This lecture explains how to collect cryo-EM data efficiently and why detectors + sampling rules strongly influence resolution.
It focuses on:
A fundamental concept in microscopy is the Nyquist sampling theorem.
To resolve a structure, you must sample it with:
At least 2 pixels per smallest resolvable feature.
This is called Nyquist sampling.
If the smallest feature size you want to see is d, then:
ext{pixel size} = rac{d}{2}
This ensures enough information is captured.
In imaging:
A common resolution test is a line-pair target:
When imaged:
Three scenarios:
Pixel size = half the feature size → correct reconstruction
Pixel too large → information lost → wrong structure perception
Pixel too small → unnecessary data → slower processing
👉 Rule of thumb in cryo-EM:
Choose pixel size ≈ ½ desired resolution
Typical effective specimen sampling can be about ~1 Å per pixel after magnification.
Two key parameters define EM detector quality.
DQE = rac{(S/N)^2}{(S/N)^2}
It measures how well signal-to-noise is preserved by the detector.
DQE depends on spatial frequency:
This matters because:
👉 Small particles rely strongly on low-frequency contrast.
MTF describes:
How well different spatial frequencies (contrast details) are transferred.
It is essentially a resolution transfer curve.
So:
Modern direct detectors have two modes:
Advantages:
If two electrons hit the same area during readout:
This occurs above roughly:
~4 electrons per pixel per second
Thus:
👉 High frame rate detectors are needed.
Example:
Even then, multiple hits can occur during ~2.5 ms integration time.
Direct detection cameras record movies, not single micrographs.
Why?
Because beam exposure causes:
Movies allow:
This is a major reason cryo-EM resolution improved dramatically.
Key advantages:
✅ Precise electron detection ✅ Motion correction via movies ✅ Sub-pixel positioning ✅ Automated fast acquisition ✅ Much higher resolution structures
Drawbacks:
❌ Very expensive ❌ Large data volumes ❌ Slower data readout
Still — they revolutionized cryo-EM.
When an electron hits the detector:
This allows:
👉 Dividing one pixel into four virtual sub-pixels
Result:
Effective resolution improves by ~2×.
This is extremely important for high-resolution reconstructions.
| Detector | Advantages | Disadvantages |
|---|---|---|
| 📼 Film | Cheap, huge field of view, sensitive | Manual, slow, low throughput |
| 🖥 CCD | Digital, automated, fast | Poor SNR, small FOV |
| 🚀 Direct detection | Highest resolution, motion correction, counting | Expensive, massive data |
⭐ Resolution depends on correct Nyquist sampling ⭐ Detector quality determined by DQE + MTF ⭐ Electron counting drastically improves SNR ⭐ Motion correction from movies is essential ⭐ Super-resolution enables sub-pixel localization ⭐ Proper dose rate is critical to avoid coincidence loss