Magnetic resonance imaging for quantifying regional adipose tissue content in HIV-infected people at risk for developing cardiometabolic syndrome

In this technical brief, we describe a Magnetic Resonance Imaging (MRI) technique to quantify regional adipose tissue content using commercially available MR systems. An advantage of MRI over more conventional methods, such as anthropometry and dual energy X-ray absorptiometry (DXA), for quantifying body fat content or body composition is that regional differences in fat or muscle distribution can be assessed. Computed Tomography (CT) can also provide reasonable estimates of regional adipose content, but it exposes the patient to potentially harmful ionizing radiation. MRI is a non-invasive technique that can quantify adipose tissue content using established clinical imaging sequences on standard 1.5T clinical MR systems in approximately 30 minutes. One potential shortcoming is that image analysis currently requires off-line processing with advanced non-standard software. However, these advanced image analysis tools are becoming available as a standard tool on newer clinical MR systems.

Background

The amount and regional distribution of adipose tissue on the body is considered an important biomarker for the cardiometabolic syndrome (1, 2). Excessive visceral fat (visceral adiposity) is associated with impaired glucose tolerance, insulin resistance, an increased risk for type 2 diabetes, atherosclerosis, and hypertension - components of the cardiometabolic syndrome (3-5).

Approximately 50% of HIV-infected people treated with highly active antiretroviral therapy (HAART) that includes certain HIV-protease inhibitors and nucleoside reverse transcriptase inhibitors, develop changes in fat distribution, including regional lipohypertrophy (e.g. visceral adiposity, breast enlargement in women, cervical adipose deposition (“buffalo-hump”), and benign lipomas) and lipoatrophy (e.g. subcutaneous fat loss in the abdomen, limbs, face, and buttocks); these changes are sometimes referred to as lipodystrophy. Changes in the distribution of abdominal fat include; loss of subcutaneous adipose tissue (SAT) between the skin and the outer margin of the abdominal wall; and an accumulation of visceral adipose tissue (VAT) within the inner margin of the abdominal wall and surrounding the abdominal organs (6-9). In HIV, the presence of visceral adiposity and peripheral lipoatrophy tend to be associated with insulin resistance (7). In the general population, visceral adiposity and abdominal subcutaneous adipose accumulation appear more frequently than peripheral lipoatrophy, and they are associated with insulin resistance. Regardless, the ability to image and partition the total, subcutaneous, and visceral adipose tissue compartments, and to accurately and reproducibly quantify SAT, VAT, and peripheral subcutaneous adipose tissue content is crucial to the diagnosis of adipose tissue redistribution syndromes, and necessary to evaluate the effectiveness of interventions focused on reducing visceral adiposity and restoring peripheral subcutaneous adipose tissue, and potentially, reducing cardiometabolic disease risk in HIV.

MRI

Magnetic resonance imaging (MRI) is a widely used imaging tool that uses a strong magnetic field and radiofrequency (rf) waves to produce detailed images of the body's organs and structures without the use of ionizing radiation. Typically, MRI images are created by exciting the labile hydrogen (¹H) atoms in tissues using a short duration (few milliseconds) radiofrequency (rf) pulse in the presence of the strong magnetic field. The excited protons emit radio waves that are detected by receivers and processed to generate grey-scale images. By applying additional magnetic field gradients, information can be directly related to the signal position in the body, thus generating an image (10).

The signals emitted by the ¹H atoms after rf excitation decay with two characteristic time constants, the longitudinal relaxation time constant (T₁) and the transverse relaxation time constant (T₂). Different tissues and pathology exhibit different relaxation time constants and these differences in time constants are used to generate contrast in MR images. By changing data acquisition parameters, the MRI system can generate images that highlight or alter the grey-scale intensity or appearance of the different tissues in the body. Water and fat ¹H atoms have very different relaxation time constants and therefore will have different appearances on relaxation weighted MR images. Specifically, water has a longer T₁ than protons in fat, so on a T₁-weighted image, where the image intensity is inversely proportional to T₁, the fat appears brighter than water.

In addition to different characteristic time constants, fat and water protons exist in different chemical environments. Therefore, when placed in a strong magnetic field they resonate at slightly different frequencies. This frequency difference can also be used to selectively highlight water (muscle) or fat tissue in an image. For example, muscle tissue can be highlighted in an image by using either fat suppression (where the signal from fat tissue is selectively eliminated by rf pulses), or water excitation (where only water protons in muscle are excited by rf pulses). These selective water and fat tissue images are advantageous in signal processing for visually partitioning and quantifying muscle and fat volumes.

Study Subjects

In an effort to highlight the divergence in regional adipose tissue distribution that can be quantified using MRI in HIV-infected people, 2 HIV infected men treated with HAART were recruited for the study (Table 1). The adipose tissue distribution of Subject A is similar to the metabolic syndrome in the general population, while that for Subject B is typical of the HIV-related metabolic syndrome or lipodystrophy (Figs 1-2). The Washington University School of Medicine Human Research Protection Office approved the study. The study risks and benefits were explained, and each volunteer signed an approved consent document.

Table 1.

Descriptive Characteristics.

