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Chapter Ten
MR Image Contrast

10-01 Introduction

verybody involved in medical imaging shares one common dream: to be able to distinguish the structures of the object examined with such ac­­cu­­ra­­cy and sharp­­ness that there is no room for diagnostic speculation (Fi­gu­re 10-01).

Contrast is one of the major concerns in medical imaging. The ability to distinguish and characterize certain structures in the image is the goal of imaging. In con­­ven­ti­o­nal x-ray and in x-ray CT distinction and char­acterization of lesions are often based up­on in­di­rect signs.

The aim of medical imaging, in particular of MR imaging, is going one step further than this ex­amp­le. Contrast should be good enough to both highlight and cha­rac­te­ri­ze lesions. We do not want to rely up­on in­direct signs.

Figure 10-01:
In this picture, a glass is filled with a liquid; how­ever, we do not know what kind of liquid this might be. The bottom picture shows the same glass with a red wine bottle next to it. Although the quality of this image is worse than the top one, we can de­duce that the glass con­tains red wine.

Definition of normal anatomy and pathological changes should be easy and exact. This means that in addition to excellent spatial resolution, high contrast is a pre­re­qui­site for a good imaging method.

Magnetic resonance imaging has drawn the attention of many researchers, fas­ci­nat­ed by the manifold possibilities of influencing contrast.

In the early years of MR imaging it was believed that image contrast of such qua­li­ty could be ob­tai­ned that problems in lesion delineation and even lesion-typing would not oc­cur any more.

The early enthusiasm was rapidly replaced by disillusionment and partial dis­­ap­­point­­ment. It is still not clear whether the method itself is incapable of un­co­ve­ring all the states and diseases it was intended for or whether poor un­der­stan­ding of the theo­re­ti­cal background of MR imaging led to misguided ap­­pli­­ca­­tions.

Today, many of the early mistakes and misunderstandings can be explained. How­ever, there is enough space for new mistakes. ‘Urban myths’ without sound scien­ti­fic background appeared and are spreading.

10-01-01 Main Contrast Factors

This chapter provides an overview of the main factors and parameters in­flu­en­cing the magnetic resonance image. We will introduce, one by one, the main pulse se­quen­ce pa­ra­me­ters and see how they influence image contrast.

Contrast in conventional radiographs and CT images is essentially based on small den­si­ty differences. It can only be changed by adding contrast agents such as ba­rium and iodinated substances that influence electron density within a cer­tain or­gan.

MR imaging possesses many more contrast-influencing factors and parameters than other imaging methods. One can compare x-ray imaging with radio broad­cast­ing and MR imaging with color television: the former relies on one factor, sound, the latter on sound and moving color pictures. This makes the contrast behavior of MR imag­ing more complex than that of any other medical imaging modality.

The numerous factors influencing contrast can be divided into two groups: intrinsic and extrinsic parameters. Table 10-01 gives an overview of the most im­por­tant of these parameters.

Table 10-01:
Principle intrinsic and extrinsic contrast pa­ra­me­ters in magnetic resonance imaging. Please note that this list is long, but not exhaustive.

Many of the extrinsic factors can influence the in­trin­sic factors. For the clinical ap­pli­ca­tion of MR ima­ging, it is necessary to be aware of all of their interactions if one is to react rapidly and efficiently to a gi­ven diagnostic question.

The re­la­ti­ve abundance of factors creates a ple­tho­ra of data, which can impede rather than facilitate the diagnosis, especially if there is a lack of know­ledge on how to exploit the in­for­ma­tion.

One of the main advantages of MR imaging is the possibility to change contrast by choos­ing special pulse se­quen­ces and pulse-sequence pa­ra­me­ters.

By emphasizing one factor or mixing several factors in a spe­ci­fic way, the contrast be­ha­vior of a certain morphological region or pa­tho­lo­gi­cal lesion can be highlighted.

One should always bear in mind that even changing minor factors can cause se­vere contrast changes.

The comparison of two images of the same patient ta­ken with two different ma­chi­nes, apparently using the same parameters, often reveals different contrast pat­terns.