Head & Neck CT Angiography

Head and Neck
CT Angiography
Seyedeh Shokoofeh Mousavi Gezafroudi
Med.Phys M.Sc

Table of
Content
• Introductio
n
• Physics
• Anatomy
• Indication
• Preparatio
n
• Injection
• Acquisitio
n
• Reconstru

Introduction
Medical Physisc M.Sc
4

 Rapid advances in CT technology and image postprocessing
software
 CT angiography: improved by increasing scan speed and
decreasing section thickness >> a powerful tool in neurovascular
imaging
 CTA: - a rapidly evolving field in CT imaging
- a noninvasive tool for visualizing blood vessels
- acquisition of thin-slice continuous images of the vessels
- to create three-dimensional reformations
An Introduction to CTA
5

 Greater volume covered per unit time
 Faster scanning
 Potential for CTA and Cardiac
Advantages of MDCT
HIGH TEMPORAL RESOULUTION
• Vascular work eg. CTA, CTV
• Phased scanning eg. Liver, Kidneys
• Functional work eg. CT Perfusion, Cardiac CT
AS WELL AS for
• Uncooperative patients
• Pediatrics
6

Physics
7

 Attenuation of monochromatic x-ray
 Inhomogenous object
Principle of Operation
8

 Quantitative parameter in CT images
CT Number
9

 WW: range of CT numbers
 WL: center of the scale
Window Width & Window Level
Effect of window width and level on CT image display. (a) Level = 50; Width = 200. (b) Level =
50; Width = 400. The image in (a) is displayed with greater contrast and appears noisier than
that in (b).
10

 Bone kernels: less high-frequency roll-off >> accentuate
higher frequencies at the expense of increased noise
 Soft tissue kernels: more roll-off at higher frequencies
>> reduced noise but lower spatial resolution
Compromise Of Spatial And Contrast Resolution
Kernels
11

 For Single Slice Scanners
 For Multi Slice Scanners
Pitch Factor
table motion during one rotation
Slice Pitch = ---------------------------------------
slice thickness
table motion during one rotation
Beam Pitch = ---------------------------------------
Beam thickness
12

 Lower Pitch >> more radiation dose
>> more details in images
Pitch Factor (cont.)
Beam Pitch > 1 Beam Pitch = 1
13

Anatomy
14

Surface Anatomy
15

Surface Anatomy (cont.)
16

Skull X-ray
17

Sectional Anatomy
18

Sectional Anatomy (cont.)
19

Brain Vascularization
20

Brain Vascularization
21

22

23

The highest background Radiation has been recorded in ……
24

Indication
25

Intra/Extra Cranial CTA
Indication
• CVA & TIA
• One-sided weakness
• Inability to speak
• Vision defects or loss
• Severe dizziness and vertigo
• Head and neck trauma
• Suspected arterial insufficiency
• Suspected arterial dissection
• Aneurysms
• Blockages
• Blood clots
• Vascular malformations e.g. AVM
• Surgery planning r.g. for tumors
• Vessel rupture or tears
• Carotid-cavernous sinus fistulas
• Detect atherosclerotic plaques
26

Intra/Extra Cranial CTA
Indication (cont.)
Contraindication:
known hypersensitivity to iodinated contrast media
Known renal insufficiency
taking metformin
You have had an X-ray test using contrast material in at least past 4 days
Risks:
a small chance of cell/tissue damage from being exposed to radiation
Potential risks to the developing CNS for fetuses in pregnant patients
CM leakage from veins and spread under skin
ACR & ESUR Vs. FDA in breastfeeding mothers
27

Image quality
28

CT images
29

CT images
Factors affecting spatial resolution
Detector Pitch/ Aperture
Number of Rays/ Views
Object magnification
Focal Spot size
Slice thickness
Pixel Matrix
30

CT images
Factors affecting Contrast resolution
Slice thickness
Patient size
mAs
FOV/ Pixel size
Reconstruction filter
Gantry rotation speed
31

CT images
Factors affecting Noise
mAs
kVp
Algorithm
Object size
Slice thickness
Pixel Matrix
32

