Real-time spatial compound imaging improves reproducibility in the evaluation of atherosclerotic carotid plaques

A Review of Medical Doppler Ultrasonography of Blood Flow in General and Especially in Common Carotid Artery

Medical Doppler ultrasound is usually utilized in the clinical adjusting to evaluate and estimate blood flow in both the major (large) and the minor (tiny) vessels of the body. The normal and abnormal sign waveforms can be shown by spectral Doppler technique. The sign waveform is individual to each vessel. Thus, it is significant for the operator and the clinicians to understand the normal and abnormal diagnostic in a spectral Doppler show. The aim of this review is to explain the physical principles behind the medical Doppler ultrasound, also, to use some of the mathematical formulas utilized in the medical Doppler ultrasound examination. Furthermore, we discussed the color and spectral flow model of Doppler ultrasound. Finally, we explained spectral Doppler sign waveforms to show both the normal and abnormal signs waveforms that are individual to the common carotid artery, because these signs are important for both the radiologist and sonographer to perceive both the normal and abnormal in a spectral Doppler show.

Spatial Compounding of 3-D Fetal Brain Ultrasound Using Probabilistic Maps

A new method to address the problem of shadowing in fetal brain ultrasound volumes is presented. The proposed approach is based on the spatial composition of multiple 3-D fetal head projections using the weighted Euclidean norm as an operator. A support vector machine, which is trained with optimal textural features, was used to assign weighting according to the posterior probabilities of brain tissue and shadows. Both phantom and real fetal head ultrasound volumes were compounded using previously reported operators and compared with the proposed composition method to validate it. The quantitative evaluations revealed increases in signal-to-noise ratio ≤35% and in contrast-to-noise ratio ≤135% using real data. Qualitative comparisons made by obstetricians indicated that this novel method adequately recovers brain tissue and improves the visibility of the main cerebral structures. This may prove useful both for fetal monitoring and in the diagnosis of brain defects. Overall this new approach outperforms spatial composition methods previously reported.

Precision imaging of focal liver lesions: comparison with conventional sonography in terms of image quality

OBJECTIVES

The purpose of this study was to compare the Precision Imaging sonographic technique (Toshiba Medical Systems Corporation, Tokyo, Japan) with conventional sonography of liver lesions in terms of lesion conspicuity, margin sharpness, overall image quality, and posterior enhancement.

METHODS

Sixty-nine focal liver lesions in 60 patients (age range, 14-76 years; mean age, 43 years) were prospectively evaluated. Each lesion was examined with Precision Imaging and conventional sonography. All images were reviewed and graded on a 3-point scale by 2 readers for lesion conspicuity, margin sharpness, and overall image quality. Posterior acoustic enhancement was also analyzed in cystic lesions. A Wilcoxon signed rank test was used for statistical comparisons of the techniques for all parameters.

RESULTS

Statistical analysis showed that for margin sharpness, lesion conspicuity, and overall image quality, Precision Imaging was superior to conventional sonography (P< .05). In addition, according to lesion types and dimensions, Precision Imaging was significantly superior to conventional sonography for all parameters. For posterior enhancement, there was no significant difference between Precision Imaging and conventional sonography (P ≥ .05).

CONCLUSIONS

In sonography of focal liver lesions, Precision Imaging provides better lesion conspicuity, better margin sharpness, and better overall image quality than conventional sonography. With respect to posterior enhancement of cystic lesions, Precision Imaging is not significantly different from conventional sonography. Precision Imaging may be used as a complementary method in the sonographic evaluation of focal liver lesions.

Evaluation of the accuracy of articular cartilage thickness measurement by B-mode ultrasonography with conventional imaging and real-time spatial compound ultrasonography imaging

