Mapa ADC El ADC (apparent diffusion coefficient) es el coeficiente promedio de libertad de difusión de las moléculas de H20 incluídas en un voxel “Aparente”: por afectarse por volumen parcial, perfusión y errores de cálculo Permite cuantificación de la difusión Reduce el efecto T2 shine-through Necesita la adquisición de al menos 2 secuencias de DWI con distinta duración y amplitud (valores b) de gradientes No contiene contaminación T1 o T2* ADC: Apparent Diffusion Coefficient (or Constant) The ADC map shows the average diffusion-freedom water molecules have in each voxel. This parameter can be calculated from all the diffusion weighted images which are acquired in DTI. Note that it is an average of all directions acquired, when performing DTI. The “A” for Apparent is there because the ADC is affected by partial volume averaging, perfusion, and some measurement errors. The average ADC map is sometimes called the trace map, which is to do with the mathematics of how it is calculated. It is not the same as the Isotropic Image; the ADC uses the mathematics of the tensor (the sum of the scalar values of the eigenvalues of the tensor, divided by three (which is the trace/3), sometimes called simply the “trace” image, but let’s not get into the maths now). The ADC is a useful DTI map. On an ADC map, good diffusion is bright. This is opposite to the diffusion weighted images, where good diffusion is dark. The ADC removes the effect of T2 shine-through. Tsuda K, Murakami T, Sakurai K, et al. Preliminary evaluation of the apparent diffusion coefficient of the kidney with a spiral IVIM sequence. Nippon Igaku Hoshasen Gakkai Zasshi 1997; 57:19-22

ADC exponencial El ADC exponencial es un mapa ADC al que se le sustrae completamente el T2 shine through Se calcula como exp (-b*ADC) eADC: enhanced (or exponential) Apparent Diffusion Coefficient (or Constant) The eADC shows the attenuation of the signal due to diffusion. On an eADC map, good diffusion is dark, just like the diffusion weighted source images. This is opposite to the ADC. Like the ADC, the eADC also removes the effect of T2 shine-through. Clinicians can choose to use ADC or eADC maps depending on whether they want contrast to match (or be opposite to) the diffusion weighted source images.

METÁSTASIS HEPÁTICA: RESTRICCIÓN Difusión b 1000 s/mm2 T2 Mapa ADC exp-ADC

EFECTO T2 SHINE TROUGH Secundario a la hiperintensidad de señal de los tejidos con tiempo de relajación T2 largo, por la potenciación intrínseca en T2 de DWI Pitfall común de falsa restricción So what are b values? b values are actually a neat way of summing up into one parameter how much dephasing we are going to allow (size of diffusion weighting gradients etc). We need images with different b values to be able to work out diffusion-related parameters such as the ADC (see below). Note that we can’t say what diffusion is from the signal intensity alone, but from the signal intensity loss, which is why we usually acquire a b=0 (baseline) image to compare with.

Hemangiomas hepáticos: T2 shine through Parikh T, Drew SJ, Lee VS, Wong S, Hecht EM Focal liver lesion detection and characterization with diffusion-weighted MRI: comparison with standard breath-hold T2-weighted imaging. Radiology 2008;246:812-822

Metástasis y quiste Semiología en T2 y DWI T2 DWI ADC Esta es la apariencia típica de la metástasis y el quiste hepáticos en T2 y en las secuencias de difusión. Ambos muestran hiperintensidad cuando potenciamos en difusión. Sin embargo el mapa ADC nos informa que la lesión que restringe es la metástasis. La hiperintensidad que puede mostrar el quiste al potenciar en difusión es debida al efecto residual T2. ADC 6