Spectroscopy

Considerations

The READY View Brain and Prostate MR spectroscopy protocols are used to display functional maps for metabolites and metabolite ratios in the brain and prostate.

READY View extracts and displays spectra from the CSI that can be acquired with PROBE-P and PROSE pulse sequences. The CSI data can be displayed as spectra, metabolite maps, and metabolite ratio maps. In addition, numeric values of relative signal intensities and metabolite ratios display.

An MR spectroscopy data set (acquired with the PROBE or PROSE sequence) consists, for each location in the exam, of 256 low–resolution (8 × 8, 16 × 16 or 32 × 32) images, numbered 1 .. 256, where each of the 256 images represents a point of the spectrum in the range of 4.3 .. 0.40 ppm for all protocols. These 256 sub–images are recorded within a single image file for each location. READY View automatically splits each single image into 256 separate images.

A double spin echo method, referred to as PRESS is used to excite each point within a given region of interest. This region (referred to as the PRESS ROI) is shown as a rectangle on the images. Some metabolites will show a response outside this region, but only the data within the region should be considered as valid.

Algorithms

Metabolites

The relative concentrations for metabolites such as choline, creatine, N–acetyl, myoinositol, and citrate residues, or combinations of metabolites such as choline+creatine or lipid and lactate, are computed from the pixel values within defined ppm ranges of the spectrum (image ranges). A global (composite) map for the metabolites is also computed.

You can select whether the concentrations are computed as the sum of each image range (this is the default selection), the average of absolute value of each image range, or as the maximum (peak) value within each image range.

Ratios

When computing ratios of relative metabolite concentrations, such as choline/creatine or N–acetyl/choline, the division of two concentrations with small values can result in spuriously high-ratio values that will appear as noise in the functional maps.

To reduce or eliminate such noise, you can define a threshold (advanced settings from the Ratio panel). For a ratio A/B, the threshold sets a lower limit on the denominator B. Pixels in the functional map for which B<threshold are displayed in black (transparent in composite views).

Signal/Noise Ratio

The 3D protocols compute a signal–to–noise ratio (displayed as a separate functional map) which provides an indication of the signal–to–noise ratio during acquisition.

In this context, for each pixel location:

• Signal is defined as the peak value of the N–acetyl metabolite.
• Noise is defined as the standard deviation of the signal over the 0.9..0.4 ppm frequency range. This frequency range does not contain any significant metabolite spectral lines.

You can change the definition of signal and noise by means of the controls in the advanced settings Signal/Noise panel.

Image Ranges

By default, the protocols use the sum of the pixel values in a given image range to compute and display the metabolite functional maps. This is indicated by the (Sum) button in the ”advanced settings” Metabolites panel.

You can select to use the average of the absolute value or the maximum (peak) value within each image range instead [by activating the (Abs.) or (Max.) button in the ”advanced settings” Metabolites panel].

For the signal–to–noise ratio, the signal level is computed as the peak value and the noise level as the standard deviation of the pixel values in the respective image ranges. This is independent of the setting of the (Sum) / (Abs.) / (Max.) control in the ”advanced settings” panel.

The controls in the advanced settings panels of the protocols allow you to adjust the image ranges defining each metabolite or combination of metabolites as required, e.g., to correct for spectral line shift during acquisition.

To find the image number that corresponds to a given ppm value, use the rank active annotation on the series view to move through the images until the image with the desired ppm value is displayed, then read off the corresponding image number.

Alternatively you can calculate the image number from:

image = 1 + (4.30 – ppm) * 66.93 for the 2D Brain protocol,

image = 1 + (4.30 – ppm) * 65.38 for the 3D protocols,

then round off the resulting value to the nearest integer.

Spectroscopy measurement units

The Spectroscopy functional maps have the following units of measurement.

READY View protocols that use spectroscopy scan data

• Brain Spectro
• Prostate Spectro
• Brain Spectro - SVQ
• Breast SVQ
• MR Brain
• MR Breast
• MR Pelvis