Angiography

1.  The structures in a CT image are represented by
varying shades of gray.
 The creation of these shades of gray is based on the
attenuation (degree to which a beam is reduced).
 An x-ray photon may pass through a structure,
may be scattered(redirected) by a structure,
absorbed by a given structure in varying amounts,
depending on
- the strength (average photon energy) of the x-ray
beam
- the characteristics of the structure in its path.

2.  X-ray photons that pass through objects are
represented by a black area on the image.
These areas on the image are commonly
referred to as having low attenuation.
 An x-ray beam that is completely absorbed by
an object cannot be detected; the place on the
image is white and the object is said to have
high attenuation.

3.  The number of the photons that interact
depends on the thickness, density, and atomic
number of the object that the beam is passing
through.
 Density can be defined as the degree to which
matter is crowded together, or concentrated.
 The number of photons that interact increases
with the density, thickness, and atomic number
of the object

4.  The amount of the x-ray beam that is scattered
or absorbed per unit thickness of the absorber
is expressed by the linear attenuation
coefficient, represented by the Greek letter μ.
 For example, if a 125-kVp x-ray beam is used,
the linear attenuation coefficient for water is
approximately 0.18/cm, meaning that about
18% of the photons are either absorbed or
scattered when the x-ray beam passes through
1 cm of water

5.  If the kVp is kept constant, the linear
attenuation coefficient will be higher for bone
than it would be for lung tissue.
 That is, bone attenuates more of the x-ray beam
than does lung, allowing fewer photons to
reach the CT detectors.
 this results in an image in which bone is
represented by a lighter shade of gray than that
representing lung

6.  Differences in linear attenuation coefficients
among tissues are responsible for x-ray image
contrast.
 In CT, the image is a direct reflection of the
distribution of linear attenuation coefficients

7.  metallic objects and bone are represented on
the CT image as white areas.
 Air (gas) has very low density, so it has little
attenuation capacity.
 Air-filled structures (such as lungs) are
represented on the CT image as black areas.
 To differentiate an object on a CT image from
adjacent objects, there must be a density
difference between the two objects.

8.  An oral or intravenous administration of a
contrast agent is often used to create a
temporary artificial density difference between
objects.

9.  In CT we are able to quantify the beam
attenuation capability of a given object.
 Measurements are expressed in Hounsfield
units (HU), named after Godfrey Hounsfield,
one of the pioneers in the development of CT.
 These units are also referred to as CT numbers.
 Hounsfield units quantify the degree that a
structure attenuates an x-ray beam

10.  Hounsfield arbitrarily assigned distilled water
the number 0
 He assigned the number 1000 to dense bone
and −1000 to air.
 Objects with x-ray beam attenuation less than
that of water have an associated negative
number.
 Substances with an attenuation greater than
that of water have a proportionally positive
Hounsfield value

11.  Using the system of Hounsfield units, a
measurement of an unknown structure that
appears on an image is taken and compared
with measurements of known structures. It is
then possible to approximate the composition
of the unknown structure

12.  In general, the smaller the object being
scanned, the thinner the CT slice required.
 As an example lung nodules; the chest is sliced
and examined , some slices contain nodules
and others do not.
 If the slices are thick, it increases the possibility
that even though a given slice contains a
nodule, it will be obscured by the normal lung
tissue. If the slices are thin, the likelihood of
missing a nodule decreases, but the total
number of slices increases.

13.  Thinner slices result in a higher radiation dose
to the patient.
 In addition, if the area to be scanned is large, a
huge number of slices are produced.
 Scanning procedures are designed to provide
the image quality necessary for diagnosis at an
acceptable radiation dose

14.  If the structures being investigated are very small
(coronary arteries) and the region to be scanned is
not extensive (the heart versus the entire
abdomen), then slice thickness can be quite thin.
 Conversely, scan protocols that span a longer
anatomic region (such as the abdomen and pelvis)
typically use a slice thickness of 5 to 7 mm.
 In addition, spiral-scanning techniques have
allowed options for using data sets to
retrospectively adjust the slice thickness when
circumstances dictate.

15.  The terms scan data and raw data are used
interchangeably to refer to computer data
waiting to be processed to create an image.
 The process of using the raw data to create an
image is called image reconstruction.
 Once raw data have been processed so that
each pixel is assigned a Hounsfi eld unit value,
an image can be created.

