemporal Dementia (FTD), Epilepsy

Infection/inflammatory FDG indications

Graft Infection, Fever of Unknown Origin (FUO), Endocarditis, Sarcoidosis

FDG Contraindications

Food in < 4 hours, Physical Exertion within 24 hours (subjective), BGL > 250 mg/dl. For Diabetic patients: Not off rapid-acting insulin for at least 2 hours, or Not off regular-acting insulin for at least 4 hours.

FDG Normal Distribution

Brain, bladder, salivary glands, myocardium, liver, spleen, kidneys, thyroid, muscles, blood vessels, thymus, brown fat.

FDG Body dose

10 mCi

FDG Body dose for Quadra

0.06 mCi/kg, 8 mCi max

FDG Brain dose

5 mCi

FDG Peds dose

0.1 mCi/kg (min of 1 mCi)

FDG uptake

body- 60 mins

brain- 30 mins

FDG Scan Prep

• NPO for 4-6 hours (water is okay)

•Tube Feeding (TPN) off for 6 hours

•Nutritional Supplements (Ensure, Boost) off for 8 hours

• No workout for at least 24 hours

• For Diabetic patients:

• Off insulin for 2-4 hours depending on type taken

Cardiac Sarcoidosis

inflammatory conditioning affecting heart. Formation of granulomas in myocardium

Why does cardiac sarcoid scan prep include keto diet (no carbs high fat/protein)?

• When patients are on a high fat/protein diet, the body relies more on ketone bodies for energy (aka keto diet)

•This include cardiac myocytes

• Thus, we need to have patients follow dietary restrictions of carbohydrates to have the cardiac muscle rely heavily on ketone bodies and not on carbohydrates for energy

• This would limit cardiac muscle uptake, while immune cells’ metabolism remain unaltered and they will still take in FDG

How Can FDG Scan Detect Cardiac Sarcoidosis?

• Fluorodeoxyglucose (FDG) is capable of getting transported into the immune cells because they are actively causing the inflammatory condition, needing energy in the source of FDG.

• However, cardiac myocytes (muscle cells) are also demanding FDG for metabolic processes.

FDG Prep Questions

• Fasting within last 4 hours?

• Diabetic?

•Taking Insulin?

• Vaccines within last 2 months?

• Superficial medical devices on skin?

• GLP-1 Medications?

Non-fasting patient

Non-fasting patient

chest workout prior

Rapid acting insulin 90min before FDG injection in diabetic patient with fasting bgl over 160 mg/dL

know this

mA effect

quantity of photons (bean intensity)

kVp

kilovolt peak

kVp effect

effects quality of photons (beam energy)

Image quality mA

higher = lower noise

Image quality kVp

higher = lower contrast

mA range

50-800 (CT 10-440)

kVp range

80-140

Computed tomography

combines X-rays images taken from different angles to create cross sectional images. spatial resolution reduced by an order of magnitude over radiography.

3 components of CT

Gantry

Operating Console

Computer and Array Processor

Gantry of CT

frame housing of the x-ray tube, collimators, detectors, DAS, slip rings

Data Acquisition system in gantry

Measures transmitted intensity of radiation beam, turns data into digital signal via analog to digital converter ADC, sends binary data to computer.

Operating console functions:

Select protocol, change image factors, move table, patient info. Post process images.

Computer and Array Processor functions

Image reconstruction system. Receives info from the DAS and creates image.

the opening of the gantry is called

the Aperture

Typical generator power

20-100 kW

total collimation =

beam width

Purpose of CT Filters

remove unnecessary low energy photons, produce uniform distribution.

Inherent + added = total

3 CT scanning methods

CT localizer radiography, axial/conventional, helical/spiral

CT localizer radiography scan

scout, tube stationary and patient moves.

Axial/conventional scan

better special resolution. Bed is stationary, tube rotates. Data can be overlapping, contiguous, or with gaps.

What is a projection

view, set of ray sums at fixed angle

What is a ray and ray sum

ray=one x ray path

Ray sum= line integral, total attenuation along one ray

Helical/spiral scan

Tube and bed in motion. Needs interpolation to reconstruct image.

Effective mA:

mAs per slice, mAs divided by pitch

average photon energy (keV) is

30-40% of applied kilovoltage.

Focal spot size

0.5-1.2 mm