L10 - Bioimaging

What is imaging?

The science of extracting spatially and temporally resolved information at all physical scales.

Resolution

The smallest thing that can be seen.

The lower the number, the better the resolution.

Quality Characteristics

• Spatial resolution
• Details
• Contrast
• Noise
• Fidelity

Sensitivity and Specificity

$$\begin{array}{rl}& \text{Sensitivity}=\frac{TP}{TP+FN}\\ & \\ & \text{Specificity}=\frac{TN}{TN+FP}\end{array}$$

	Actual True	Actual False
Tested True	TP	FP
Tested False	FN	TN

• High sensitivity $\to$ sensitive in detecting abnormality $\to$ good for abnormal patients
• High specificity $\to$ not diagnosing normal as abnormal $\to$ good for normal patients

Receiver Operating Characteristics (ROC) Curve

$$\begin{array}{r}\text{FP Fraction (FPF)}=1-\text{Specificity}\\ \\ \text{Accuracy}=\frac{TP+TN}{TP+TN+FP+FN}\end{array}$$

X-ray

Production

1. Current heats up cathode (filament)
2. Cathode emit electrons
3. Anode (tungsten) attracts high speed electrons
4. The electron hits a tungsten atom
5. An electron in a lower orbital falls out of the atom
6. An electron in a higher orbital falls to the lower energy level
7. Extra energy is released as X-ray

Linear Attenuation coefficient

$$\mu \propto \frac{1}{\mathrm{Number}\mathrm{of}\mathrm{photons}\mathrm{reaching}\mathrm{the}\mathrm{screen}}$$

Components

• Low-energy filter
• Collimator $\to$ make X-ray parallel
• Collimating scatter grid

Applications

• Diagnostic imaging $\to$ detection of structures and pathology of tissues (e.g. dental radiography)
• Orthopedics (bones)
• Chest
• Breasts
  • Less penetrating, low energy source for soft tissues
• Fluoroscopy
  • Moving images with X-ray
• Radiation therapy

Pros and Cons

Pros

• Obtain image of tissues inside body
• Detect diseases at early stage
• Widely accepts
• Low cost
• Quick

Cons

• A potential carcinogen (cancer)
• Soft tissues cannot be shown clearly
• Not recommended for pregnant women
• Image affected by position of patient and tissues

Computed Tomography (CT) Scan

Uses X-ray and computed tomography to obtain a 3D image of a body section.

Supplement X-ray and medical ultrasonography.
Gold standard in diagnosis.

Working Principle

1. Measure 2D transmission projections at different angles, obtain 2D images around a single axis of rotation
2. Obtain 3D image with digital geometry processing

Single slide and axial scan	Helical (spirlal) scan
Scan one slice, translate table, repeat	Continuous linear movement between scans

Pros and Cons

Pros

• Widely available
• Show bone changes best
• Detect life-threatening conditions
• Quick, painless, reasonably priced
• Only method that provides details images of bones, soft tissues, and blood vessels

Cons

• Radiation
• Misinterpret test results
• Allergic reaction to contrast material