Secrets of Diffusion-For Radiology Resident

This is a special documentary prepared for Radiology residents preparing for various exams. Submitted by Dr MGK Murthy. 

1.Molecular Movements

-          3 Types

-          intravascular, intracellular or extracellular .

-           

2.Techniques of DW sequence

-          Normal Spin echo T2 sequence  consists of 90 deg  RF pulse followed by 180 deg pulse and signal is collected thereafter which largely depends on T2 decay related to transversal relaxation

-           In DW sequence ,  a  Diffusion sensitizing Gradient (dephasing gradient) is applied before 180 deg RF pulse and another one symmetrically after 180 deg RF pulse

-          can be performed with SE, FSE, Gradient echo, and EPI(most commonly used)

Two scenarios are possible:

-          In Scenario  “1”,  tissues with limited impedence of water molecules ,first dephasing gradient signal effect , would be canceled by second symmetrically applied rephasing gradient  and hence T2 bright signals  would remain.( possible with increased cellularity ,or intact cell membrane , as in  tumour, abscess,  cytotoxic oedema, fibrosis etc)

-          In Sccenario “2”,tissues with  increased mobility of  water molecules , the molecules move considerable distance between the dephasing and Rephasing Diffusion gradient pulse  and hence can not be completely canceled , and therefore net loss of T2  bright signal occurs (possible in decreased cellular content or broken cell membrane  permitting movement )

3.“b” Value

·         Refers to the strength of gradient  pulse applied

·         Is proportional to amplitude of the gradient, duration of application and time interval between dephasing and rephrasing applications(Parameter adjusted to increase the  “b”value  is amplitude)

·         Atleast two sets of images are needed , one with b=0 and other b=500 or 1000sec/square mm

·         Small b value (50-100sec/sq mm) would affect the fast moving molecules say in bloodvessels , because  they move before the rephasing pulse could take effect and hence lose the T2 bright signal  due to dephasing pulse already applied (also called “black blood” sequence )

4.ADC

·         Represents the slope of  “log of relative tissue signal”on “Y” axis  versus “b” value on “X”axis

·         Atleast two “b” value acquisitions are needed(multiple will enhance  information, but takes time)(amount of “b” value varies with organs for eg Liver is ideal with = 0 and 500-600 sec/sq mm, pelvis goes to 800 and brain with =0 and 1000)

5.T2 Shine Through

·         Some lesions like cysts/fluid in gall bladder etc show  high signal on diffusion , however can be differentiated from lesions by  lack of corresponding ADC  low signal(T2 signal retained)

·         Slow flowing blood is an exception with high signal on b=0 as well as 500 sec/sqmm, and sometimes even with reduced ADC values mimicking tumours – we need to depend on other sequences  in these situations , particularly in   liver haemangioma

6.Limitations

·         Low SNR and  Susceptible for artifacts

·         Can help by (a) increasing magnetic strength(b) increase No of acquistions  (c) minimizing echotime(<100msec) (d) coarse acquisition matrix(256 by 256 at 3.0T)(e) increase slice thickness(at least 6-7 mm)