The relevance of T2 relaxation time in interpreting MRI apparent diffusion coefficient (ADC) map for musculoskeletal structures

Age-and gender-related variations of liver diffusion metrics apparent diffusion coefficient (ADC) and diffusion derived vessel density (DDVD), and explanations with the known physiological T2 relaxation time variations among different volunteers' groups

BACKGROUND

Age-related liver diffusion metrics changes have been described. We aim to further clarify these questions: 1) whether an age-related reduction of liver perfusion can be observed by DDVD (diffusion derived vessel density) in older males; 2) whether there is a male female difference in liver perfusion; 3) whether liver ADC values and spleen ADC values are correlated. It is known that, physiologically, males' liver has a higher iron level (thus a shorter T2) than females' liver; pre-menopausal females have a lower liver iron level (thus a longer T2) than post-menopausal females. The observations of this study will be interpreted with the recently gained knowledge of the T2 contribution to diffusion metrics.

METHODS

Included in this healthy volunteer's study were 68 males (mean age:50.22 years, range: 25-70 years) and 43 females (mean age 45.56 years, range:20-71 years). DWI images with b-values of 0, 2, 10, 20, 60, and 600 s/mm2 were acquired at 1.5T. DDVD were calculated with b = 0, b = 2, b = 10, and b = 20 s/mm2 images. ADC were calculated with b = 0, b = 2, b = 60 and b = 600 s/mm2 images.

RESULTS

There was a statistically significant age-related decline of liver DDVD values for females (p = 0.024). A similar trend was observed for males, though statistical significance was not achieved (p = 0.113). Liver DDVD values were all higher in females than in males (p < 0.001). There was a statistically significant age-related decline of liver ADC values both for males (ADC(b0b600), p = 0.009) and for females (ADC(b0b600), p = 0.016). Liver ADC values and spleen ADC values were positively correlated (ADC(b0b600), r = 0.33 for males and 0.31 for females, p < 0.05). When the spleen ADC was used to normalize the liver ADC, then the age-related trend was largely removed, both for males and for females (p > 0.05).

CONCLUSION

Females have a larger liver perfusion volume than males. There is an age-related decrease of DDVD and ADC, both for males and females. Liver ADC values and spleen ADC values are positively correlated. These gender and age-related changes are unlikely mainly caused by the liver T2 relaxation time variations.

CLINICAL TRIAL NUMBER

Not applicable.

The rationale for proposing a magnetic resonance slow diffusion metric and its proof-of-concept testing showing spleen parenchyma and hepatocellular carcinoma have faster diffusion than liver parenchyma

Background

The liver and spleen have a similar amount of blood perfusion, and the spleen is waterier than the liver. The spleen tissue has a higher contrast-enhanced computed tomography (CT) extracellular volume fraction than the liver. The spleen has been reported to have a much lower apparent diffusion coefficient (ADC), intravoxel incoherent motion (IVIM)-perfusion fraction (PF), and IVIM-Dslow than those of the liver, which appears to be unreasonable. As hepatocellular carcinomas (HCCs) are mostly associated with increased blood supply and increased proportion of arterial blood supply and with edema, HCC has been reported to have a lower ADC, lower IVIM-PF, and lower IVIM-Dslow than liver parenchyma, which appears to be unreasonable. Cysts are known to have a longer T2 and a higher ADC than hemangiomas. Due to the 'flushing' of blood flow inside the hemangioma, we hypothesize that the actual diffusion of hemangioma liquid is faster than the more 'static' liquid of the cysts. As ADC measure is heavily affected by T2 and in order to minimize the T2 effect, we propose a new metric reflecting tissue slow diffusion which is termed 'slow diffusion coefficient (SDC)': SDC = [S(b1 ) - S(b2 )]/(b2 - b1 ), where b1 and b2 refers to a high b-value (e.g., 400 s/mm2 for liver) and a higher b-value (e.g., 600 s/mm2) respectively, where S(b1 ) and S(b2 ) denote the corresponding diffusion weighted image signal intensity.

