Chapter 1: Physics and Instrumentation
What are the 2 distinct reasons that the image plane is indicated as either axial or coronal?
1) the anatomy of interest lies vertically rather than horizontally (ex- ethmoid sinuses), so that axial images would not capture them, but coronal would. 2) to reduce artifacts created by surrounding structures (ex- coronal plane is preferred for pituitary gland due to number of streak artifacts that occur when scanned in the axial plane).
What are the main advantages of CT over conventional radiography?
1- elimination of superimposed structures 2-ability to differentiate small differences in density of anatomic structures and abnormalities 3-superior quality of images
What is the most common matrix size in NM for SPECT acquisitions with 2 detectors?
128x128
Scan protocols that span a longer anatomic region, such as the abdomen or pelvis, typically use slice thickness' ranging from:
5 to 7mm
What is the most common matrix size in CT?
512
What is the range of matrices used in NM?
64x64 up to 256x256 Choose matrix based on pixel size Pixel Size Formula: FOV/# of pixels
What does image data have that scan data does not?
A Hounsfield Unit number assigned, and sectioned pixels.
Define matrix-
A matrix is the grid formed from the rows and columns of pixels. In CT, the most common matrix size is 512. This translates into 512 pixels down and 512 columns of pixels across.
What is a picture element?
A pixel
Define voxel-
A three dimensional cube (volume element), with x, y and z axis data sets.
What is a volume element?
A voxel
How can you adjust the image plane?
Adjust positioning of patient Adjust positioning of gantry Adjust positioning of both gantry + patient Reformatting the image data
What are image artifacts?
Artifacts are objects seen on the image but not present in the object scanned. Artifacts ALWAYS degrade the image quality. They are either: Physics based Patient based Equipment induced
What is a beam hardening artifact?
Artifacts that result from low energy x-ray beams being absorbed, or attenuated, leaving the higher energy x-ray beams to strike the camera. These happen when the x-ray beams must first penetrate a very dense structure, so you see them on images of the base of the skull. Beam hardening artifacts appear as dark streaks or vague areas of decreased density (cupping artifacts).
Most CT scans are performed in which plane?
Axial
Explain beam attenuation. What determines a structure's ability to attenuate the x-ray beam?
Beam attenuation is the phenomenon by which an x-ray beam passing through a structure is decreased in intensity or amount because of absorption and interaction with matter. The alteration in the beam varies with the density of the structure it passes through. Hounsfield units (HU)- measures the ability of a material to cause beam attenuation. HU -1000 - air HU 0 - distilled water HU +1000 - dense bone
Dark streaks or vague areas of decreased density on your images are likely what type of artifact?
Beam hardening
What are the additional terms for Hounsfield Units?
CT numbers Density values
Most head protocols are performed in which plane?
Coronal
What are the 3 Standard Planes?
Coronal Sagittal Axial
What are the 3 steps in the CT process?
Data acquisition (Obtaining Data) Image reconstruction (Using Data) Image display (Displaying Data)
Define density.
Density can be defined as the mass of a substance per unit volume. It is the degree to which matter is crowded together, or concentrated.
What is an additional term for posterior?
Dorsal
X-ray photons are created when:
Fast moving electrons slam into a metal target. Their kinetic energy is transformed into electromagnetic energy.
How can you minimize beam hardening/cupping artifacts?
Filtering the x-ray beams with a substance like teflon or aluminum helps to reduce the range of x-ray energies that reach the patient, eliminating the weak x-rays. Filtering the soft (low energy) x-ray beams reduces the radiation dose to the patient.
What unit quantifies a structure's ability to attenuate the x-ray beam?
Hounsfield Units Hounsfield units (HU)- measures the ability of a material to cause beam attenuation. HU -1000 - air HU 0 - distilled water HU +1000 - dense bone
In the anatomic position, are the palms facing in front of the person, or behind?
In front of.
Caudal is another term for:
Inferior
Which type of x-ray is easier to attenuate, high energy or low energy?
Low energy
Is scan data measured in voxels or pixels?
Neither. Scan data is raw data, and raw data is information that has not yet been sectioned off to create pixels or voxels. No HU units have been assigned, either.
What are the factors that contribute to inaccurate Hounsfield measurements?
Poor equipment calibration image artifacts volume averaging
What is the reconstruction that is automatically produced during scanning often called?
Prospective reconstruction
If the same raw data is used later on to generate new images, what is that process referred to as?
Retrospective reconstruction
What is another term for raw data?
Scan data
Why does the slice thickness vary among examination protocols?
Selecting a slice thickness limits the x-ray beam so that it passes only through this volume, hence, scatter radiation and superimposition of other structures are greatly diminished. In general, the smaller the object being scanned, the thinner the CT slice required. This decreases the likelihood of missing very small items, but it increases the number of slices needed, which increases exposure. Thicker CT slices increase the likelihood of missing very small objects. Scanning procedures and protocols are designed to provide the image quality necessary for diagnosis at an acceptable radiation dose.
What are the 3 types of Scan Modes?
Step and Shoot Helical (Spiral) Multidetector Row CT
What is the relationship between Hounsfield units and the linear attenuation coefficient?