The present study aimed to quantify the thickness of articular cartilage (Tc) in vitro using both conventional and real-time spatial compound B-mode ultrasonography (US) with a clinically used transducer and to evaluate the accuracy of measurement by comparing the results with values obtained microscopically. Femoral condyle samples were obtained from a 6-month-old pig and a 3-year-old pig. B-mode US images with conventional imaging and real-time spatial compound imaging (RTSCI) of osteochondral blocks were acquired. Tc determined using US (Tc-US) was measured from line data parallel to US beam direction acquired from B-mode images with an objective method for determining cartilage surface and bone-cartilage interfaces at the peak brightness values. Tc was also determined under microscopy (Tc-optical) using the corresponding points from US measurement. Tc-US was compared with Tc-optical to assess accuracy. Tc-US correlated significantly with Tc in both conventional imaging and RTSCI (r = 0.961, 0.976, respectively). Bland-Altman plots showed mean differences between Tc-optical and Tc-US were -0.0073 mm and 0.0139 mm with standard deviations of 0.171 mm and 0.131 mm for conventional imaging and RTSCI, respectively. Our results show that Tc-US measurement using B-mode US allows accurate measurement of Tc. Considering correlation coefficients between Tc-US and Tc-optical, RTSCI US may offer higher accuracy for measuring Tc than conventional methods when an objective tissue border determination algorithm is used, even though both showed good accuracy in our study.

Noninvasive imaging of carotid arteries in stroke: emerging value of real-time high-resolution sonography in carotid occlusion due to cardiac embolism

OBJECTIVE

Comprehension of the pathophysiologic characteristics of atherosclerosis has focused its attention on the study of dynamic and metabolic processes involving the vessel wall as possible causes of stroke. When compared with conventional radiologic techniques, sonography has the main advantage of being a real-time imaging modality. We report 2 acute stroke cases in which carotid sonography showed some dynamic features that could not be identified with computed tomography (CT) and magnetic resonance angiography (MRA).

METHODS

Carotid sonography with high-resolution probes (9-14 MHz) was compared with CT and MRA findings showing carotid axis occlusion in 2 patients with acute stroke.

RESULTS

In case 1, the internal carotid artery occlusion observed on CT and MRA was interpreted as a dissection on a clinical basis, but sonography showed a mobile embolus originating from the heart in the internal carotid artery. In case 2, the occlusion of the whole carotid axis observed on CT and MRA was instead related to a heart-originating embolus floating in the common carotid artery.

CONCLUSIONS

The evaluation of dynamic aspects of atherosclerosis is fundamental to understanding the pathophysiologic characteristics of stroke. Sonography is fundamental in carotid artery imaging for its possibility of showing dynamic processes that could be misdiagnosed with "static" imaging. The correct identification of the pathophysiologic characteristics of stroke in these cases could have led to different diagnostic and therapeutic algorithms.

Low-density lipoprotein particle size in hepatic steatosis and metabolic syndrome

BACKGROUND

Hepatic steatosis (HS), the most frequent liver disorder, was reported to be an independent predictor of cardiovascular disease. HS, if combined with the metabolic syndrome (MetS), might have a synergistic effect on low-density lipoprotein (LDL) particle size.

METHODS

Carotid intima-media thickness (IMT) and plaque formation, and HS were diagnosed ultrasonographically, and the MetS was diagnosed using the ATP III criteria in 274 healthy workers (mean age +/- SD, 43.5 +/- 7.1 yrs). LDL particle size was measured with density gradient ultracentrifugation, and subfractions were classified as large, buoyant LDL I (27.2 to approximately 28.5 nm) and small, dense LDL III (24.2~25.5). All participants were grouped into three categories: control, subjects with HS alone and those with both HS and the MetS.

RESULTS

The subjects with HS alone were 84 (30.7%), whereas those with HS and the MetS were 46 (16.8%). LDL peak particle sizes showed significant negative correlations with carotid mean IMTs. LDL peak particle size and LDL I (%) decreased significantly in the HS, showing the lowest values in the subjects with both HS and the MetS, and their association was independent, even adjusted for potential confounders. LDL III also showed independent associations across the groups.

CONCLUSION

HS alone was more prevalent than HS combined with the MetS in general population. For the patients with HS alone, LDL particle size and carotid atherosclerosis were found to fall in the middle of the control and those with both HS and the MetS.

