Journal of Medical Imaging and Radiation Oncology •• (2015) ••–••
RADIOLO GY—P I CTO R I A L E SSAY
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Recognising pitfalls in assessment of tumours by diffusion-weighted MRI: A pictorial essay Bimal Kumar Parameswaran,1 Eddie Lau1,2 and Nicholas J Ferris3,4 1 2 3 4
Department of Cancer Imaging, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia Department of Radiology, University of Melbourne, Melbourne, Victoria Monash Imaging, Monash Health, Melbourne, Victoria, Australia Monash Biomedical Imaging, Monash University, Melbourne, Victoria, Australia
BK Parameswaran FRANZCR, FRCR, MD, DNB; E Lau BPharm, MB BS, FRANZCR, FAANMS; NJ Ferris MB, BS(Hons), M. Med, FRANZCR. Correspondence Dr Bimal Kumar Parameswaran, Department of Cancer Imaging, Peter MacCallum Cancer Centre, St. Andrews Place, East Melbourne, Vic. 3002, Australia. Email: [email protected]
Summary Diffusion-weighted imaging (DWI) has become an integral part of MRI. Knowledge of the basic principles of DWI and its pitfalls are imperative in the proper application of this technique. We illustrate potential pitfalls of DWI in oncologic imaging. Key words: diffusion; MRI; oncologic imaging; pitfall; tumour.
Conflict of interest: None. Submitted 5 May 2014; accepted 25 November 2014. doi:10.1111/1754-9485.12278
Introduction Diffusion-weighted imaging (DWI) is a useful MRI tool for detection,1 characterisation,2,3 and staging of tumours,4–6 as well as assessment of treatment response.7 We illustrate some important pitfalls in the use of DWI for tumour assessment.
Imaging technique DW MRI is a measure of the Brownian motion of water molecules in the body and reflects the state of the cellular environment. To obtain DW images, gradient pulses are applied in various directions, causing increased signal dephasing in the selected direction, and hence signal loss; where water molecules are not able to diffuse freely, there is less (or no) dephasing, and signal intensity is not affected. The strength of the diffusionsensitising gradient pulse is referred to as its ‘b value; b = 0 implies images with no diffusion weighting. Comparison of images obtained with and without diffusion weighting allows calculation of ‘apparent diffusion coefficient’ (ADC) maps of a tissue. A good description of the basic principles of DWI can be found in reviews by Padhani et al.8 and Koh and Collins.9
© 2015 The Royal Australian and New Zealand College of Radiologists
Most DWI of tumours uses fat-suppressed echo-planar T2-weighted sequences; free breathing techniques allow larger areas of coverage, with fewer acquisitions. Although 1.5 T units yield lower SNR than 3T systems, 1.5 T images also show fewer susceptibility artefacts, and fewer failures of fat suppression.8 Initially two b values (0, 1000) were used, as for stroke, but acquiring images at three b values, though more time-consuming, yields more accurate ADC values.8 Images with b = 0 are essentially just T2-weighted images, and there is evidence that using a non-zero lower b value reduces perfusion effects on the ADC calculation8,9 (see below). Tumours are generally hypercellular, with high nuclear/cytoplasmic ratios, and these features are thought to account for diffusion restriction within tumours, seen as high signal intensity in high b-value images, and low calculated ADC values8,9 (Fig. 1).
Pitfalls Need for more than 1 b value, and correlation with ADC maps The signal intensity of a mass in a DW image depends both on its cellularity, and on its T2 relaxation time. With increasing b values, a cellular tumour, with restricted
1
BK Parameswaran et al.
Fig. 1. Carcinoma in the right peripheral zone of prostate (arrow). Axial T2-weighted small field of view scan of the prostate performed on a 3T unit shows a hypointense lesion in the right posterolateral peripheral zone, indicated by an arrow in (a). The lesion is hyperintense in diffusion-weighted image obtained with a b value of 800 (b) and hypointense in the ADC map (c) denoting restricted diffusion, as expected for prostate carcinoma.
diffusion, shows increasing hyperintensity, relative to less cellular surrounding tissue, with low values in the derived ‘ADC map’8,9 (Fig. 2). The relative signal intensity of the lesion must be correlated with the b value of the image, and the lesion’s ADC value, for correct interpretation.
