Thyroid Ultrasound Physics and Doppler Advanced AACE-ACE US training course 2017 Dev Abraham MD, MRCP(UK), ECNU, FACE Professor of Medicine, University of Utah No Disclosures
Natural Ability to see with sound
Sound Waves Physics of Sound Role of the propagating medium Acoustic Impedance Density, Stiffness, and Speed of Sound Reflection Occurs at Boundaries of Impedance Refraction Attenuation
Principles of Sound Sound is the Propagation of mechanical energy through matter. No sound in a vacuum Qualities of the transmitting medium influence sound transmission Density Stiffness Acoustic Impedance Speed of Sound (SOS)
Sound Waves Sound waves propagate by compression and rarefaction of molecules in space. Molecules of the medium vibrate around their resting position and transfer their energy to neighboring molecules. Waves carry energy, not matter. Frequency and period Hertz (Hz) = cycles/second Propagation speed Wavelength
Pulsed Waves Pulse Duration, Repetition Period and Frequency (Hz) Spatial Pulse Length (m) Determinant of axial resolution (1/2 SPL) Duty Factor (%) Clinical Correlate: Pulse duration (frequency) determines axial resolution of an ultrasound image.
Propagation Velocity of Common Tissues
Q 1: One of the characteristic features of a cyst is the following: a. Echogenic texture with sharp margins b. Speckling with intra lesional sub 2 mm calcifications c. Anechoic with posterior enhancement d. Acoustic shadowing posterior to the lesion
Properties of Transmitting Material Acoustic Impedance Reflection of sound occurs at interfaces of Acoustic Impedance Homogeneous medium does not reflect sound Cyst Anechoic Most tissues have heterogeneous impedance Acoustic Impedance depends on the speed of sound, density and stiffness of a material
Reflection Reflection is the redirection of a portion of a sound beam back at the interface of tissues of unequal acoustic impedance. The greater the difference in impedance, the greater the reflection.
Benign Findings The Halo Sign Hassani, 1977 Propper, 1980
Suspicious Findings: Microcalcifications
Illustrating Physics by Understanding Artifacts Reflection Artifacts Shadowing and Enhancement Edge Artifact Reverberation Artifact Comet Tail Artifact
Shadowing and Enhancement
Enhancement is seen behind cysts - But can be seen behind homogeneous solid objects as well Benign hyperechoic nodule seen in Hashimoto s Thyroiditis
Reflection Artifacts Edge Artifact
Reverberation Artifact Meritt, 1998
Reverberation Artifact
Q 2: One of the following is a typical example of reverberation artifacts: a. Sub 2 mm calcifications that do not cast shadows b. Popcorn type calcification with acoustic signal drop out c. Comet tail and cats eye signs d. Mural mass within the wall of the nodule
Comet Tail Artifact
Comet Tail Artifacts
Attenuation Attenuation of acoustic energy results from Reflection Scatter Absorption Attenuation is frequency dependent Higher frequencies have greater attenuation Less depth of imaging with higher frequencies Artifacts due to attenuation Shadowing Enhancement
Q 3: Which of the following is true regarding ultrasound frequency? a. Increasing frequency will increase depth of penetration. b. Increasing frequency will improve resolution. c. Increasing frequency will increase noise. d. High PRF (pulse rep frequency) is useful in the study of lymph nodes
Resolution AZIMUTHAL (up-down) AXIAL (distance from transducer) LATERAL (side to side)
Resolution Ability to discriminate two adjacent objects as separate entities. The narrower the beam, the better the lateral resolution. The higher the frequency the better the axial resolution. Poor focus Image Good focus
Focus and Resolution Focused beam width determines Lateral and Azimuthal Resolution Near field (Fresnel Zone) Large variations of intensity Far field (Fraunhofer Zone) Intensity more uniform Focal Zone - Area of maximal narrowing Pulse duration (frequency) determines Axial Resolution (axial resolution = 1/2 SPL) Practical Consideration - As frequency increases, axial resolution improves, but depth of imaging decreases. The depth of the focal zone is often adjustable and indicated on the display
Frequency and Resolution With higher frequency the resolution of the near field is improved, but the deep structures are better visualized with the lower frequency.
