View
217
Download
0
Category
Preview:
Citation preview
7/30/2019 Spiral CT Dr.shravani
1/82
By
Dr. Shravani
7/30/2019 Spiral CT Dr.shravani
2/82
Basic principle
The internal structure of an object can bereconstructed from multiple projections of the same.
The ray projections are formed by thin cross sections
of the scanning part with a narrow x-ray beam
The transmitted radiation from this beam is
measured by using a sensitive radiation detector.
This measured radiation is fed into a computer and
analysed by using mathematical algorithms andreconstructed as a tomographic image.
7/30/2019 Spiral CT Dr.shravani
3/82
X-ray goes through
collimator therefore
penetrate only an
axial
layer of the object,
called
"slice"
How does CT Work?
7/30/2019 Spiral CT Dr.shravani
4/82
Patient is placed in the centerof the measurement field
X-ray is passed through thepatients slice from many
direction along a 360 path
The transmitted beams arecaptured by the detectorswhich digitizes these signals
These digitized signals calledraw data are sent to acomputer which create the CTimage
7/30/2019 Spiral CT Dr.shravani
5/82
The object slice is divided
into small volume
elements called voxels.
Each voxel is assigned a
value which is dependent
on the average amount of
attenuation
How is CT Image generated?
7/30/2019 Spiral CT Dr.shravani
6/82
The attenuation values aretransferred to the computer where
they are coded & used to create aslice image
7/30/2019 Spiral CT Dr.shravani
7/82
CT Data Acquisition Components
7/30/2019 Spiral CT Dr.shravani
8/82
DATA ACQUISITIONThe scanning process begins with
data acquisition.
Data Acquisition refers to amethod by which the patient issystematically scanned by the X
ray tube and detectors to collectenough information for imagereconstruction.
A basic data acquisition schemeconsists of
X ray tube
Filters
Collimators
Detectors
7/30/2019 Spiral CT Dr.shravani
9/82
Image reconstruction
Substance HU
Air -1000
Fat -120
Water 0Muscle +40
Contrast +130
Bone +400 or more
9
The pixel itself is displayed according to the mean attenuation
of the tissue(s) that it corresponds to on a scale from +3071
(most attenuating) to -1024 (least attenuating) on the
Hounsfield scale
Voxel is a volumetric pixel
The HU of common
substances:
7/30/2019 Spiral CT Dr.shravani
10/82
CT Gantry
7/30/2019 Spiral CT Dr.shravani
11/82
CT gantry internal components
1.X-ray tube &
collimator
2.Detector assembly
3.Tube controller
4.High freq. generator
5.Onboard computer6.Stationary computer
7/30/2019 Spiral CT Dr.shravani
12/82
X-RAY TUBE
Rotating anode type
More heat loading and
heat dissipationcapabilities
Small focal spot size
(0.6mm) to improvespatial resolution
7/30/2019 Spiral CT Dr.shravani
13/82
FILTERS
Compensation filter is being used
To absorb low energy x rays
To reduce patient dose
To provide a more uniform beam
7/30/2019 Spiral CT Dr.shravani
14/82
COLLIMATORS
To decrease scatter radiation
To reduce patient dose
To improve image quality Collimator width determines
the slice thickness
7/30/2019 Spiral CT Dr.shravani
15/82
DETECTORS The detectors gather information by measuring the x-ray
transmission through the patient.
Two types:
Scintillation crystal detector
(Cadmium tungstate+ Si Photodiode)
Can be used in third and fourth generation scanners
Xenon gas ionisation chamber
Can be used in third generation scanners only
7/30/2019 Spiral CT Dr.shravani
16/82
Scintillation crystal detector used in I & II gen. CT scanners
When X ray Photons are absorbed by these materials the
convert them into light photons and again converted into
electric signals by photo multiplier diode.
7/30/2019 Spiral CT Dr.shravani
17/82
Scintillation crystal detector used in III and IV gen. CT
scanners
7/30/2019 Spiral CT Dr.shravani
18/82
First Generation Scanners
Translate - rotate one detector.
Total Scan time : 25-30 minutes (5 minutes per section).
Uses pencil beam to scan the body.Arc of rotation of the gantry by 1 degree.
d
7/30/2019 Spiral CT Dr.shravani
19/82
Second Generation Scanners
Translate rotate multiple detector (30).