T2 shine-through As the signal intensity of a tissue in DWI depends on its T2 characteristics as well as its diffusion properties, some lesions may show high signal in high b-value images without having restricted diffusion. This is known
a
b
c
d
e
f
Fig. 2. 3T Axial T2 (a), b = 50, 400, 800 (b, c, and d), ADC map (e) and FDG PET scan (f) of the pelvis in a 77-year-old male with pelvic recurrence following abdominoperineal resection of rectal cancer four years ago. Note how the two small foci of tumour recurrence (arrows) are progressively bright compared with areas with no restricted diffusion in higher b-value images and hypointense in ADC map. Both lesions show tracer uptake on FDG PET scan.
2
© 2015 The Royal Australian and New Zealand College of Radiologists
Recognising pitfalls in DW MRI
a
c
b
Fig. 3. Carcinoma rectum with metastatic vaginal nodule: Transverse T2 scan of the pelvis (a) showing catheter in tumour-bearing rectum (long arrow) and in vagina with metastatic nodule (short arrow) with injected gel (double arrows) around the catheter in the vagina. b 1000 image (b) shows the rectal tumour and the vaginal nodule as well as the gel in the vagina to be hyperintense; ADC map (c) shows low values in rectal tumour and the vaginal nodule, while the gel in vagina and within the catheters in situ is bright, indicating T2 shine-through.
as ‘T2 shine-through’. Such a lesion will be hyperintense on both high b-value images and the ADC map8–10 (Fig. 3). This potential pitfall also emphasises the importance of correlating the ADC images with the high b-value image.
Sites normally showing restricted diffusion Normal structures that show high signal in high b-value images, and low values in ADC maps, include the spinal cord, normal lymph nodes, the spleen, normal bowel, endometrium and ovaries (Fig. 4).
be suppressed in all the DW images (regardless of b value), no meaningful ADC value can be calculated for fatty lesions; they may appear with spuriously high or low values in ADC maps (Fig. 8).
Haemorrhage Haematomas contain high concentrations of macromolecules such as fibrin; these can restrict the diffusion of water molecules with resulting hyperintensity at DWI, and low ADC values11 (Fig. 6).
Benign lesions mimicking malignancy
Fibrosis
Several benign lesions can show restricted diffusion, due to their high cellularity or macromolecular content, and mimic malignancy.9 These include non-neoplastic lesions like abscess (Fig. 5), and haematoma (see below) (Fig. 6), and benign tumours like hemangioma, hepatic adenoma, fibronodular hyperplasia9,10 and rectal adenoma (Fig. 7).
Fibrosis, both inherent and as a response to therapy, returns low signal intensity in all sequences. As with fatty lesions, calculation of an ADC value from two near-zero signal intensities is unreliable.
Fat
a. Mucinous tumours
The assessment of fat containing lesions with DW echo planar imaging (EPI) is difficult. Since the fat signal must
Mucin in a tumour shows relatively high ADC8 (Fig. 9); this can also cause difficulties in treatment response
High ADC lesions: potential false negative for malignancy
Fig. 4. b = 800 image (a) and corresponding ADC map (b) showing hyper and hypointense appearances, respectively, of the normal spinal cord and spleen (arrows).
© 2015 The Royal Australian and New Zealand College of Radiologists
3
BK Parameswaran et al.
Fig. 5. Known residual abscess: 3T Axial T2-weighted follow-up scan of the pelvis (a) in a patient who had undergone radiotherapy for rectal cancer and had a presacral abscess (baseline scan not shown) demonstrates the residual abscess as a smaller well defined T2 hyperintense lesion (arrow). b = 800 (b) image and ADC (c) map show the residual collection (arrow) as an area of high and low signal respectively, mimicking a cellular tumour.
assessment, given that mucinous transformation may be a favourable response.12
Technical issues a. Movement artefacts
b. Cystic/necrotic tumours Inherently cystic tumours, and necrotic areas in tumours, can also show high ADC values9,10 (Fig. 10).
As with conventional MRI, movement artefacts from inherent motion or poor breath-holding significantly degrade DWI. Lesions high in the left lobe of the liver are
Fig. 6. Pelvic hematoma: Axial T1FS (a), T2FS (b), b = 800 images (c), and ADC map (d) of a postoperative haematoma (arrow). DWI characteristics mimic hypercellular tumour.
