Computerized Computerized TomographyTomography

CTCT

CT is an X-ray image CT is an X-ray image

The gray scale is the same as an x-rayThe gray scale is the same as an x-ray– Except…CT utilizes a computer to enhance the Except…CT utilizes a computer to enhance the

contrast of mid or water grays to create a much better contrast of mid or water grays to create a much better differentiation of soft tissue densities.differentiation of soft tissue densities.

CT uses multiple exposures, merged by a CT uses multiple exposures, merged by a computer, to produce a single image. The result computer, to produce a single image. The result is a 2 or 3 dimensional appearance of the image is a 2 or 3 dimensional appearance of the image compared to plain film radiology.compared to plain film radiology.

In CT the x-ray beam is moved around the patient, and the radiation strikes detectors that are either stationary or moving in coordination with the x-ray tube. A computer then creates a 2 (or 3)-dimensional image of the 3-dimensional area scanned.

CT is imaged cross-sectionally, like a loaf of bread, with the slice thickness ranging from 7-10mm.

Tracing of Image pathway in Tracing of Image pathway in spiral or helical CTspiral or helical CT

Coronal view of femoral neck Coronal view of femoral neck fracturefracture

Axial view of same femoral neck Axial view of same femoral neck fracturefracture

CT CT Density is the same as X-rayDensity is the same as X-ray

DensityDensity

AirAir

BlackBlack

DensityDensity

FatFat

Dark GrayDark Gray

““Neutral” or Neutral” or Water Water DensityDensity

Mid-GrayMid-Gray

DensityDensity

MineralMineral

Light GrayLight Gray

DensityDensity

Heavy Heavy MetalsMetals

WhiteWhite

LungsLungs

TracheaTrachea

BowelBowel

Thin fatThin fat

Thin C.T.Thin C.T.

AdiposeAdipose

Thicker Thicker Adipose,Adipose,

Multiple Multiple layers of layers of thin tissuethin tissue

OsteoporotiOsteoporoticc

Bone Bone

Muscle, Muscle, tendon, thin tendon, thin bones, bones, overlapping overlapping soft tissues, soft tissues, blood, vasc-blood, vasc-ulatureulature

Cancellous Cancellous bone, bone, thinner thinner muscle, muscle, tendon, tendon, organ organ tissues,supetissues,super-imposition r-imposition of thin soft of thin soft tissues, tissues, large blood large blood vesselsvessels

Thick Thick cortical cortical bone, bone, Dental Dental fillings, fillings, jewelry, jewelry, orthopedic orthopedic hardware, hardware, zippers and zippers and buttonsbuttons

CT elbow axial view: note bone and soft CT elbow axial view: note bone and soft tissue appearance. This is a “bone window”.tissue appearance. This is a “bone window”.

Material Air Fat Water Soft Tissue

Calcium or bone

Metal

HU 1500 -40 0 +80 +400 +1500

In CT the density of a substance is rated in In CT the density of a substance is rated in “Hounsfield Units” (HU) after the inventor Sir “Hounsfield Units” (HU) after the inventor Sir Goeffrey Hounsfield. A continuum of tissue density Goeffrey Hounsfield. A continuum of tissue density expressed in HU looks similar to an x-ray scale.expressed in HU looks similar to an x-ray scale.

The human eye can discern only limited shades of gray, whereas a computer can detect and differentiate thousands. By enhancing the gray contrast in desired density regions of a CT scan tissue details can be made more appreciable to the eye. By setting a tissue “window” grays are displayed only for a limited range of densities.

Anything more dense than the window setting-in HU units- will appear white and structures less dense than the window setting appear black.

For example: Here is Microsoft’s gray scale. For example: Here is Microsoft’s gray scale. Your eyes can beat this: you can distinguish about 26 gray shades.Your eyes can beat this: you can distinguish about 26 gray shades.

Original Water/soft tissue Window

New Water Window

CT enhances various gray tones within the soft tissue “window” selected.

The “window” setting on CT machines is standardized. If you need to see a muscle the tech sets the machine to “soft tissue window” to spread that zone of gray into more distinct shades.

Air

Bone

Thick soft tissue

Fat

Body Section “slices”

Single Slice Selected

Final Image seen on film

CT Image: 7-10mm thick “slice”: just like CT Image: 7-10mm thick “slice”: just like MRIMRI

Superior Inferior

Dorsal

Ventral

D

LR

V

D

Scout Film showing Axial Scout Film showing Axial thoracic studythoracic study

Image 2

Image 2 from previous filmImage 2 from previous film

anterior

Bone WindowBone Window

Bone Window IIBone Window II

3D CT 3D CT clavicular clavicular fracturefracture

CT Volume RenderedCT Volume RenderedCarotid Artery from General Electric’s Carotid Artery from General Electric’s

WebsightWebsight

3 D 3 D CTCT

3D CT3D CT

3D CT R shoulder3D CT R shoulder

Rotation seriesRotation series

3D CT3D CT

3D CT3D CT

•   Trauma•        Fracture detection and evaluation•        Spine Alignment •        Intracranial hemorrhage•        Abdominal Injury•        Detection of foreign bodies, especially in joints.•        Diagnosis of primary and secondary tumors•       Tumor staging

What is CT good for?What is CT good for?

CT Indications and AdvantagesCT Indications and Advantages

Good contrast in neutral density tissuesGood contrast in neutral density tissues

Spine alignment Spine alignment

TraumaTrauma

Fracture detectionFracture detection

Intra-cranial hemorrhageIntra-cranial hemorrhage

Abdominal organ injuryAbdominal organ injury

Foreign body detectionForeign body detection

Tumor detection and stagingTumor detection and staging

CT: Fracture classification and CT: Fracture classification and orientationorientation

Same tibial plateau fractureSame tibial plateau fractureaxial viewaxial view

To distinguish a CT from an MRI look at the fat:

•Fat is bright white on a T1 image and gray to black on a CT.

•Then look at bone. Bone is white on CT and gray (medullary) or black (cortical) on MRI.

•Also CT scans will display kVp and mA exposure settings alerting you to the X-ray nature of the study.

CT MRI ComparisonCT MRI Comparison

CT T1 T2 CT T1 T2BoneBone

MarrowMarrow

CortexCortex

graygray brightbright graygray

FatFat blackblack whitewhite graygray

AirAir blackblack darkdark darkdark

WaterWater graygray darkdark brightbright

CT identificationCT identification14 y/o CC: knee pain image 1/414 y/o CC: knee pain image 1/4

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CT of aortic archCT of aortic arch

Tibial Plateau fractureTibial Plateau fracture

Acetabular FractureAcetabular Fracture

View?View?Dx?Dx?

Femoral Neck FractureFemoral Neck Fracture

Widened AC JointWidened AC Joint

Benign? We’ll do tumors soon…Benign? We’ll do tumors soon…

Practice Set. Make the dx. 1/4Practice Set. Make the dx. 1/4

CT Same foot CT Same foot 2/42/4

Close-up 3/4Close-up 3/4

Image 4/4. Calcaneal fracture into the subtalor joint.

Practice Practice Set 1/3Set 1/3

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CT axialCT axial3/33/3

Sagittal Scout: Lumbar Study, Sagittal Scout: Lumbar Study, Coronal ViewsCoronal Views

Axial series leading inferiorly from pedicles to IVF