16.  The reconstruction that is automatically
produced during scanning is often called
prospective reconstruction.
 The same raw data may be used later to
generate new images. This process is referred
to as retrospective reconstruction.

17.  Step-and-Shoot Scanning:
The scanning systems of the 1980s operated
exclusively in a “step-and-shoot” mode. In this
method:
1) the x-ray tube rotated 360° around the patient to
acquire data for a single slice,
2) the motion of the x-ray tube was stoped while the
patient was advanced on the CT table to the location
appropriate to collect data for the next slice,
3)Steps one and two were repeated until the desired
area was covered.

18.  The step-and-shoot method was necessary
because the rotation of the x-ray tube entwined
the system cables, limiting rotation to 360°.
 Consequently, gantry motion had to be
stopped before the next slice could be taken,
this time with the x-ray tube moving in the
opposite direction so that the cables would
unwind

19.  Many technical developments allowed for the
continuous acquisition scanning mode most
often called spiral or helical scanning.
 Key among the advances was the development
of a system that eliminated the cables and
thereby enabled continuous rotation of the
gantry.
 This, in combination with other improvements,
allowed for uninterrupted data acquisition that
traces a helical path around the patient

20.  Anterior and ventral refer to movement forward
 Posterior and dorsal --the back surface of the body.
 Inferior and caudal –down
 Superior or cranial or cephalic toward the head
(up)
 Lateral refers to movement toward the sides of the
body.
 Medial refers to movement toward the midline of
the body
 Distal (away from) or towards the end of an
extremity and proximal (close to)near a point

22.  CT scanners are similar in that they consist of a
scanning gantry, table, x-ray generator,
computer system, operator’s console, and
physician’s viewing console
 The process includes data acquisition, image
reconstruction, and image display.

23. ▪ Components are mounted on a rotating scan frame.
▪ Gantries vary in total size as well as in the diameter of the
opening, typically 70 to 90 cm.
▪ The CT gantry can be tilted either forward or backward as
needed with the degree of tilt ±15° to ±30° is usual.
▪ The gantry also includes a laser light that is used to position
the patient within the scanner.
▪ Control panels located on either side of the gantry opening
allow the technologist to control the alignment lights, gantry
tilt, and table movement.
▪ In most scanners, these functions may also be controlled via
the operator’s console.
▪ A microphone is embedded in the gantry to allow
communication between the patient and the technologist
throughout the scan procedure

25.  Early CT scanners used recoiling system cables
to rotate the gantry frame.
 Current systems use electromechanical devices
called slip rings.
 Slip rings use a brushlike apparatus to provide
continuous electrical power and electronic
communication across a rotating surface.
 They permit the gantry frame to rotate
continuously, eliminating the need to
straighten twisted system cables

26.  High-frequency generators are currently used in CT.
 They are small enough so that they can be located
within the gantry
 Generators produce high voltage and transmit it to the
x-ray tube. The power capacity of the generator is
listed in kilowatts (kW).
 The power capacity of the generator determines the
range of exposure techniques (i.e., kV and mA settings)
available on a particular system.
 CT generators produce high kV (generally 120–140 kV)
to increase the intensity of the beam, which will
increase the penetrating ability of the x-ray beam and
thereby reduce patient dose.

27.  Compensating filters are used to shape the x-ray
beam.
 They reduce the radiation dose to the patient and
help to minimize image artifact.
 The radiation beam emitted by CT x-ray tubes is
polychromatic. Filtering the x-ray beam helps to
reduce the low energy range of x-ray photons.
 These photons are absorbed by the patient,
therefore they do not contribute to the CT image
but do contribute to the radiation dose to the
patient.

28.  The compensating filters used in CT are often
called bowtie filters because of their shape.
 The filter is thinnest at the central ray of the beam
and thicker toward the edges of the fan beam.
 Thus the thinnest part of the filter is in line with
the thickest path length through the patient’s body
as the gantry rotates, and the thicker part of the
filter is in line with the thinner peripheral
anatomy. The filter partially compensates for the
patient’s body thickness contour
 The bowtie filter is selected by the scanner and
switched into the beam path automatically based
on the scan setup chosen by the operator.