4
© 2015 The Royal Australian and New Zealand College of Radiologists
Recognising pitfalls in DW MRI
a
c
b
Fig. 7. 3T Axial T2 (a) scan of proven rectal adenoma. b = 800 image (b) and ADC map (c) show restricted diffusion, mimicking carcinoma.
Fig. 8. Intra -abdominal liposarcoma (arrow): 3T Axial T1-weighted image (a) and ADC map (b) of a large intra-abdominal liposarcoma with significant fatty component. Note the variation and aliasing in the ADC map due to the suppression of fat signal.
a
a
b
c
b
Fig. 9. T2 (a), b = 800 (b) images, and ADC map (c), of mucinous adenocarcinoma rectum (arrow). Note the high ADC values in the lesion.
a
b
c
Fig. 10. Post contrast (a), b = 50 (b) images and ADC map (c) of a partly cystic retroperitoneal sarcoma. Note high ADC in the central, cystic, non-enhancing component.
© 2015 The Royal Australian and New Zealand College of Radiologists
5
BK Parameswaran et al.
Fig. 11. 3T Axial T2 (a), b = 800 images (b) and ADC map (c) in a patient with T3 rectal carcinoma on left, almost obscured by gross susceptibility artefacts from left hip prosthesis.
notoriously likely to be affected by cardiac pulsation/ breath-hold artefacts, limiting their evaluation with DWI. b. Susceptibility artefacts Susceptibility artefacts in DWI are conspicuous, particularly at 3T, due to the use of EPI-based sequences. These artefacts occur at tissue interfaces with air, bone, or introduced metal (Fig. 11), where there are abrupt changes in tissue magnetic susceptibility in a short distance, with resulting local field distortions and signal loss. Parallel imaging techniques reduce susceptibility and magnetic field inhomogeneity-related artefacts. c. Dielectric effects
is as yet no consensus on the optimum number, or magnitudes, of b values, in oncologic imaging.8 b Values higher than 1000 have been suggested for evaluation of prostate carcinoma, while values between 750 and 1000 are recommended when imaging the breast and general abdomen including pancreas, kidneys and rectum.8 While two b values are sufficient to generate ADC maps, ADC calculation from three b values is recommended for greater accuracy and reproducibility.8 b. Choice of region of interest (ROI) As expected, the ADC value of a lesion will differ depending on the ROI chosen (Fig. 13).
Particularly in large subjects at 3T, the interaction of the RF field with the patient’s body can be inhomogeneous, with RF absorption mainly in superficial tissues, leading to signal loss (‘the dielectric effect’) deep to the body wall, e.g. in the left lobe of the liver or the mid-abdomen (Fig. 12). This can be controlled by placing a dielectric pad over the abdominal wall.
c. Confounding by perfusion effects
DWI in clinical trials
Conclusion
a. Standardisation of parameters ADC measured with b values 0 and 400 will be different from an ADC calculated with b values 0 and 800.13 There
a
b
At low b values, signal from fast-moving water (e.g. blood) is lost. ADC calculations using b values above 100/150 alone therefore provide a ‘flow insensitive ADC’, and those including b values of 0–100 provide a ‘flow sensitive ADC’.8,9
DWI has become an important tool in the MRI assessment of tumours, but we have illustrated a number of potential pitfalls, recognition of which is important for accurate interpretation.
c
Fig. 12. 3T MRI in a patient with cirrhosis, multifocal hepatocellular carcinoma and ascites: axial T2-weighted image (a) shows signal loss across the liver due to dielectric effects, worse in b = 800 (b) image and ADC map (c).
6
© 2015 The Royal Australian and New Zealand College of Radiologists
Recognising pitfalls in DW MRI
4.
5.
6.
7.
8.
9.
Fig. 13. Left renal carcinoma (a and b) shows ADC values varying with size and location of ROI, in a heterogeneous lesion.
References 1. Inada Y, Matsuki M, Nakai G et al. Body diffusionweighted MR imaging of uterine endometrial cancer: is it helpful in the detection of cancer in nonenhanced MR imaging? Eur J Radiol 2009; 70: 122–7. 2. Attariwala R, Picker W. Whole body MRI: improved lesion detection and characterization with diffusion weighted techniques. J Magn Reson Imaging 2013; 38: 253–68. 3. Caivano R, Rabasco P, Lotumolo A et al. Comparison between Gleason score and apparent diffusion
© 2015 The Royal Australian and New Zealand College of Radiologists
10.
11.
12.