Uses fan beam.
Multiple detectors are used.Scan time for single section : 10 -90 seconds.
hi d i
7/30/2019 Spiral CT Dr.shravani
20/82
Third Generation Scanners
Rotate rotate type.
No linear motion of the Gantry.
Multiple detectors are used.Single section Scan time : 4 9 seconds.
F h G i S
7/30/2019 Spiral CT Dr.shravani
21/82
Fourth Generation Scanners
Rotate fixed type.
Detectors are arranged in fixed circles around the
patient and they do not move.Single section Scan time : less then 1 second.
7/30/2019 Spiral CT Dr.shravani
22/82
Scan Modes
22
Conventional (incremental) CT(short pause , several minutes)
Spiral (helical) CT
(helical motion of X-ray tube,reducing time and radiation
exposure)
7/30/2019 Spiral CT Dr.shravani
23/82
What is Spiral Scan? -- just 4C
Continuously rotating tube/detector system
Continuously generating X-ray
Continuously table feed
Continuously data acquisition
7/30/2019 Spiral CT Dr.shravani
24/82
Newer method
X-ray tube moves continuously with slip-ring
technique, the outer fixed detector ring and
inner rotating x-ray tube ring having brushes thatremain in electrical contact with the inside of
outer ring.
X-ray tube moves continuously with patientmoving cephalad with constant rate producing a
spiral or helix around the patient.
Spiral CT
7/30/2019 Spiral CT Dr.shravani
25/82
CT SCANNING IN SPIRAL-HELICAL
GEOMETRY BASED ON SLIP RING
TECHNOLOGY
Sli i T h l
7/30/2019 Spiral CT Dr.shravani
26/82
Slip-ring Technology
Power is transmitted through parallel sets of
conductive ringsinstead of electrical cables.
Continuous Gantry RotationPrerequisite for Spiral CT
Non Slip-ring Scanner Slip-ring Scanner
7/30/2019 Spiral CT Dr.shravani
27/82
SPIRAL CT MAJOR STEPS
DATA ACQUSITION-ENTIRE TISSUE IS BEING
SCANNED DURING ONE BREATH HOLD
IMAGERECONSTRUCTION-INTERPOLATION USED TO
GENERATE SLICES. FILTERED BACK PROJECTION USED
TO REDUCE BLURR.
7/30/2019 Spiral CT Dr.shravani
28/82
DATA ACQUSITION
Z-AXIS
7/30/2019 Spiral CT Dr.shravani
29/82
Reconstruction of arbitrary
slices (either contiguous or
overlapping) within the
scanned volume
Distance between the
slices is called Increment.
A BVolume Data
Continuous dataacquisition
During the scanning procedure, the tube is
rotating & emitting x-rays at all time.
The patient is advanced at a constant speed
through the rotating gantryThis produced a volume data set from which
multiple slices can be reconstructed either in a
contiguous or overlapping manner.
The images can be produced at any position
within the scanned volume A & B
7/30/2019 Spiral CT Dr.shravani
30/82
Increment
Slice Thickness
Increment = Slice Thickness No Overlap
No Gaps
Contiguous Image
Reconstruction
7/30/2019 Spiral CT Dr.shravani
31/82
Increment
Overlap
Slice Thickness
Overlapping Image
Reconstruction
Increment < Slice Thickness Overlap of slices
Closer image interval
More images created
7/30/2019 Spiral CT Dr.shravani
32/82
Increment > Slice Thickness Gaps between slices
Images are further apart
Less images created
Image Reconstruction with
Gaps
Increment
Slice Thickness
7/30/2019 Spiral CT Dr.shravani
33/82
Deep Inspiration Shallow Inspiration
Misregistration due to differentrespiratory levels between slices
Partial Volume EffectStandard CT / Slice Imaging
Unable to reconstruct images atarbitrary position
Slice imaging is slow
Conventional CT scanning is slow because it
requires multiple slice acquisition to cover the
volume of interest
Despite the fact that scanning are performed
with consecutive slices, it is easy to miss small
lesion when the patient varies his depth of
inspiration from scan to scan.It is also difficult to reduce partial volume effect.
In order to do so, the lesion must be captured
fully, not partially within the slice. Several
overlapping scans may be repeated, with the
hope that the patient hold his breath at the
same depth each time.