33

Motion Artifact
Causes:
VOLUNTARY & INVOLUNTARY (cardiac/inspiration/pristaltic)
Correction:
Immobilization
Proper explanation of procedure
Short scan time
Motion reduction software (shimadzu –mac)
34

Streak Artifact
Causes:
Improper sampling of data
Partial volume averaging
Patient motion
Metal & beam hardening
Noise
Spiral scanning
Mechanical failure
Correction:
Software correction
Gantry angulation
35

Streak Artifact
Causes:
Improper sampling of data
Partial volume averaging
Patient motion
Metal & beam hardening
Noise
Spiral scanning
Mechanical failure
Correction:
Software correction
Gantry angulation
36

Ring Artifact
Causes:
Bad detector alignment
Correction:
Balancing algorithm utilization
Detector Calibration
Not using 3rd CT generation!
37

Beam Hardening Artifact
Causes:
Polyenergrtic x-ray spectrum
Change in average beam energy
Different length and materials on the way of beams
Correction:
Bow tie filter utilization
Special correction software
38

Partial Volume Artifact
Causes:
tissues of widely different absorption on the same CT voxel
producing a beam attenuation proportional to the average value
Correction:
Thin slices
39

Preparation
40

Before having a CT Angiogram
tell your doctor if you:
 Are or might be pregnant/ allergic to any medicines
 Have a heart condition/ kidney problems/ Asthma/ Thyroid
problems/ multiple myeloma/ Diabetes
 had a x-ray test using contrast material in the past 4 days
 Become very nervous in small spaces
 You may be asked not to eat or drink anything for several hours
before the test
 Read test information form, Fill out the informed consent form
41

During the test
 lie very still on a table that is attached to the CT scanner
 contrast material will be put in a vein in your arm or hand.
 The table will slide into the round opening of the scanner and move
slightly while the scanner takes pictures
 You may hear a click or buzz as the table and scanner move
 The technologist may ask you to hold your breath for a few seconds
at a time
 You may be alone in the scanning room. But the technologist will
watch you through a window. You will be able to talk to him or her
through an intercom.
 A CT angiogram usually takes no longer than 30 minutes
42

43

Which one is usually exposed to more radiation annually?
a) Nuclear plant workers
b) Flight crews
c) Miners
d) Nuclear medicine technologists
44

Injection
45

CTA injection protocol
 To deliver an appropriate amount of iodine, at right injection rates
of highly concentrated contrast medium
 Injection protocol varies according to the type of scanner and
velocity of acquisition
 For this type of exam the best method to deliver the contrast
medium is bolus tracking
( a designated vessel of interest is monitored in real-time with low-
dose dynamic scanning and when the selected enhancement
threshold (from 80 to 120 HU) is reach, the scan begins )
46

CTA injection protocol (cont.)
 un-enhanced and enhanced scans might be performed
 A peripheral venous access with an 18 or 20 gauge needle is obtained Usually
the right antecubital vein is selected
 The large amount of contrast material or a small amount of it followed by a
saline bolus chaser
 The injection rate should be >4 mL/sec.
 The acquisition should cover from the aortic arch in caudocranial direction
 Recent advances:
40ml, 370mgI/ml of CM using 64-slice scanners
47

Acquisition
48

CTA image aquisition
 Simultaneous enhancement of the arteries and veins on brain CTA
 enhancing the venous structures may interfere with detailed evaluation of the
arterial structures and vice versa
 The scan timing after the arrival of CM is one factor that affects the separation of
intracranial arteries and veins
 16-row MDCT: inferior to the novel wide coverage detector CT >> low temporal
resolution
 320-row MDCT: artery–vein separation on brain 3D-CTA. ( scanner equipped
with a detector coverage of 160mm per rotation >> 1.0 s scan time for the whole
brain)
 Optimal scan timing >> sufficient enhancement for entire intracranial arteries or
veins >> minimal superimposition in each phase
49

Suggested Parameters
50

Arterial/ Venal Phase CTA
51

Siemens Somatom Scope CT Scanner
Focus on clinical excellence
 Ultra Fast Ceramics detector >> crystal clear images
 Combined Applications to Reduce Exposure >> right dose to patients
 Adaptive Signal Boost >> reduces image noise
 Fully Assisting Scanner Technologies >> optimizes your process efficiency
52

