Computed Tomography (CT) Imaging

Computed Tomography (CT) Overview

• Provides radiographs with thousands of fast projections using a spinning radiograph tube
• Projections are organized as 2-D images
• High levels of constant radiation requires the room to be vacant during operation
• Patients often need sedation or general anesthesia
• Images can be sent to a radiologist for evaluation
• Can distinguish between soft tissue types
• Uses a radiopaque contrast medium to visualize vasculature and organ systems

Uses of CT Imaging

• Can diagnose nasal disease
• Can be used for mass assessment
• Can identify orthopedic disease like FCP/OCD
• Useful for identifying abnormalities
• Aid in pre-surgical planning

The Focal Plane in CT Imaging

• It is the section or layer where minimal blurring occurs
• The ROI is a fixed point in the anatomy
• Structures above and below the ROI are blurred to make it stand out

Image Processing in CT Scans

• An X-ray tube emits a fan-shaped beam and rotates 360 degrees around the patient inside a gantry
• X-ray detectors directly opposite the tube rotate at the same speed
• Detectors collect the attenuated beam after it passes through the patient
• Detectors convert photons to an electrical signal, which is then fed into an analog-to-digital converter
• The digital signal is sent to a computer that reconstructs the images
• The table moves forward in increments, allowing multiple slices to be taken at different levels
• The computer reassembles and displays images on a monitor, where each "slice" is in focus

Image Display in CT Scans

• CT numbers affect image brightness and contrast
• The "window" for CT images refers to the number of gray shades, affecting contrast
• The "level" is the midpoint of the gray range within a specific anatomical area, affecting density

Magnetic Resonance Imaging (MRI) Overview

• Provides almost real-time images
• Well-suited for visualizing soft tissue and fluid
• Slower than CT scans
• Machines are described by format and magnet strength
• Useful for understanding soft tissue positioning and identifying pathological conditions
• Provides insight into normal soft tissue positions and sizes
• Preferred for assessing neurological conditions, spinal cord injuries, and blood flow

How MRI Works

• The machine spins around the subject to get images
• Protons in tissues or liquid temporarily respond in specific ways to generate an image
• The images are constructed a series of 2D images, which may be viewed independently or as a 3D reconstruction
• A radio frequency, tuned to the processing frequency of hydrogen atoms, is applied to the subject
• Hydrogen atoms respond by resonating into a higher energy state and changing the angle of their magnetization
• Radio frequency is removed, and the molecules return to normal, emitting a radio frequency which scanner turns into an image

MRI Safety Concerns

• Magnets used in the scanners are dangerous, so personnel require proper training
• Individuals are screened for unsafe implants and loose metal before entering an MRI room
• The projectile effect, caused by metallic objects launching into the magnet, is a major danger

Types of MRI Scans

• Shades of gray are referred to as signal intensities
• Bright or white areas indicate high signal intensity
• Darker shades of gray indicate low signal intensity

T1 Scan Characteristics

• Brain and spinal cord imaging is good on T1 scans because of high levels of lipids
• Fat and bone marrow appear bright white
• Cortical bone, fluids, and solid masses appear dark or black

T2 Scan Characteristics

• Fluid, flowing blood, and cysts are bright white
• Pooling blood and fat are gray

Contrast Studies in MRI

• Use of an agent is required to highlight infection, tumors, or vascular disease
• Gadolinium is the contrast agent used
• Only T1-weighted sequences (with or without fat saturation) show enhancement of tissues

Video to Review

• Difference between CT and MRI