7/30/2019 Spiral CT Dr.shravani
34/82
Why do we need spiral/helical CT scan
It eliminates respiratory misregistration and motionartifacts by rapid acquisition in a single breath-hold.
Produces overlapping images without additionalradiation. From single helical CT scan data, multiple,overlapping axial sections can be retrospectivelyreconstructed.
Multidimensional imaging is possible 2D / 3Dreformations with maximum longitudinal resolutionoptimizing image quality.
7/30/2019 Spiral CT Dr.shravani
35/82
SPIRAL CT MODES
SINGLE SLICE
7/30/2019 Spiral CT Dr.shravani
36/82
SPIRAL CT MODES
MULTI - SLICE
7/30/2019 Spiral CT Dr.shravani
37/82
Conventional CT Alternate translation & x-
ray exposure is done.
Each rotation of x-ray tubegenerates data of atransaxial image
Prospective selection of
slice positioning andspacing
Spiral CT Both are done
simultaneously
Each rotation yields data
Specific to an angledplane of section. Fromthis, by interpolation,transaxialimage is reconstructed.
Done retrospectively .
7/30/2019 Spiral CT Dr.shravani
38/82
Protocol factors of spiral CT scan
Slice thickness / Collimation:
5 to 8mm for abdomen scan.7 to 10 mm for chest scan.
5 mm for neck scan.
2 to 3 mm to scan small structures like lung nodules, smaller
blood vessels.It affects effective slice thickness of reconstructed transaxial
scan and its longitudinal resolution.
7/30/2019 Spiral CT Dr.shravani
39/82
7/30/2019 Spiral CT Dr.shravani
40/82
Advantages of spiral overconventional CT
Shorter scanning timeReconstruction along z axis with overlapping
of successive images leading to detection of
small lesionsFewer motion artifacts
3D reconstruction of image
7/30/2019 Spiral CT Dr.shravani
41/82
Applications:
CT angiography.
Cardiac CT with gating to freeze cardiac
motion. Coronary artery imaging including calcium
scoring.
Tumor permeability studies.
Functional and perfusion imaging. In Emergency acute abdomen cases.
7/30/2019 Spiral CT Dr.shravani
42/82
Despite high heart rate of 90 bmp,
motion free visualization of the
coronary arteries.
.
7/30/2019 Spiral CT Dr.shravani
43/82
Hepatic Laceration & Adrenal hematoma due to abdominal trauma
Fig: A Fig: B
7/30/2019 Spiral CT Dr.shravani
44/82
7/30/2019 Spiral CT Dr.shravani
45/82
7/30/2019 Spiral CT Dr.shravani
46/82
7/30/2019 Spiral CT Dr.shravani
47/82
7/30/2019 Spiral CT Dr.shravani
48/82
7/30/2019 Spiral CT Dr.shravani
49/82
Hepatic focal fibrosis misdiagnosed as hepatocellular
7/30/2019 Spiral CT Dr.shravani
50/82
carcinoma. Axial helical CT images obtained during
hepatic arterial and portal venous phases reveal multiple
round, hypo attenuating areas (arrows) in right and left
liver lobes. This finding simulated hypo vascular massand was originally misdiagnosed as possible tumour.
ARTIFACTS IN COMPUTED
7/30/2019 Spiral CT Dr.shravani
51/82
ARTIFACTS IN COMPUTED
TOMOGRAPHY
7/30/2019 Spiral CT Dr.shravani
52/82
ARTIFACT
Any discrepancy between the CT numbers
represented in the image and the
expected CT number based on the linear
attenuation coefficient
7/30/2019 Spiral CT Dr.shravani
53/82
PATIENT INDUCED ARTIFACTS
Motion
Beam Hardening Metal Artifact
Out Of Field Artifact
7/30/2019 Spiral CT Dr.shravani
54/82
MOTION ARTIFACT CAUSED BY:
Voluntary motion.
Involuntary motion.
It produces ghosting effect object
Ct image appears as if composed ofSuperimposed images
MOTION
7/30/2019 Spiral CT Dr.shravani
55/82
MOTION
MOTION
http://radiographics.rsnajnls.org/content/vol24/issue6/images/large/g04nv25g16x.jpeg7/30/2019 Spiral CT Dr.shravani
56/82
MOTION
MOTION
7/30/2019 Spiral CT Dr.shravani
57/82
MOTION
MOTION
7/30/2019 Spiral CT Dr.shravani
58/82
MOTION
7/30/2019 Spiral CT Dr.shravani
59/82
REMEDY:
VOLUNTARY MOTION: Explanation Of The
Procedure And Good Communication With A
Patient.