13.
coefficient obtained from diffusion-weighted imaging of prostate cancer patients. Cancer Invest 2013; 31: 625–9. Caivano R, Rabasco P, Lotumolo A et al. The role of diffusion weighted imaging in the preoperative staging. Cancer Invest 2014; 32: 184–90. Jun. Giannarini G, Petralia G, Thoeny HC. Potential and limitations of diffusion-weighted magnetic resonance imaging in kidney, prostate, and bladder cancer including pelvic lymph node staging: a critical analysis of the literature. Eur Urol 2012; 61: 326–40. Vandecaveye V, De Keyzer F, Vander Poorten V et al. Head and neck squamous cell carcinoma: value of diffusion-weighted MR imaging for nodal staging. Radiology 2009; 251: 134–46. Kim HS, Kim CK, Park BK, Huh SJ, Kim B. Evaluation of therapeutic response to concurrent chemoradiotherapy in patients with cervical cancer using diffusion-weighted MR imaging. J Magn Reson Imaging 2013; 37: 187–93. Padhani AR, Liu G, Koh DM et al. Diffusion-weighted magnetic resonance imaging as a cancer biomarker: consensus and recommendations. Neoplasia 2009; 11: 102–25. Koh DM, Collins DJ. Diffusion-Weighted MRI in the body: applications and challenges in oncology. AJR Am J Roentgenol 2007; 188: 1622–35. Malayeri AA, El Khouli RH, Zaheer A et al. Principles and applications of diffusion-weighted imaging in cancer detection, staging, and treatment follow-up. Radiographics 2011; 31: 1773–91. Shah N, Reichel T, Fleckenstein JL. Diffusion findings in blood clot: the last word? AJNR Am J Neuroradiol 2004; 25: 157–8. Patel UB, Blomqvist LK, Taylor F et al. MRI after treatment of locally advanced rectal cancer: How to report tumor response- the Mercury experience. AJR Am J Roentgenol 2012; 199: W486–95. Kallehauge JF, Tanderup K, Haack S et al. Apparent Diffusion Coefficient (ADC) as a quantitative parameter in diffusion weighted MR imaging in gynecologic cancer: dependence on b-values used. Acta Oncol 2010; 49: 1017–22. Oct.
7
RADIOLO GY—P I CTO R I A L E SSAY
bs_bs_banner
Recognising pitfalls in assessment of tumours by diffusion-weighted MRI: A pictorial essay Bimal Kumar Parameswaran,1 Eddie Lau1,2 and Nicholas J Ferris3,4 1 2 3 4
Department of Cancer Imaging, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia Department of Radiology, University of Melbourne, Melbourne, Victoria Monash Imaging, Monash Health, Melbourne, Victoria, Australia Monash Biomedical Imaging, Monash University, Melbourne, Victoria, Australia
BK Parameswaran FRANZCR, FRCR, MD, DNB; E Lau BPharm, MB BS, FRANZCR, FAANMS; NJ Ferris MB, BS(Hons), M. Med, FRANZCR. Correspondence Dr Bimal Kumar Parameswaran, Department of Cancer Imaging, Peter MacCallum Cancer Centre, St. Andrews Place, East Melbourne, Vic. 3002, Australia. Email: [email protected]
Summary Diffusion-weighted imaging (DWI) has become an integral part of MRI. Knowledge of the basic principles of DWI and its pitfalls are imperative in the proper application of this technique. We illustrate potential pitfalls of DWI in oncologic imaging. Key words: diffusion; MRI; oncologic imaging; pitfall; tumour.
Conflict of interest: None. Submitted 5 May 2014; accepted 25 November 2014. doi:10.1111/1754-9485.12278
Introduction Diffusion-weighted imaging (DWI) is a useful MRI tool for detection,1 characterisation,2,3 and staging of tumours,4–6 as well as assessment of treatment response.7 We illustrate some important pitfalls in the use of DWI for tumour assessment.
