T1 Shortening by Gad

Why does gadolinium apparently only shorten T1? Doesn't it have an effect on T2 as well?  

Because of its paramagnetic properties, gadolinium facilitates both longitudinal and transverse magnetic relaxation, thereby shortening both T1 and T2 of tissues in which it accumulates. The dominant effect (T1 vs T2 shortening), however, depends on baseline tissue relaxation times in the absence of the agent, as well as the maximum tissue concentration achieved.

In relaxation chemistry it is often more convenient to talk about relaxation rates rather than relaxation times. Relaxation rates, measured in sec⁻¹, are simply the inverses of the relaxation times. Total relaxation rates of chemical processes are often the sum of relaxation rates of various component steps.

Accordingly, the relaxation rates observed after contrast administration, 1/T1obs and 1/T2obs, can be calculated as the sum of relaxation rates contributed by the native tissue and the paramagnetic contrast agent, respectively,

1/T1obs = 1/T1t + 1/T1c             and              1/T2obs = 1/T2t + 1/T2c

where 1/T1t and 1/T2t are the relaxation rates of the tissue without contrast, and 1/T1c and 1/T2c are the relaxation rates attributable to the paramagnetic agent.  Furthermore, 1/T1c and 1/T2c are linearly related to the concentration [C] of the paramagnetic contrast agent, according to the relationships

1/T1c = r1 [C]                   and                    1/T2c = r2 [C]

where r₁ and r₂ are called the specific relaxivities of the paramagnetic agent, measured in units of L/mmol-s. Although this formulation is strictly true only for contrast agents dissolved in pure solvents and lacking solute-solute interactions, clinical measurements have demonstrated that it is nevertheless a reasonable approximation for the relaxation effects of most extracellular gadolinium-containing contrast media in a variety of human tissues and tumors.

The relaxivities (r1 and r2) of most extracellular gadolinium contrast agents are comparable; for low molecular weight gadolinium chelates measured values are in the 3 - 5 L/mmol-s range. Because the r1 and r2 relaxivities are so similar, one might mistakenly conclude that gadolinium contrast would shorten T1obs and T2obs equally.  This is not the case, however, because it is the relaxation rates, not relaxation times, that are additive and proportional to the concentration of gadolinium.  Additionally, most biological tissues possess a marked natural disparity in their baseline relaxation times;  T1t is often 5 to 10 times longer than T2t and therefore contributes further to the unequal effects of gadolinium on observed relaxation rates.

To illustrate the apparent preferential T1 shortening effects of gadolinium at low doses, let us choose as our baseline tissue normal brain, with T1t = 800 msec (0.8 sec) and T2t = 80 msec (0.08 sec).  Let us now suppose the blood-brain barrier becomes disrupted, and gadolinium contrast with specific relaxivities r1 = 4 L/mmol-s and r2 = 4 L/mmol-sec accumulates in this tissue at a concentration of 0.1 mmol/L.  According to the data and equations above, the observed relaxation rates would be 

1/T1_obs = (1/0.8) + (4)(0.1) = 1.65 sec⁻¹, and
1/T2_obs = (1/0.08) + (5)(0.1) = 13.0 sec⁻¹

from which we calculate T1obs = 606 msec and T2obs = 77 msec.  We can see that gadolinium has reduced the observed value of T1 by almost 25%, whereas it has diminished T2 by only 4%.  In most tissues where T1 is normally much longer than T2, therefore, low concentrations of extracellular gadolinium contrast will produce a relatively greater shortening of T1 than of T2.

Advanced Discussion (show/hide)»

The simple treatment of relaxivity provided here is referred to as the "fast exchange" model which assumes all protons that contribute to the MR signal have equal and unrestricted access to the paramagnetic center on a time scale less than or equal to the T1 relaxation time of the medium. Clearly this is not the case with biological tissues where multiple compartments exist with different exchange rates that may even be different in the forward and reverse directions. Nevertheless, the simple treatment here goes a long way to explaining basic phenomena involving contrast agent behavior in tissues and solutions.

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References
      Elster AD. How much contrast is enough? Dependence of enhancement on field strength and MR pulse sequence. Eur Radiol 1997; 7:S276-S280.   
     Rohrer M, Bauer H, Mintorovitch J et al. Comparison of magnetic properties of MRI contrast media solutions a different magnetic field strengths. Invest Radiol 2005; 40:715-724.

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Related Questions
     What is the difference between relaxation rates and relaxation times?
     Can the T2-shortening effects of gadolinium ever be observed on routine MR imaging? 
     What is meant by the relaxivity of a contrast agent? How is it measured?

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