Computerized Tomography

CT CT is an X-ray image The gray scale is the same as an x-ray The gray scale is the same as an x-ray –Except…CT utilizes a computer to enhance the contrast of mid or water grays to create a much better differentiation of soft tissue densities. CT uses multiple exposures, merged by a computer, to produce a single image. The result is a 2 or 3 dimensional appearance of the image 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 spiral or helical CT

Coronal view of femoral neck fracture

Axial view of same femoral neck fracture

CT Density is the same as X-ray  Density AirBlack  Density Fat Dark Gray “Neutral” or Water Density Mid-Gray  Density Mineral Light Gray  Density Heavy Metals White LungsTracheaBowel Thin fat Thin C.T. Adipose Thicker Adipose, Multiple layers of thin tissue OsteoporoticBone Muscle, tendon, thin bones, overlapping soft tissues, blood, vasc- ulature Cancellous bone, thinner muscle, tendon, organ tissues,super- imposition of thin soft tissues, large blood vessels Thick cortical bone, Dental fillings, jewelry, orthopedic hardware, zippers and buttons

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

Material AirFat WaterSoft Tissue Calcium or bone Metal HU In CT the density of a substance is rated in “Hounsfield Units” (HU) after the inventor Sir Goeffrey Hounsfield. A continuum of tissue density 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. 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. AirAir BoneBone Thick soft tissue FatFat

Body Section “slices” Single Slice Selected Final Image seen on film CT Image: 7-10mm thick “slice”: just like MRI Superior Inferior Dorsal Ventral D L R V D

Scout Film showing Axial thoracic study Image 2

Image 2 from previous film anterior

Bone Window

Bone Window II

3D CT clavicular fracture

CT Volume Rendered Carotid Artery from General Electric’s Websight

3 D CT

3D CT R shoulder

Rotation series

3D 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?

CT Indications and Advantages Good contrast in neutral density tissues Spine alignment Trauma Fracture detection Intra-cranial hemorrhage Abdominal organ injury Foreign body detection Tumor detection and staging

CT: Fracture classification and orientation

Same tibial plateau fracture axial 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 Comparison CT T1 T2 Bone Marrow Marrow Cortex Cortexgraybrightgray Fatblackwhitegray Airblackdarkdark Watergraydarkbright

CT identification 14 y/o CC: knee pain image 1/4

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CT of aortic arch

Tibial Plateau fracture

Acetabular Fracture

View? Dx?

Femoral Neck Fracture

Widened AC Joint

Benign? We’ll do tumors soon…

Practice Set. Make the dx. 1/4

CT Same foot 2/4

Close-up 3/4

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

Practice Set 1/3

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CT axial 3/3

Sagittal Scout: Lumbar Study, Coronal Views

Axial series leading inferiorly from pedicles to IVF