Imaging technique DW MRI is a measure of the Brownian motion of water molecules in the body and reflects the state of the cellular environment. To obtain DW images, gradient pulses are applied in various directions, causing increased signal dephasing in the selected direction, and hence signal loss; where water molecules are not able to diffuse freely, there is less (or no) dephasing, and signal intensity is not affected. The strength of the diffusionsensitising gradient pulse is referred to as its ‘b value; b = 0 implies images with no diffusion weighting. Comparison of images obtained with and without diffusion weighting allows calculation of ‘apparent diffusion coefficient’ (ADC) maps of a tissue. A good description of the basic principles of DWI can be found in reviews by Padhani et al.8 and Koh and Collins.9
© 2015 The Royal Australian and New Zealand College of Radiologists
Most DWI of tumours uses fat-suppressed echo-planar T2-weighted sequences; free breathing techniques allow larger areas of coverage, with fewer acquisitions. Although 1.5 T units yield lower SNR than 3T systems, 1.5 T images also show fewer susceptibility artefacts, and fewer failures of fat suppression.8 Initially two b values (0, 1000) were used, as for stroke, but acquiring images at three b values, though more time-consuming, yields more accurate ADC values.8 Images with b = 0 are essentially just T2-weighted images, and there is evidence that using a non-zero lower b value reduces perfusion effects on the ADC calculation8,9 (see below). Tumours are generally hypercellular, with high nuclear/cytoplasmic ratios, and these features are thought to account for diffusion restriction within tumours, seen as high signal intensity in high b-value images, and low calculated ADC values8,9 (Fig. 1).
Pitfalls Need for more than 1 b value, and correlation with ADC maps The signal intensity of a mass in a DW image depends both on its cellularity, and on its T2 relaxation time. With increasing b values, a cellular tumour, with restricted
1
BK Parameswaran et al.
Fig. 1. Carcinoma in the right peripheral zone of prostate (arrow). Axial T2-weighted small field of view scan of the prostate performed on a 3T unit shows a hypointense lesion in the right posterolateral peripheral zone, indicated by an arrow in (a). The lesion is hyperintense in diffusion-weighted image obtained with a b value of 800 (b) and hypointense in the ADC map (c) denoting restricted diffusion, as expected for prostate carcinoma.
diffusion, shows increasing hyperintensity, relative to less cellular surrounding tissue, with low values in the derived ‘ADC map’8,9 (Fig. 2). The relative signal intensity of the lesion must be correlated with the b value of the image, and the lesion’s ADC value, for correct interpretation.
T2 shine-through As the signal intensity of a tissue in DWI depends on its T2 characteristics as well as its diffusion properties, some lesions may show high signal in high b-value images without having restricted diffusion. This is known
a
b
c
d
e
f
Fig. 2. 3T Axial T2 (a), b = 50, 400, 800 (b, c, and d), ADC map (e) and FDG PET scan (f) of the pelvis in a 77-year-old male with pelvic recurrence following abdominoperineal resection of rectal cancer four years ago. Note how the two small foci of tumour recurrence (arrows) are progressively bright compared with areas with no restricted diffusion in higher b-value images and hypointense in ADC map. Both lesions show tracer uptake on FDG PET scan.
2
© 2015 The Royal Australian and New Zealand College of Radiologists
Recognising pitfalls in DW MRI
a
c
b
Fig. 3. Carcinoma rectum with metastatic vaginal nodule: Transverse T2 scan of the pelvis (a) showing catheter in tumour-bearing rectum (long arrow) and in vagina with metastatic nodule (short arrow) with injected gel (double arrows) around the catheter in the vagina. b 1000 image (b) shows the rectal tumour and the vaginal nodule as well as the gel in the vagina to be hyperintense; ADC map (c) shows low values in rectal tumour and the vaginal nodule, while the gel in vagina and within the catheters in situ is bright, indicating T2 shine-through.
as ‘T2 shine-through’. Such a lesion will be hyperintense on both high b-value images and the ADC map8–10 (Fig. 3). This potential pitfall also emphasises the importance of correlating the ADC images with the high b-value image.
Sites normally showing restricted diffusion Normal structures that show high signal in high b-value images, and low values in ADC maps, include the spinal cord, normal lymph nodes, the spleen, normal bowel, endometrium and ovaries (Fig. 4).
be suppressed in all the DW images (regardless of b value), no meaningful ADC value can be calculated for fatty lesions; they may appear with spuriously high or low values in ADC maps (Fig. 8).
Haemorrhage Haematomas contain high concentrations of macromolecules such as fibrin; these can restrict the diffusion of water molecules with resulting hyperintensity at DWI, and low ADC values11 (Fig. 6).