Siemens Somatom Scope CT Scanner
Focus on cost efficiency
 eCockpit – enables more cost-efficient operation with:
• eStart: extends the tube lifetime by pre-warming the tube before a scan
• eMode: reduces wear and tear on the scanner by instantly optimizing scan
parameters
• eSleep: saves energy by stopping gantry rotation during scan breaks
 Small footprint – reduced space occupancy costs and low power requirements,
making installation quick and easy
 Innovative service offerings – delivering sustainable investment protection and
predictable total cost of ownership (TCO)
53

MOVIE TIME
54

Reconstruction
55

CTA post
processing
Reconstruction
3D
SSD
VR
2D
MIP
MPR
Reformatting
Various
Recomputation
56

MPR (Multiplanar Reformation)
 Creates views in arbitrary planes without loss of information
 Similar resolution to the original source images
 No editing is required, but only 2D views can be generated
 Applied for precise measurements
 Be combined with another visualization technique
57

MIP (Maximum Intensity Projection)
 Created by displaying only the highest attenuation value from the
data The depth information along the projection ray would lost
 Bone elimination techniques are essential for processing vascular
MIP images
 Thin-slab MIP images viewed interactively may be an alternative, as
the necessity for bone elimination is limited
 Not suitable for the evaluation of stenosis in cases of dense
calcification or stents
 Thin-slab MIP can provide an excellent “road map” of the vessel
course for further evaluation with MPR.
58

SSD (Shaded surface display)
 Surface rendering: an algorithm that provides a good 3D impression
 The surface is separated from other structure
 Is done by thresholding: meeting the condition>> be represented
 As with MIP, bone elimination techniques have to be applied to
extract the vascularstructures.
 With binary data, densitometric information gets lost and makes
the method prone to undesirable artifacts
 Volume rendering has supplanted shaded surface display in
virtually all CT angiography indications
59

VR (Volume Rendering)
 creates a 3D impression and provides densitometric
Information
 all acquired data may be used>> requires greater
processing power than other techniques
 Visualization of CT angiography data with volume
rendering is based on transfer functions that map
measured intensities to colors and opacities
 total transparency to total opacity are assigned along
artificial rays that pass through the data
 Separation of different tissue types can be
performed by applying multiple trapezoids, which can
be color encoded.
 does not correlate with the linear progression of
gray-scale values on conventional CT images Seyedeh Shokoofeh Mousavi Gezafroudi
60

Protection
61

General principles of ALARA
 Justification: The exam must be medically indicated
 Optimization: The exam must be performed using doses that are
As Low As Reasonably Achievable
 Limitation: While dose levels to occupationally exposed individuals
are limited to levels recommended by consensus organizations,
 limits are not typical for medically-necessary exams or procedures
62

Why it’s so important?
 To address concerns on the increased population exposure during
CT procedures
 Various effective strategies for dose reduction in CT
63

Brief tutorial on Radiation Dose in CT
CTDI
• Primary
metric of
scanner
output
• Using acrylic
phantoms
• One axial
scan
DLP
• Overall
energy
delivered by a
given scan
protocol
• Scan length is
incorporated
E
• reflects the
stochastic risk
• Uses
conversion
factor
64

65

Simple Strategies
Avoid unnecessary crisp images
Check if patient is pregnant
Encourage alternative modalities
Perform if it’s indicated
66

Thoughtful Strategies
Using Shielding
Higher pitch factor
Single phase imaging
Lower Radiation Parameters
67

Pros and Cons
68

69

70

MRA at a glance
 TOF
o Derives contrast between stationary tissues and flowing blood by manipulating
the magnitude of magnetization
o Moving spins have greater magnitude
o Uses longitudinal magnetization vector for imaging
 PC
o Derives contrast between stationary tissues and flowing blood by manipulating
the Phase of magnetization
o Moving spins have nonzero phase
o Uses transverse magnetization vector for imaging
 CE
Similar to CTA
Shortens T1 of blood vessels
Uses rapid bolus of Gd
71

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