TwinTree Insert

09-04 Contrast-to-Noise Ratio

decisive criterion for comparing the con­trast of dif­fe­rent ima­ges and dif­fe­rent MR ma­chines is the contrast-to-noise ratio. Fig­ure 09-10 explains the contrast-to-noise re­lationship.

Without noise, two neighboring tis­sues with different signal in­ten­si­ties can be easily distinguished from each other. If the level of noise is low (Figure 09-10 center), contrast-to-noise is sufficient and the tis­sues are distinguishable.

In case the noise level is high and sig­nal-to-noise is poor, it is difficult or im­pos­­si­ble to dis­tin­guish the tissues from each other (Figure 09-10 bot­tom). Even if there is suf­fi­cient contrast bet­ween two tissues, noise may ob­li­te­rate this contrast and no dis­­cri­­mi­­na­­tion will be possible.

Figure 09-10:
Contrast-to-noise. SI = signal intensity.

With the increase of field strength, T1 relaxation times increase too. This also has a ne­ga­tive im­pact on signal-to-noise, be­cause the repetition time TR has to be in­­creas­ed to ob­tain the same signal-to-noise ratio. Only when sufficient time is grant­ed for the spin system to recover after the ini­tial excitation pulse will sig­nal in­ten­si­ty be sufficient.

If we administer the next pulse after a shorter time than 5×T1, our sample will be sa­tu­rat­ed and its signal intensity will be lower (Figure 09-11). Thus, TR must be suf­fi­ci­ent­ly long to receive a strong signal. However, in general the increase in sig­nal de­riv­ing from the higher field strength compensates the T1-related signal loss.

Figure 09-11:
The longer the repetition time TR, and thus the more recovered the system will be, the stronger the signal intensity.

It is worth noting, that for any given spatial resolution and contrast the signal-to-noise ratio needs only reach a level for secure detection of a lesion. Above that level, further increases in signal-to-noise make the image more pleasant but, on the other hand, 'beautiful' images do not guar­antee the accuracy of the di­ag­no­sis.

When trying to optimize imaging condi­tions, one must never forget that there are nu­me­rous in­ter­de­pen­den­cies between the different factors influencing the image and image contrast.

If we choose imaging speed as the main factor, there is a straight con­nection to the signal-to-noise ratio and spa­tial resolution. Spatial resolution is linked to con­­trast and ar­ti­fact reduction. Contrast is also related to signal-to-noise and artif­acts.

As we will discuss in the following chapter, altering one minor parameter can af­fect an entire chain of other parameters.