Benign lesions mimicking malignancy
Fibrosis
Several benign lesions can show restricted diffusion, due to their high cellularity or macromolecular content, and mimic malignancy.9 These include non-neoplastic lesions like abscess (Fig. 5), and haematoma (see below) (Fig. 6), and benign tumours like hemangioma, hepatic adenoma, fibronodular hyperplasia9,10 and rectal adenoma (Fig. 7).
Fibrosis, both inherent and as a response to therapy, returns low signal intensity in all sequences. As with fatty lesions, calculation of an ADC value from two near-zero signal intensities is unreliable.
Fat
a. Mucinous tumours
The assessment of fat containing lesions with DW echo planar imaging (EPI) is difficult. Since the fat signal must
Mucin in a tumour shows relatively high ADC8 (Fig. 9); this can also cause difficulties in treatment response
High ADC lesions: potential false negative for malignancy
Fig. 4. b = 800 image (a) and corresponding ADC map (b) showing hyper and hypointense appearances, respectively, of the normal spinal cord and spleen (arrows).
© 2015 The Royal Australian and New Zealand College of Radiologists
3
BK Parameswaran et al.
Fig. 5. Known residual abscess: 3T Axial T2-weighted follow-up scan of the pelvis (a) in a patient who had undergone radiotherapy for rectal cancer and had a presacral abscess (baseline scan not shown) demonstrates the residual abscess as a smaller well defined T2 hyperintense lesion (arrow). b = 800 (b) image and ADC (c) map show the residual collection (arrow) as an area of high and low signal respectively, mimicking a cellular tumour.
assessment, given that mucinous transformation may be a favourable response.12
Technical issues a. Movement artefacts
b. Cystic/necrotic tumours Inherently cystic tumours, and necrotic areas in tumours, can also show high ADC values9,10 (Fig. 10).
As with conventional MRI, movement artefacts from inherent motion or poor breath-holding significantly degrade DWI. Lesions high in the left lobe of the liver are
Fig. 6. Pelvic hematoma: Axial T1FS (a), T2FS (b), b = 800 images (c), and ADC map (d) of a postoperative haematoma (arrow). DWI characteristics mimic hypercellular tumour.
4
© 2015 The Royal Australian and New Zealand College of Radiologists
Recognising pitfalls in DW MRI
a
c
b
Fig. 7. 3T Axial T2 (a) scan of proven rectal adenoma. b = 800 image (b) and ADC map (c) show restricted diffusion, mimicking carcinoma.
Fig. 8. Intra -abdominal liposarcoma (arrow): 3T Axial T1-weighted image (a) and ADC map (b) of a large intra-abdominal liposarcoma with significant fatty component. Note the variation and aliasing in the ADC map due to the suppression of fat signal.
a
a
b
c
b
Fig. 9. T2 (a), b = 800 (b) images, and ADC map (c), of mucinous adenocarcinoma rectum (arrow). Note the high ADC values in the lesion.
a
b
c
Fig. 10. Post contrast (a), b = 50 (b) images and ADC map (c) of a partly cystic retroperitoneal sarcoma. Note high ADC in the central, cystic, non-enhancing component.
© 2015 The Royal Australian and New Zealand College of Radiologists
5
BK Parameswaran et al.
Fig. 11. 3T Axial T2 (a), b = 800 images (b) and ADC map (c) in a patient with T3 rectal carcinoma on left, almost obscured by gross susceptibility artefacts from left hip prosthesis.
notoriously likely to be affected by cardiac pulsation/ breath-hold artefacts, limiting their evaluation with DWI. b. Susceptibility artefacts Susceptibility artefacts in DWI are conspicuous, particularly at 3T, due to the use of EPI-based sequences. These artefacts occur at tissue interfaces with air, bone, or introduced metal (Fig. 11), where there are abrupt changes in tissue magnetic susceptibility in a short distance, with resulting local field distortions and signal loss. Parallel imaging techniques reduce susceptibility and magnetic field inhomogeneity-related artefacts. c. Dielectric effects
is as yet no consensus on the optimum number, or magnitudes, of b values, in oncologic imaging.8 b Values higher than 1000 have been suggested for evaluation of prostate carcinoma, while values between 750 and 1000 are recommended when imaging the breast and general abdomen including pancreas, kidneys and rectum.8 While two b values are sufficient to generate ADC maps, ADC calculation from three b values is recommended for greater accuracy and reproducibility.8 b. Choice of region of interest (ROI) As expected, the ADC value of a lesion will differ depending on the ROI chosen (Fig. 13).