The Hounsfield unit value is directly related to the linear attenuation coefficient: 1HU equals a 0.1% difference between the linear attenuation coefficient of the tissue as compared with the linear attenuation coefficient of water. Hounsfield units (HU)- measures the ability of a material to cause beam attenuation. HU -1000 - air HU 0 - distilled water HU +1000 - dense bone
Which axis has the most significant effect on volume averaging?
The Z axis, though all three axis' have an impact. The larger the X and Y dimensions (ie the larger the pixel), the more chance the pixel will contain different densities within it.
What happens in the Image reconstruction phase of the CT process?
The data is sorted so that each pixel has one associated Hounsfield value. The goal of this phase is to use information collected in the previous segment and prepare it for display.
What is kinetic energy?
The energy of motion
What role does the size of the matrix play in the number of pixels in the image?
The larger the matrix, the smaller the individual pixels. 124x124 matrix would be 15,376 pixels. 512x512 matrix would be 262,144 pixels. Remember, the dimension of the image remains the same. Your camera heads are not getting any bigger or smaller, so the actual size is not changing regardless of the matrix selection. If you are taking a 4x6 selfie for your desk, that image is 4x6 regardless of the matrix size. The matrix is going to determine how many pixels are in that 4x6.
In CT, what is the contrast difference between objects typically characterized by?
The percentage linear attenuation coefficient: 1% contrast difference corresponds to a difference of 10HU. ***Note the difference in values between linear attenuation coefficient and contrast difference, versus linear attenuation coefficient and Hounsfield Units.*** The Hounsfield unit value is directly related to the linear attenuation coefficient: 1HU equals a 0.1% difference between the linear attenuation coefficient of the tissue as compared with the linear attenuation coefficient of water. Do not confuse the two!
What happens in the Image Display phase of the CT process?
The processed data is converted into shades of gray for viewing.
What defines the Z axis?
The z-axis is defined by the slice thickness. Plane that correlates to the slice thickness, or depth, of the CT slice. The length of the patient is also known as the z-axis. Be aware that including the Z axis (with your x and y axis data) is what changes a pixel into a voxel. The result is a cube, rather than a square.
The number of the photons in an x-ray that interact with the material being imaged depends on:
Thickness Density Atomic Number of the object being imaged Dense objects provide more opportunities for photon interaction.
Define Pixel-
Two dimensional squares (picture elements), with x and y axis data sets. NO Z AXIS.
What are additional terms for anterior?
Ventral Forward Front Toward the face
Width is indicated by which axis?
X axis
What happens in the Data Acquisition phase of the CT process?
X-ray photons are created and directed thru the patient, where they are absorbed (attenuated) or penetrate thru to strike the camera's detectors. The goal of this phase is to acuire information.
Height is indicated by which axis?
Y axis
Each CT slice represents:
a specific plane in the patient's body
X-rays are produced when:
a substance is bombarded by fast-moving electrons
The Coronal Plan divides the body into:
anterior and posterior sections.
Oblique planes are slanted and lie:
at an angle to one of the three standard planes (3 Standard Planes: Coronal Sagittal Axial
The degree to which an x-ray beam is reduced by an object is referred to as:
attenuation
What is another name for cupping artifacts?
beam hardening
The ability of the x-ray tube to withstand head is called it's:
heat capacity
The length and frequency of scans can be determined in part by the x-ray tube's:
heat capacity heat dissipation rate
The ability of the x-ray tube to rid itself of heat is called it's:
heat dissipation
Black or very dark areas on a CT image are referred to as having:
low attenuation. Think about the lungs on a chest x-ray. They are super dark. The darker the area, the less dense the material, the lower the attenuation, the lower the Hounsfield Unit number.
The sagittal plane directly in the center of the body is referred to as:
midsagittal
Inferior refers to:
movement toward the feet (down)
Proximal, which is the opposite of distal, may be defined as situated:
near the point of attachment
Transverse planes are:
parallel to the floor
A sagittal plane that is located to either the left or right of the midline is called:
parasagittal
An x-ray beam consists of bundles of energy known as:
photons
All x-ray beam sources for CT and conventional radiography produce x-ray energy that is _____________.
polychromatic. This means that the x-ray beam is made of photons with varying energies. They are not a singular energy level.
The sagittal plane divides the body into:
right and left sections.
Cephalic is another term for:
superior
The Hounsfield unit of a single pixel is comprised of:
the average of all data measurements within that pixel. This is called volume averaging. (This can lead to inaccuracies in the image.) Using a smaller pixel size reduces the likelihood of volume averaging by limiting the amount of data to be averaged.
Each detector cell is sampled and converted to a digital format by:
the data acquisition system (DAS).
Distal refers to movement toward:
the ends.
The terms distal and proximal are most often used in referring to:
the extremities (limbs)
Superior defines movement toward:
the head
The intensity of the x-ray beam is controlled by what setting?
the kVp setting
Medial refers to movement toward:
the midline of the body
Lateral refers to movement toward:
the sides of the body
Another name for horizontal is:
transverse
Axial planes divide the body into:
upper and lower sections.
The process in CT by which different tissue attenuation values are averaged to produce one less accurate pixel reading is called:
volume averaging Using a smaller pixel size reduces the likelihood of volume averaging by limiting the amount of data to be averaged.