Image Optimization - Frequency Choose highest frequency (12-15 MHz) that allows adequate depth penetration. Lower frequencies (7-10 MHz) for deep structures or very obese subjects
Advances in Technology Signal Processing Image Enhancement Noise reduction Edge sharpening Utilization of CT and MRI reconstruction algorithms Beam Steering Spatial compounding
Signal Processing
Comparison of standard and processed images
Effect of Compound Imaging on Artifacts Comet Tails
Effect of Compound imaging on Artifacts Edge Artifact and Enhancement
Image Optimization Create the sharpest image to allow tissue discrimination. Equipment factors: Quality of Transducer Quality of Electronics Image Enhancement and Compound Imaging User Adjustments: Depth, Gain, Frequency Focal zones Number and Location Compound Imaging Tissue Harmonic Imaging Dynamic range
Optimal Gain
Adjustment of number and position of focal zones
Image Optimization Dynamic range May increase conspicuity of subtle lesions
Doppler Shift
Color and Power Doppler Meritt, 1998
Color and Power Doppler Meritt, 1998
Color and Power Doppler Color Doppler Provides information regarding direction and velocity. More useful in vascular studies Power Doppler No information regarding velocity Less angle dependence Less noise Increased sensitivity for detection of flow
Color Doppler - Follicular Carcinoma Inferior right lobe - sagittal
Color Doppler Benign Nodule
Does Doppler play a role in the prediction of malignancy? (1) 494 consecutive patients with nonpalpable nodules measuring 8 to 15 millimeters. 31 malignant. 87% of cancers were solid and hypoechoic. 77% of cancers had irregular or blurred margins. Intra nodular vascular pattern was seen in 74% of cancers. Microcalcifications were seen in 29 percent of cancers. Independent risk factors for malignancy included irregular margins (RR = 16.83) intra nodular flow (RR = 14.29) microcalcifications (RR = 4.97) Recommendation for ultrasound guided fine needle aspiration if any of the independent risk factors are present, and if no risk factors are present follow-up with clinical and ultrasound evaluation. Papini E, Guglielmi R, Bianchini A, Crescenzi A, et al. Risk of malignancy in nonpalpable thyroid nodules: predictive value of ultrasound and color Doppler features. J Clin Endocrinol Metab. 2002;87(5):1941-1946.
Does Vascularity play a role in the prediction of malignancy? (2) Retrospective evaluation of 1083 nodules in 1024 patients. 814 benign. 269 malignant (97.4% PTC) Marked hypoechogenicity, noncircumscribed margins, microcalcifications and taller than wide considered suspicious by gray scale. Vascularity graded as none, peripheral or intranodular. Vascularity was frequently seen in benign nodules (31%) No vascularity was more frequent in malignant nodules (60% v 43%) (p<.001) Vascularity of nodules was NOT related to risk of malignancy. Moon HJ, et al. Can Vascularity at Power Doppler US Help Predict Thyroid Malignancy? Radiology, 2010; 255(1) 260-269
Why the discrepancy? What to do? Papillary versus Follicular Cancer. Power Doppler may be predictive for follicular cancer. In Moon s study 97.4% were papillary Vascularity does not seem to be predictive in papillary cancer. Consider whether features are suspicious for papillary or follicular cancer. Levine RA, Endocrine Practice 2006 Iared W, J Ultrasound Med 2010
Value of Doppler in the prediction of malignancy following follicular biopsy 310 follicular nodules Color Doppler flow mapping prior to surgery Grade 1: No color flow mapping inside the nodule Grade 2: Color flow mapping only in peripheral area; PI index < 1.3 Grade 3: Penetrating color flow mapping, vascularity moderate-rich Grade 4: High velocity penetrating color flow mapping, vascular rich; PI index >1.3 Sensitivity 86% Specificity 85% Fukunari N, Clinical evaluation of color Doppler imaging for the differential diagnosis of thyroid follicular lesions World J Surg, 2004; 28(12):1261-1265.
Value of Doppler in Prediction of Malignancy - Conclusions Doppler analysis does not appear useful regarding malignant potential for papillary cancer. Doppler may be of use in lesions with indeterminate (follicular) biopsies. Power Doppler has greater sensitivity and is able to detect lower degrees of internal flow. Doppler information must be interpreted along with other ultrasonographic characteristics including echogenicity, edge definition, calcifications, etc.