29. A beam composed of different photon energies passing through one half-
value layer of absorber will have a lower intensity (fewer photons) and a
higher average energy in the transmitted beam, because the low-energy
photons are more easily absorbed and the high-energy photons are better able
to penetrate the absorber.

30.  Collimators restrict the x-ray beam to a specific
area, thereby reducing scatter radiation.
 Scatter radiation reduces image quality and
increases the radiation dose to the patient.
 Reducing the scatter improves contrast
resolution and decreases patient dose.
 Collimators control the slice thickness by
narrowing or widening the x-ray beam

31.  The source collimator is located near the x-ray
source.
 Because it acts on the x-ray beam before it
passes through the patient it is sometimes
referred to as pre-patient collimation.
 The source collimator affects the slice thickness.
 In MDCT systems, slice thickness is also
influenced by the detector element
configuration.

34.  Some CT systems also use pre-detector
collimation.
 This is located below the patient and above the
detector array.
 Because this collimator shapes the beam after it
has passed through the patient it is sometimes
referred to as post-patient collimation.
 The primary functions of pre-detector
collimators are to ensure the beam is the proper
width as it enters the detector and to prevent
scatter radiation from reaching the detector.

35.  As the x-ray beam passes through the patient it
is attenuated to some degree.
 To create an x-ray image we must collect
information regarding the degree to which
each anatomic structure attenuated the beam.
 We use detectors to collect the information
 The detector array comprises detector elements
situated in an arc or a ring, each of which
measures the intensity of transmitted x-ray
radiation along a beam projected from the x-
ray source to that particular detector element

36.  Detectors can be made from different substances,
each with their own advantages and
disadvantages.
 The optimal characteristicsof a detector are as
follows:
1) high detector efficiency, defined as the ability of
the detector to capture transmitted photons and
change them to electronic signals;
2) low, or no, afterglow, defined as a brief, persistent
flash of scintillation that must be taken into
account and subtracted before image
reconstruction;
3) high scatter suppression;
4) high stability, which allows a system to be used
without the interruption of frequent calibration

37.  Detector efficiency can be measured as
- geometric efficiency, defined as the amount of
space occupied by the detector collimator plates
relative to the surface area of the detector;
- Absorption efficiency refers to the number of
photons absorbed by the detector and is dependent
on the physical properties of the detector face (e.g.,
thickness, material)
- Response time is the time required for the signal
from the detector to return to zero after
stimulation of the detector by x-ray radiation so
that it is ready to detect another x-ray event

38.  The relative
placement, shape, and
size of the detectors
affect the amount of
scatter radiation that
reaches the image.
 A deep, narrow
detector design will
accept less scatter
than short, wide
detectors Low scatter acceptance is desirable

39.  Solid-state detectors are also called scintillation
detectors because they use a crystal that fluoresces
when struck by an x-ray photon
 These detectors use crystals composed of fast
scintillator ceramics such as cadmium telluride
and gadolinium oxysulfide.
 The crystals convert x-rays to light pulses whose
intensity is proportional to the total energy of
photons striking the crystal.
 The light is converted to electrical current which in
turn is detected as digital data for use by the
reconstruction system.

40.  Detectors are separated using spacing bars. This
allows the detectors to be placed in an arc or circle.
 Ideally, detectors should be placed as close
together as possible, so all x-rays are converted to
data.
 The size of the detector opening is called the
aperture.
 A small detector is important for good spatial
resolution and scatter rejection.
 Maximum utilization and small aperture are
desirable

41.  Detector arrays continue to grow as individual
detector elements remain small, providing
small pixel size with ever-greater anatomic
coverage.
 Common detector arrays accommodate
scanning of 80 to 160 mm of coverage in a
single tube rotation, with resolution of 0.5 to
0.6 mm, so that entire sections of patient
anatomy can be captured in a single revolution
of the CT scanner’s rotating frame, taking less
than 1 second.

42.  Computed Tomography for Technologists. Louis
E. Romans
 Principles of CT: Multislice CT, Lee W. Goldman,
Journal of Nuclear medicine Technology
 Scan Acquisition Settings - Trade Offs Between
Speed, Resolution and Dose, James M. Kofler,
Ph.D., Radiology, Mayo Clinic Rochester,
Minnesota
 http://xrayphysics.com/ctsim.html
 Radiologykey.com
 Radiopedia.org