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What Is MTF? (Modulation Transfer Function)

MTF summarizes an imaging system's spatial resolution in a single curve: it shows how well the system preserves contrast as a function of object fineness (spatial frequency). Large objects transfer easily; fine detail transfers progressively more weakly. What is MTF, how is it read, and why is it the gold standard of resolution? Concise, grounded in Bushberg.

Intermediate · radiologists, technologists & physicists· ~8 min read· Sourced

"How small a detail can this system show?" has no single-number answer, because it depends on the object's fineness. The MTF (modulation transfer function) captures this dependence in one curve: it shows how well the system preserves contrast as a function of spatial frequency (object fineness). MTF is the most complete measure of spatial resolution. Broader picture: Image Quality.

What is MTF?

Imagine stimulating an imaging system with a pure sinusoidal pattern (light–dark stripes at a given frequency). The system produces an image at the same frequency, but usually with reduced contrast.1 This contrast reduction comes from the system's resolution losses and grows as frequency rises. In Bushberg's example, the recorded contrast for 1, 2 and 4 cycles/mm inputs is 87%, 56% and 13% respectively.1 MTF gives the ratio of output to input contrast at each frequency: ~1.0 at low frequency, declining at high frequency.

Spatial frequency

Spatial frequency is the spatial counterpart of temporal frequency: its unit is millimeters (instead of seconds).1 The intuitive rule: large objects are low frequency, fine detail is high frequency (unit: cycles/mm). A system transfers large structures (low frequency) at almost full contrast; but as detail gets finer (frequency rises), contrast drops. The highest frequency a system can show is limited by sampling at the Nyquist frequency.1

MTF · contrast transfer vs spatial frequency1.00spatial frequency (cycles/mm)1 cy/mm · 87%2 cy/mm · 56%4 cy/mm · 13%large objectsfine detail
MTF is ~1.0 at low frequency (large objects) and falls as frequency rises (fine detail). How far the curve extends to the right tells how fine a detail the system can show with contrast.1

Reading the curve

When comparing two systems, the MTF curve tells all: the further right and higher the curve stays, the better the system transfers fine detail with contrast — i.e. the higher its spatial resolution. In practice MTF is computed from the point/line spread function of a small object (e.g. a tungsten bead or a sharp edge); in CT quality control a medical physicist typically measures it at acceptance/commissioning.

In a nutshell
MTF = a map of contrast transfer vs spatial frequency → the full measure of spatial resolution. High MTF at high frequency = ability to see fine detail. Far more informative than a single-number (lp/mm) summary. For the noise side → NPS; for the efficiency that combines both → DQE.
Related articles
Image Quality · What Is NPS? · What Is DQE? · Quality Control in CT

References

  1. Bushberg JT, Seibert JA, Leidholdt EM, Boone JM. The Essential Physics of Medical Imaging, 3rd ed. Lippincott Williams & Wilkins, 2011. §4 (Image Quality): MTF, bir görüntüleme sisteminin uzaysal çözünürlüğünü uzaysal frekansın fonksiyonu olarak gösterir; sinüzoidal girdiler azalan kontrastla aktarılır — 1, 2 ve 4 çevrim/mm için kontrast sırasıyla %87, %56, %13 (Şekil 4-13, s.70–72); Nyquist frekansı sınırı. Sayfa numaraları bu baskıya aittir.
  2. İlişkili: Görüntü Kalitesi (kontrast, çözünürlük, gürültü) · NPS Nedir? · DQE Nedir?
Note: This content is for education; for clinical decisions or regulatory compliance, consult a qualified medical physicist and current regulations.

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