Intro to Imaging - CT/MRI (10/18b) [Biomedical] Flashcards

1
Q

Computerized Tomography (CT) - Overview

A

CT also uses x-rays to acquire the image

Displayed in voxels (3D pixels)

Attenuations — in Hounsfield units (HU) → higher number is more radiodense, appears whiter

Acquired axially but can be reformatted

Have better contrast and are more sensitive for imaging soft tissue than plain films

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2
Q

CT - Hounsfield Unit Trend

A

Most radiodense (white) → Most radiopaque (black)

Dense metals
Cortical bone
Spongy bone
Muscle
Blood 
Water
Fat
Soft tissues 
Lungs
Air
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3
Q

Some uses of CT

A

Routinely used for trauma, generally more available than MRIs in rural areas

Conventional Axial Tomography (CAT) scan — can be used for bone issues/fractures if more contrast is needed than an x-ray
- Especially where geometry of the bone might be tricky (EX: knee, spine)

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4
Q

How would a clot vs a hemorrhagic stroke appear on CT?

A

Clot will appear darker, hemorrhagic stroke appears bright white

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5
Q

CT - Windowing

A

set the level of the shades of gray of interest around the tissue they’re interested in

Must be done when the imaging occurs

Bone windowing can enhance ability to distinguish between cortical and spongy bone

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6
Q

___ tissue is the most negative substance in the body

A

Lung

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7
Q

Uses/Advantages of CT

A

Used for bone issues or head trauma

Subtle or complex fractures

Degenerative changes in bone (EX: spinal stenosis)

Acute intracranial bleeding (EX: hemorrhagic strokes or subdural hematomas)

Quantitative bone mineral analysis (used for measure of OP)

Heterotopic Ossification (HO)

More available, less expensive and quicker than MRI and more information than radiographs

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8
Q

Disadvantages of CT

A

More exposure to radiation than radiograph (still very low), but cumulative

Not as good for soft tissue differentiation — cartilage, tendons and ligaments better with MRI

Patient movement and metal implants will produce artifacts

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9
Q

Magnetic Resonance Imaging (MRI) - Overview

A

MR Images are produced by the interaction of hydrogen containing molecules within tissues and a magnetic field produced by the magnet

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10
Q

MRI Steps

A

1) Hydrogen lines up with the field of the magnet
2) A radio wave is delivered to the tissue which causes the hydrogen nuclei to absorb energy and change its alignment within the magnet (resonance)
3) The radio pulses are stopped, the absorbed energy is released and measured by the computer detector
4) This info is converted to an image.

5) Unique images are produced because each tissue has a different
amount of hydrogen and so relax at different rates

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11
Q

What is MRI based on?

A

Not based on density, but on water content

Increased sensitivity and ability to differentiate soft tissues

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12
Q

Fat vs cortical bone in T1 MRI

A

Fat has highest amount of water in our body, appears bright white in T1

Cortical bone will appear black in T1

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13
Q

In an MRI, the ___ and ___ of the pulses produces different quality images of the same tissues

A

length and sequence

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14
Q

MRI - Repetition time (TR)

A

time that elapses between 2 consecutive radio wave pulses

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15
Q

MRI - Echo Time (TE)

A

time selected to wait after the start of the TR to receive the signal or “echo” from the patient

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16
Q

MRI - Varying TE and TR

A

Varying the TR and TE will accentuate different tissues

T1 and T2 images are different for each tissue and produce different intensities (degrees of whiteness) of the same tissue

17
Q

MRI - T1 Weighted Image

A

Using a shorter TE

Better for viewing anatomy

18
Q

MRI - T2 Weighted Image

A

Using a longer TE

Better for pathology since the images accentuate still fluid

Suppresses fat and shows pathology – increase in water brightness (abnormal water uptake) and fat suppressed

19
Q

Uses/Advantages of MRI

A

Superior soft tissue visualization, especially muscles, tendons, ligaments, nerves, articular cartilage and menisci

Okay for spongy bone (high fat content)

No ionizing radiation, no known harm

20
Q

Disadvantages of MRI

A

Can’t use with patients who work with metal shavings, metal pacemakers, certain types of fixative devices (ferrous metals)

21
Q

CT vs MRI

A

CT IS BETTER FOR

  • cortical bone: subtle and complex fractures
  • calcifications in any tissue
  • acute intracranial bleeding

MRI IS BETTER FOR

  • spongy bone
  • soft tissue (ligaments, tendons)
  • cartilage
22
Q

MRI - T1 Weighted Appearance

A

High intensity (white) → Low intensity (black)

subcutaneous fat 
spongy bone 
still fluid
cartilage
tumor
muscle
fluid
23
Q

MRI - T2 Weighted Appearance

A

High intensity (white) → Low intensity (black)

still fluid (inflammation)
tumor
fluid
muscle
cartilage
spongy bone 
subcutaneous fat