Parameter	Subj A	Subj B
Age (yrs)	27	54
Ethnicity	African American	Caucasian
Current anti-HIV medications	Norvir, Reyataz, Truvada	Combivir, Sustiva
Height (cm)	175.3	175.3
Weight (kg)	123	84
BMI (kg/m2)	40.0	27.3
Waist Circumference (cm)	120	96
Body Fat (%)	31	22
Trunk/Limb Fat Ratio	1.1	2.9
Glucose (mg/dL)	99	92.5
Insulin (μU/mL)	18	19
HOMA-IR	4.4	4.3
Total-cholesterol (mg/dL)	163	172
LDL-cholesterol (mg/dL)	108	98
HDL-cholesterol (mg/dL)	34	53
Triglycerides (mg/dL)	104	107
Blood Pressure	140/74	112/77

HOMA-IR= homeostasis model for insulin resistance; all hormone, metabolite, and blood pressure measurements were made after an overnight fast; trunk/limb fat ratio and body fat (%) were measured using dual energy X-ray absorptiometry.

Figure(1).

T₁-weighted abdominal MR images from two HIV-infected men. On the left, Subj A without visceral adiposity, but with large subcutaneous adipose tissue content. On the right, Subj B with visceral adiposity and subcutaneous lipoatrophy. Fat appears bright white-gray. Total abdominal fat volume in Subj A is greater than Subj B (5166 cm³ vs 3325 cm³). However, the ratio of visceral to total abdominal fat volume is less in Subj A (30%) than in Subj B (72%).

Figure(2).

T₁-weighted thigh MR images of the two HIV-infected men without (Panels A and C, Subj A) and with (Panels B and D, Subj B) peripheral lipoatrophy. C and D show the same images using a fat suppression signal acquisition protocol where the fat signal is eliminated and only the water signal from muscle is visualized. Subj B has lipoatrophy because panels B and D show almost complete absence of thigh subcutaneous adipose tissue. By comparison, Subj A (A and C) does not have lipoatrophy (thigh fat to muscle ratio = 60%), but Subj B (B and D) the thigh fat to muscle ratio = 6%. The small circular regions of high intensity on the fat-suppressed images represent blood vessels and are not included in the quantification of fat.

Study Protocol

Thigh subcutaneous fat and abdominal fat volumes (subcutaneous (SAT) and visceral adipose tissue (VAT) were quantified using ¹H MRI. Data were acquired on 1.5T whole body Siemens Sonata system (Siemens Medical Systems, Erlangen, Germany) using the body coil. The subjects were positioned supine for scanning. Three plane reference images were obtained to identify anatomical landmarks and to optimize the participant's position in the MR scanner. For the quantitation of abdominal fat, 27 contiguous axial slices of the abdomen were obtained during a single breathhold using a standard T₁-weighted, two-dimensional (2D), multislice, spoiled gradient-echo sequence. The inferior border of the imaging volume was centered over the spine at the L₅-sacrum intervertebral space. The other scan parameters used were TR = 209 ms, TE = 4.1 ms, slice thickness = 10 mm and flip angle = 70^°. The field of view was 400 mm with an in plane image resolution of 2.4 mm × 1.6 mm. Scan time was 20 sec. Fat suppression was not used for abdominal scanning. It requires additional time and can be limited by the participant's ability to sustain a breathhold for >20s.

For the quantitation of thigh fat and muscle content, two sets of 10 serial axial images, with and without fat suppression, were obtained starting from 10 cm above the superior border of the medial condyle of the tibia. Ten slices were obtained with 8 mm slice thickness. Other scan parameters were TR = 1500 ms, TE = 13 ms, flip angle = 90^°. In plane resolution was 2.1 mm × 1.4 mm.

Data Analysis and Results

Images were analyzed using Analyze® 7.0 software package (Rochester, MN). Abdominal fat volumes were obtained from 8 (sequential) of the 27 axial images in a semi-automated fashion based upon the pixel intensity and location, and separated into VAT and SAT regions. The VAT and SAT volumes were calculated on the basis of operator-defined adipose tissue location and pixel intensities, and the total abdominal adipose tissue (TAT) volume was calculated as the sum VAT+SAT (Figure 1). Using 8 serial axial images of the thighs, right and left thigh muscle and fat volumes were quantified (separately) using Analyze® software and operator-defined threshold pixel intensities for muscle and fat (Figure 2). In our hands, the day-to-day variability for quantifying fat and muscle volumes is <6% when the same operator identifies and quantifies the regions of interest in the same series of abdomen or thigh images.

Summary

The quantity and distribution of adipose tissue and muscle are important biomarkers in our understanding of cardiometabolic disorders. The examples presented here clearly illustrate the utility of MRI for providing insight into relationships between body fat content, distribution, and metabolism. Of note, both HIV-infected participants were insulin resistant on the basis of their fasting insulin level and HOMA-IR index, but their adipose tissue distribution was very different; subject A was obese and had a large volume of subcutaneous adipose tissue, while subject B was leaner, but had visceral adiposity and subcutaneous lipoatrophy. On the basis of their regional adiposity and insulin resistance, both subjects are at risk for developing the cardiometabolic syndrome. In Subject B, abdominal MRI was useful for making an important distinction. On the basis of NCEP ATP III criteria, waist circumference for Subject B (96cm) would not be considered central adiposity, and he would not be considered obese on the basis of his BMI (27kg/m²). However, on the basis of quantitative MRI, Subject B clearly has visceral adiposity that would not have been captured using the typical indirect indicators (waist circumference or BMI). This is especially important in HIV-related alterations in regional adipose tissue distribution. Compared with subjective estimates, MRI provides a non-invasive quantitation of regional adipose tissue volume, including the differentiation of subcutaneous and visceral fat volumes. These measurements use established imaging protocols, and may be used as clinical or research tools to identify HIV-infected people with visceral adiposity, peripheral lipoatrophy, and to evaluate the effectiveness of therapeutic interventions (exercise, diet, hormonal, anti-HIV medication changes, glucose-, lipid-, blood pressure-lowering medications) aimed at reducing visceral adiposity and potentially reducing cardiometabolic disease risk.