Particularly in large subjects at 3T, the interaction of the RF field with the patient’s body can be inhomogeneous, with RF absorption mainly in superficial tissues, leading to signal loss (‘the dielectric effect’) deep to the body wall, e.g. in the left lobe of the liver or the mid-abdomen (Fig. 12). This can be controlled by placing a dielectric pad over the abdominal wall.
c. Confounding by perfusion effects
DWI in clinical trials
Conclusion
a. Standardisation of parameters ADC measured with b values 0 and 400 will be different from an ADC calculated with b values 0 and 800.13 There
a
b
At low b values, signal from fast-moving water (e.g. blood) is lost. ADC calculations using b values above 100/150 alone therefore provide a ‘flow insensitive ADC’, and those including b values of 0–100 provide a ‘flow sensitive ADC’.8,9
DWI has become an important tool in the MRI assessment of tumours, but we have illustrated a number of potential pitfalls, recognition of which is important for accurate interpretation.
c
Fig. 12. 3T MRI in a patient with cirrhosis, multifocal hepatocellular carcinoma and ascites: axial T2-weighted image (a) shows signal loss across the liver due to dielectric effects, worse in b = 800 (b) image and ADC map (c).
6
© 2015 The Royal Australian and New Zealand College of Radiologists
Recognising pitfalls in DW MRI
4.
5.
6.
7.
8.
9.
Fig. 13. Left renal carcinoma (a and b) shows ADC values varying with size and location of ROI, in a heterogeneous lesion.
References 1. Inada Y, Matsuki M, Nakai G et al. Body diffusionweighted MR imaging of uterine endometrial cancer: is it helpful in the detection of cancer in nonenhanced MR imaging? Eur J Radiol 2009; 70: 122–7. 2. Attariwala R, Picker W. Whole body MRI: improved lesion detection and characterization with diffusion weighted techniques. J Magn Reson Imaging 2013; 38: 253–68. 3. Caivano R, Rabasco P, Lotumolo A et al. Comparison between Gleason score and apparent diffusion
© 2015 The Royal Australian and New Zealand College of Radiologists
10.
11.
12.
13.
coefficient obtained from diffusion-weighted imaging of prostate cancer patients. Cancer Invest 2013; 31: 625–9. Caivano R, Rabasco P, Lotumolo A et al. The role of diffusion weighted imaging in the preoperative staging. Cancer Invest 2014; 32: 184–90. Jun. Giannarini G, Petralia G, Thoeny HC. Potential and limitations of diffusion-weighted magnetic resonance imaging in kidney, prostate, and bladder cancer including pelvic lymph node staging: a critical analysis of the literature. Eur Urol 2012; 61: 326–40. Vandecaveye V, De Keyzer F, Vander Poorten V et al. Head and neck squamous cell carcinoma: value of diffusion-weighted MR imaging for nodal staging. Radiology 2009; 251: 134–46. Kim HS, Kim CK, Park BK, Huh SJ, Kim B. Evaluation of therapeutic response to concurrent chemoradiotherapy in patients with cervical cancer using diffusion-weighted MR imaging. J Magn Reson Imaging 2013; 37: 187–93. Padhani AR, Liu G, Koh DM et al. Diffusion-weighted magnetic resonance imaging as a cancer biomarker: consensus and recommendations. Neoplasia 2009; 11: 102–25. Koh DM, Collins DJ. Diffusion-Weighted MRI in the body: applications and challenges in oncology. AJR Am J Roentgenol 2007; 188: 1622–35. Malayeri AA, El Khouli RH, Zaheer A et al. Principles and applications of diffusion-weighted imaging in cancer detection, staging, and treatment follow-up. Radiographics 2011; 31: 1773–91. Shah N, Reichel T, Fleckenstein JL. Diffusion findings in blood clot: the last word? AJNR Am J Neuroradiol 2004; 25: 157–8. Patel UB, Blomqvist LK, Taylor F et al. MRI after treatment of locally advanced rectal cancer: How to report tumor response- the Mercury experience. AJR Am J Roentgenol 2012; 199: W486–95. Kallehauge JF, Tanderup K, Haack S et al. Apparent Diffusion Coefficient (ADC) as a quantitative parameter in diffusion weighted MR imaging in gynecologic cancer: dependence on b-values used. Acta Oncol 2010; 49: 1017–22. Oct.
7