Amiodarone-induced Thyrotoxicosis Color Doppler (1) Type 1 Preexisting Thyroid abnormality Graves-like Treatment with thionamides and perchlorate Normal or increased vascularity Type 2 Preexisting Normal Thyroid Destructive thyroiditis-like Treatment with glucocorticoids Absent vascularity +/- elevated Interleukin 6 Preserved echotexture
Amiodarone-induced Thyrotoxicosis Color Doppler (2) Flow Absent (CFDS 0) 58% Prednisolone response rate Flow Present (CFDS 1-3) 14% Prednisolone response rate Treatment Algorithm CFDS 0: Prednisolone CFDS 1-3: Thionomides and perchlorate Combined therapy/surgery if unresponsive Wong, et. al., Intern Med J, 2003 33(9-10) 420-426
Doppler Graves and Thyroiditis Graves Thyroid inferno Peak systolic velocity (PSV) 8-20 cm/sec Thyroiditis Micronodulation in Hashimoto s Various vascular patterns absent to hypervascular Subacute thyroiditis focally may appear as suspicious nodule Thyrotoxicosis Factitia Minimal intrathyroidal vascular flow Peak systolic velocity (PSV) 3-5 cm/sec
Graves Disease Thyroid Inferno
Thyroiditis Not always hypovascular
Graves Disease versus Destructive Thyoiditis. Multiple studies have attempted to separate Graves from thyroiditis using variety of Doppler techniques. Power Doppler Images Thyroid blood flow area Thyroid artery velocity
Thyroid Blood Flow Ota H et. al., Clin Endocrinol (Oxf) 67:41 2007
Thyroid Blood Flow Area Kurita S et al. Thyroid. 2005;15(11) 1249
Inferior Thyroid Artery Peak Velocity Kumar H et. al., Endocrine Practice 15(1) 6-9, 2009
Differential Diagnosis of Thyrotoxicosis Conclusions The distinction between Graves Disease and thyroiditis can be based on a combination of clinical features, RAI uptake, TsIg levels, and ultrasound features. RAI uptake remains the gold standard but the behavior over time is often the final determinant. Recognize the limitation of Doppler techniques and do not use in isolation.
Clarification of Interpretation - Cyst?
Doppler Before Biopsy
Doppler Lymph nodes In normal nodes vessels enter centrally at the hilus, and spread along the long axis. In malignant nodes aberrant vessels enter peripherally in the node capsule. Increased (disordered) vascularity may be seen peripherally and centrally.
0.4 (AP) by 1.0 (long axis) CM Left level 4
Jugular Vein
FNA TG 2067 ng/ml
Q4: One of the following US findings has high sensitivity and specificity in the prediction of metastatic lymph-nodes a) Intra lymph-nodal punctate calcification b) Cystic degeneration of lymph-nodes c) Peripheral / chaotic vascularization d) Hyperechoic texture within the lymph node e) Hypoechoic texture within the lymph node
US findings characteristic of metastasis Loss of hilum (100% sensitivity 29% specificity) Rounded shape short axis > 5mm (96 % specificity) Hyperechoic texture - (100% specificity) Calcification (100 % specificity) low sensitivity much less than <50% Peripheral blood flow (82% specificity) 86 % sensitivity Cystic areas 100% specificity low sensitivity Leboulleux, Giirard E, et al, JCEM, 2005 : US criteria of malignancy for cervical LN in patients with differentiated thyroid cancer
Doppler of Nodes Demonstration of Chaotic or peripheral vascularity in malignant nodes Can be seen in reactive nodes Normal vascularity is reassuring Power Doppler for high sensitivity Use low wall filter. Use PRF < 800. Low wall filter and low PRF both increase the sensitivity for detection of low flow.
Physics of Ultrasound Summary Sound transmission is dependent on the conducting medium. Sound is reflected at interfaces of acoustic impedance mismatch. Resolution is dependent on frequency and beam focal width. Artifacts such as shadowing and enhancement provide useful information. Advances in image processing, Doppler analysis, harmonic imaging, and planar reconstruction of images have improved ultrasound images. Doppler imaging of thyroid nodules does not appear to be correlated with risk of malignancy.