IMAGING WITH X-RAYS

RATIONALE:

X-ray imaging is one of the fastest and easiest ways for a physician to view the internal organs and structures of the body. Highlight the importance of some effects, such as the effect of scattered radiation, the grid ratio, and the effect of the direct rays, to obtain the highest quality of the image. These topics should be taught in depth to get the full knowledge of such effects.

X-ray imaging works because different tissues attenuate (absorb) X-rays differently, producing an image with varying shades of gray.

👉 The goal:
Maximize image quality by balancing:

• Contrast (difference in gray levels)

• Sharpness (resolution)

• Noise (random fluctuations)

1. IMAGE CONTRAST

✔ Definition

Contrast = difference in brightness between two regions

• High contrast → black & white (bone vs air)

• Low contrast → many shades of gray (soft tissues)

🔬 Types of Contrast

1. Radiographic Contrast (Subject Contrast)

Depends on the patient/tissue:

• Atomic number (Z)

• Density (ρ)

• Thickness (x)

• X-ray energy (kVp)

👉 Governed by attenuation:

Ix=I0e−μxI_x = I_0 e^{-\mu x}Ix​=I0​e−μx

Where attenuation depends on:

• μ (attenuation coefficient)

• density (ρ)

• thickness (x)

2. Detector Contrast

The ability of the detector to convert radiation into visible differences.

3. Display Contrast

Adjusted digitally (window/level).

4. Contrast Reduction Factors

• Scatter radiation

• Veiling glare

• Film fog

🧠 Key Insight

👉 Higher kVp → lower contrast (more penetration, less difference)
👉 Lower kVp → higher contrast (more photoelectric effect)

2. RADIOGRAPHIC CONTRAST OF TISSUES

🔥 Important Concept:

• Bone contrast ≈ 6× greater than soft tissue

• Soft tissues are very similar → need contrast agents

3. CONTRAST AGENTS

Used when natural contrast is insufficient.

Types:

• Iodine (Z=53) → blood vessels

• Barium (Z=56) → GI tract

• Air → negative contrast

💡 Why they work:

• High atomic number → ↑ photoelectric absorption

• K-edge enhances contrast at specific energies

👉 Example:

• Iodine K-edge ≈ 33 keV → optimal imaging near this energy

4. SCATTERED RADIATION

✔ Definition

Radiation that changes direction after interaction

🚫 Effects:

• Adds uniform “fog.”

• Reduces contrast

• Carries no useful information

📊 Scatter-to-Primary Ratio (S/P)

• Chest: 4:1

• Pelvis: 9:1

👉 Meaning:
Only ~10–20% of the image is a useful signal

🔥 Key Formula:

Contrast reduction ∝ 1 + S/P

5. SCATTER REDUCTION METHODS

1. Field Size

• ↓ field size → ↓ scatter

• Most effective simple method

2. Kilovoltage (kVp)

• ↓ kVp → ↑ contrast

• But it may increase the patient dose

3. Grid (MOST IMPORTANT)

📐 What is a Grid?

• Lead strips that absorb scatter

• Allow primary rays to pass

⚙ Grid Performance:

• Absorbs ~90% scatter

• Transmits ~70% primary rays

6. GRID RATIO

✔ Definition

Grid Ratio=height of lead stripsdistance between strips\text{Grid Ratio} = \frac{\text{height of lead strips}}{\text{distance between strips}}Grid Ratio=distance between stripsheight of lead strips​

🔥 Effects:

🧠 Key Insight:

👉 Higher grid ratio =
✔ Better contrast
❌ More patient dose
❌ More alignment sensitivity

7. CONTRAST IMPROVEMENT FACTOR

CIF=Contrast with gridContrast without grid\text{CIF} = \frac{\text{Contrast with grid}}{\text{Contrast without grid}}CIF=Contrast without gridContrast with grid​

👉 Typical: 3–5× improvement

8. EFFECT ON DIRECT RAYS

Grids don’t just affect scatter—they also affect primary radiation.

⚠ Key Issues:

1. Focused vs Unfocused Grids

• Focused grid → aligned with beam → better transmission

• Unfocused grid → more cutoff

2. Grid Cut-off ❗

Loss of primary rays due to:

• Wrong distance

• Misalignment

• Tilted grid

• Upside-down grid

👉 Result:

• Image appears underexposed

3. Moving Grid (Bucky)

• Moves during exposure

• Removes grid lines

• Produces cleaner image

4. Selectivity

Selectivity=Primary transmittedScatter transmitted\text{Selectivity} = \frac{\text{Primary transmitted}}{\text{Scatter transmitted}}Selectivity=Scatter transmittedPrimary transmitted​

👉 Higher = better grid

5. Exposure Factor (Bucky Factor)

Exposure increase=3–5×\text{Exposure increase} = 3–5\timesExposure increase=3–5×

👉 Because the grid absorbs some primary rays too

9. OTHER IMAGE QUALITY FACTORS

Magnification

M=FFDFFD−OFDM = \frac{FFD}{FFD - OFD}M=FFD−OFDFFD​

• ↓ OFD → ↓ magnification

• ↑ FFD → ↓ magnification

Unsharpness (Blur)

Types:

1. Geometric → focal spot size

2. Motion → patient movement

3. Absorption → gradual edges

10. EXPOSURE FACTORS

kVp

• ↑ kVp → ↓ contrast, ↓ dose

mAs

• Controls image density

Exposure Time

• Shorter = less motion blur

🧠 FINAL MASTER SUMMARY (WHAT EXAMINERS LOVE)

👉 Best image quality =

• High contrast (but not too high)

• Minimal scatter

• Proper grid use

• Correct exposure factors

🔑 Golden Relationships:

• ↑ kVp → ↓ contrast, ↑ penetration

• ↑ scatter → ↓ contrast

• ↑ grid ratio → ↑ contrast, ↑ dose

• ↓ field size → ↓ scatter

• Air gap = natural scatter reduction