INVOLUNTARY MOTION: Short Scan Time.
7/30/2019 Spiral CT Dr.shravani
60/82
BEAM HARDENING
It occurs when the average energy of an x-raybeam passing through the patient increases.
Beam is hardened high energy photons are
attenuated less by the tissue. As a result, they
are pass through the patient and reach thedetectors.
This artifact is also called cupping artifactbecause the hardening is most pronounced in
the center and less at the periphery. It resembles a cup.
7/30/2019 Spiral CT Dr.shravani
61/82
COMMON AREAS OF MANIFESTATION:
Skull Petrous Pyramids
Upper Chest And Shoulders Hips
BEAM HARDENING
7/30/2019 Spiral CT Dr.shravani
62/82
BEAM HARDENING
7/30/2019 Spiral CT Dr.shravani
63/82
BEAM HARDENING - CONTRAST
http://radiographics.rsnajnls.org/content/vol24/issue6/images/large/g04nv25g04x.jpeg7/30/2019 Spiral CT Dr.shravani
64/82
BEAM HARDENING
7/30/2019 Spiral CT Dr.shravani
65/82
REMEDY
INCREASE kvp
Decrease slice thickness
Increase filtration bowtie filter
7/30/2019 Spiral CT Dr.shravani
66/82
METAL ARTIFACT
Manifest itself as star
streaking artifact. Its
caused by presence of
metallic objects inside oroutside the patient.
Metallic object absorbs
the photons causing anincomplete profile
METAL ARTIFACT
7/30/2019 Spiral CT Dr.shravani
67/82
METAL ARTIFACT
REMEDY
Removal of external metallic objects
7/30/2019 Spiral CT Dr.shravani
68/82
OUT OF FIELD ARTIFACT
Patient is not entirely enclosed in the scanningfield of view. Patients body can obstruct
detectors. In addition, patient tissue outsidethe sfov will further harden the x-ray beam.Artifact appears as streaks and shading.
7/30/2019 Spiral CT Dr.shravani
69/82
OUT OF FIELD ARTIFACT
7/30/2019 Spiral CT Dr.shravani
70/82
7/30/2019 Spiral CT Dr.shravani
71/82
OUT OF FIELD ARTIFACT
7/30/2019 Spiral CT Dr.shravani
72/82
REMEDY:
Selection of larger sfov
Taping patient tissue
Raising patients arms above their headon the scan of chest and abdomen
RING
7/30/2019 Spiral CT Dr.shravani
73/82
RING
RING
http://radiographics.rsnajnls.org/content/vol24/issue6/images/large/g04nv25c19x.jpeg7/30/2019 Spiral CT Dr.shravani
74/82
7/30/2019 Spiral CT Dr.shravani
75/82
REMEDY
Detector calibration
Detector replacement
7/30/2019 Spiral CT Dr.shravani
76/82
TUBE ARCING
Tungsten vapor from anode and
cathode intercepts the projectile
electrons intended for collisions withthe target. Crackling sound!!!!
TUBE ARCING
7/30/2019 Spiral CT Dr.shravani
77/82
TUBE ARCING
7/30/2019 Spiral CT Dr.shravani
78/82
REMEDY
GAS BURNOFF
TUBE REPLACEMENT
LINE IN TOPOGRAM
7/30/2019 Spiral CT Dr.shravani
79/82
LINE IN TOPOGRAM
BAD DETECTOR CAUSESCONTINUOUS LINE ONTHE TOPOGRAM
REMEDY:
DETECTOR REPLACEMENT
STAIRCASE
7/30/2019 Spiral CT Dr.shravani
80/82
STAIRCASE
Improper selection of slice thicknessand slice incrementation when
generation mpr and 3-d images
7/30/2019 Spiral CT Dr.shravani
81/82
REMEDY
THIN SLICE USE
50% OVERLAP ON RECON SLICE
INCREMENTATION
7/30/2019 Spiral CT Dr.shravani
82/82
Thank You
Recommended