Methods

This study utilized a random selection of authors' historical liver IVIM magnetic resonance imaging (MRI) data. For 1.5T data, SDC was calculated with b=600 and 800 s/mm2 images. For 3.0T data, SDC was calculated with b=400 and 600 s/mm2 images. With 1.5T data, SDC was calculated for 10 healthy volunteer cases' liver and spleen parenchyma, as well as two simple liver cysts and their corresponding liver parenchyma. With 3.0T data, SDC was calculated for 14 cases' liver and spleen parenchyma, as well as 13 HCC masses, 9 simple liver cysts, 13 hemangiomas and their corresponding liver parenchyma. As in vivo diffusion metrics can be only measured with MRI thus external validation is not possible, the measures of liver parenchyma were used to normalize the measures of spleen, HCC, cyst, and hemangioma, and the ratios were expressed in median value.

Results

The median ratio of SDCspleen/SDCliver was 2.47 for 1.5T data and 1.97 for 3.0T data. Two cysts at 1.5T had a median SDCcyst/SDCliver ratio of 2.92. For 3.0T data, the median ratios of SDCHCC/SDCliver, SDCcyst/SDCliver, SDChemangioma/SDCliver were 2.83, 4.23, and 5.37, respectively. However, the ADCspleen/ADCliver ratios were always <0.81 even when calculated with various combinations of high b-values.

Conclusions

The spleen has a faster diffusion than the liver, HCCs have a faster diffusion than the adjacent liver parenchyma, and hemangiomas have a faster diffusion than simple cysts. Although it is known that cysts have a substantially longer T2 than hemangiomas, SDC of hemangioma was higher than that of cysts, suggesting 'T2 effect' is minimized for SDC.

Influences of the second motion probing gradient b-value and T2 relaxation time on magnetic resonance diffusion-derived 'vessel density' (DDVD) calculation: the examples of liver, spleen, and liver simple cyst

Background

Magnetic resonance (MR) diffusion-derived 'vessel density' (DDVD) is calculated according to: DDVDb0b2 = Sb0/ROIarea0 - Sb2/ROIarea2, where Sb0 and Sb2 refer to the tissue signal when b-value is 0 or 2 s/mm2. Sb2 and ROIarea2 can also be approximated by other low b-values diffusion-weighted imaging (DWI). This study investigates the influence of the second motion probing gradient b-value and T2 on DDVD calculations of the liver, spleen, and liver simple cyst. Literature analysis shows the liver and spleen have very similar amounts of perfusion. At 3T, liver and spleen have a T2 of around 42 and 60 ms respectively, while cyst has a very long T2.

Methods

Twenty-eight subjects had 1.5T DWI data with b-values of 0, 1, 2, 15, 20, 30 s/mm2. Twenty-one subjects had 3.0T DWI data with b-values of 0, 2, 4, 7, 10, 15, 20, 30 s/mm2. DDVDb0b1, DDVDb0b2, DDVDb0b4, DDVDb0b7, DDVDb0b10, DDVDb0b15, DDVDb0b20, and DDVDb0b30 were calculated from b=0 and b=1 images, b=0 and b=2 images, b=0 and b=4 images, b=0 and b=7 images, b=0 and b=10 images, b=0 and b=15 images, b=0 and b=20 images, b=0 and b=30 s/mm2 images, respectively. For liver simple cyst, two cysts totaling six slices scanned at 1.5T were available for DDVD measurement.

Results

At 1.5T, when the second b-value was 1 s/mm2, DDVDspleen value was slightly higher than DDVDliver value; when the second b-value was 2 s/mm2, DDVDliver value was slightly higher than DDVDspleen value. After that, the absolute difference between DDVDliver value and DDVDspleen value became increasingly larger, with DDVDliver value being consistently higher. DDVDcyst showed values close to 0 when the second b-value was 1 s/mm2. When the second b-value was 20 or 30 s/mm2, DDVDcyst value was higher than DDVDliver value. The absolute DDVD values measured higher at 3.0T than at 1.5T. However, the ratio of DDVDspleen to DDVDliver did not apparently differ between 1.5T and 3.0T. From the second b-value being 2 s/mm2 onward, an increasingly larger second b-value was associated with a trend of slow decreasing of the ratio of DDVDspleen to DDVDliver.

Conclusions

When a very low second b-value is applied, the liver and spleen measure similar perfusions by DDVD, and cysts measure DDVD close to zero. When a higher second b-value is applied, relative to the liver, the DDVD of spleen is suppressed while the DDVD of cyst is artificially promoted, which we consider are related to the T2 relaxation times of the liver, spleen, and cyst.