Image sharpness

The sharpness of an image describes how clearly edges and fine detail are displayed.
Any blurring that reduces edge definition is termed unsharpness.

While spatial resolution describes how much detail can theoretically be resolved, sharpness refers to how distinctly that detail is represented in practice.

Unsharpness can arise from geometric, motion, or system-related causes.

Definition

• Sharpness – the visual impression of edge clarity and fine detail.
• Unsharpness – the spreading or blurring of image boundaries due to limitations in the imaging system or patient motion.

Total unsharpness is the combined effect of several components:

U_total = √(U_g² + U_m² + U_s²)

where:

• U_g​ = geometric unsharpness
• U_m = motion unsharpness
• U_s​ = system (detector) unsharpness

Let’s look at these three factors.

1. Geometric Unsharpness

Cause

Geometric unsharpness arises because the X-ray focal spot has a finite size (it is not a true point source).

Each point in the object is projected as a small blurred region rather than a perfect point.

Formula

U_g= F × OID/SOD

where:

• F = effective focal spot size
• OID = object–image distance
• SOD = source–object distance

Implications

• Larger focal spot → more blur
• Larger OID → more blur
• Smaller SOD → more blur

To reduce geometric unsharpness

• Use a small focal spot (fine focus).
• Reduce OID (bring patient closer to detector).
• Increase SOD (use longer source–image distance, e.g. 180 cm for chest radiographs).

Trade-off

• Smaller focal spots have limited heat capacity → can’t be used for high mA exposures (e.g. thick body parts).

2. Motion Unsharpness

Cause

Motion of the patient, organ, or tube during exposure causes blurring along the direction of movement.

Magnitude

U_m = V × t

where:

• V = velocity of motion (mm/s)
• t = exposure time (s)

To reduce motion unsharpness

• Use short exposure times (increase mA if necessary).
• Provide clear breathing instructions or use breath-hold technique.
• Employ mechanical supports or immobilisation for limbs.
• In cardiac or respiratory imaging, use gating or high-frame-rate systems.

3. System Unsharpness (Detector-Related)

Causes

• Scintillator light spread in indirect DR or CR systems (e.g. CsI, Gd₂O₂S).
• Phosphor thickness: thicker plates cause more lateral light diffusion.
• Laser beam diameter in CR readers.
• Pixel size and sampling frequency (sets Nyquist limit).
• Backscatter within the detector assembly.

Minimised by

• Using needle-structured CsI scintillators (channelled to reduce light spread).
• Direct DR (a-Se) systems (no light spread).
• Maintaining optimal detector calibration and alignment.

Total System Sharpness

Sharpness is often assessed using the Edge Spread Function (ESF) and its derivative, the Line Spread Function (LSF).

A broader LSF corresponds to more unsharpness. Its Fourier transform yields the MTF, which quantitatively describes how contrast varies with spatial frequency.

U_total ∝ 1/MTF

A system with a high MTF maintains high contrast at fine spatial frequencies → sharp image.

Key Takeaways and Exam Tips:

• Sharpness refers to edge clarity; unsharpness quantifies the blur.
• Geometric unsharpness = F×OID/SOD​; reduce with small focal spot, small OID, large SOD.
• Motion unsharpness = V×t; reduce with short exposures and immobilisation.
• System unsharpness = detector-dependent; minimise with direct conversion and small pixel size.
• Total unsharpness combines all sources.
• Common exam question: “List and explain the causes of unsharpness in radiographic images, and describe how each can be reduced.”

Up Next

Next, we’ll move on to Scatter and Contrast-to-Noise Ratio (CNR). We’ll explain how scattered radiation degrades image contrast, how CNR quantifies detectability, and the practical methods used to control scatter.