A motion compounding technique for speckle reduction in ultrasound images

The quality of ultrasound images is usually influenced by speckle noise and the temporal decorrelation of the speckle patterns. To reduce the speckle noise, compounding techniques have been widely applied. Partially correlated images scanned on the same subject cross-section are combined to generate a compound image with improved image quality. However, the compounding technique might introduce image blurring if the transducer or the target moves too fast. This blurring effect becomes especially critical when assessing tissue deformation in clinical motion examinations. In this paper, an ultrasound motion compounding system is proposed to improve the quality of ultrasound motion sequences. The proposed motion compounding technique uses a hierarchical adaptive feature weighted motion estimation method to realign the frames before compounding. Each frame is first registered and warped to the reference frame before being compounded to reduce the speckle noise. Experimental results showed that the motion could be assessed accurately and better visualization could be achieved for the compound images, with improved signal-to-noise and contrast-to-noise ratios.

Relationship between ultrasonic attenuation, size and axial strain parameters for ex vivo atherosclerotic carotid plaque

Many ultrasonic parameters, primarily related to attenuation and scatterer size, have been used to characterize the composition of atherosclerotic plaque tissue. In this study, we combine elastographic (axial strain ratio) and ultrasonic tissue characterization parameters, namely the attenuation coefficient and a scattering parameter associated with an "equivalent" scatterer size to delineate between fibrous, calcified, and lipidic plaque tissue. We present results obtained from 44 ex vivo atherosclerotic plaque specimens obtained after carotid endarterectomy on human patients. Our results in the frequency range 2.5 - 7.5 MHz indicate that softer plaques (with higher values of the strain ratio) are usually associated with larger equivalent scatterer size estimates (200 - 500 microm) and lower values of the attenuation coefficient slope (<1 dB/cm/MHz). On the other hand, stiffer plaques (with lower strain ratio values) are associated with smaller equivalent scatterer size estimates (100 - 200 microm) and higher values of the attenuation coefficient slope (1 - 3 dB/cm/MHz). These results indicate that ultrasonic tissue characterization and strain parameters have the potential to differentiate between different plaque types. These parameters can be estimated from radio-frequency data acquired under in vivo conditions and may help the clinician decide on appropriate interventional techniques.

Paired-angle multiplicative compounding

Ultrasound compounding is a method of combining multiple images from different angles in order to create a single image with improved resolution and reduced angular-dependent artifactual detail. Compounding methods traditionally calculate each pixel in the compound image as a simple functional relationship between pixel elements in the component image set. In order to achieve an even better resolution and further reduce angular-dependent artifacts, this paper investigates a new type of compounding we call paired angle multiplicative compounding (PAMC), in which compound images are produced by a summation of multiplied pairs of component images acquired at different angles. A PAMC image of a breast phantom demonstrates improved delineation ofmicrocalcifications in comparison to the mean operation. Images of the forearm are used to investigate larger angles of paired multiplication, the best improving contrast ratio (28%) and signal to noise ratio (24%) when compared to the mean method. The PAMC method is found to be similar to the MEM and median compounding operations, the largest difference being that it is better preserving contributions from normal incidence but at the expense of needing a larger angular range.

The benefits of comparing conventional sonography, real-time spatial compound sonography, tissue harmonic sonography, and tissue harmonic compound sonography of hepatic lesions

This study aimed to compare conventional sonography, tissue harmonic imaging (THI), spatial compound sonography (SONOCT), and SONOCT+THI for overall image quality, lesion conspicuity, and elimination of artifacts of hepatic lesions. Forty-five patients were randomly selected, and 51 different hepatic lesions were scanned using each of the four techniques. The combined images of SONOCT+THI exhibited the best image quality for solid and cystic lesions, while conventional images were the worst for most hepatic lesions (P<.001). SONOCT was the best for fatty liver.

Ultrasound approach to the canine distal tibia and trochlear ridges of the talus

The aim of the present study was to evaluate to what extent the distal tibia and the trochlear ridges of the talus can be examined with ultrasound (US) in the dog and to establish a protocol for an optimal US examination of these ridges. Six hind limbs of deceased adult mixed-breed dogs were used. In two limbs, needles were placed using US guidance on the trochlea of the talus, just dorsal to and plantar to the distal tibia: one with the tarsal joint in extension and one with the joint in flexion. Then mediolateral (ML) radiographs of both joints were made with the needle in place to determine the percentage of the trochlear ridge of the talus that can be seen using US imaging. An US examination of the tarsal joint was performed on the four other limbs using microconvex (8 MHz) and linear (12 MHz) transducers (Logiq 7) and compound imaging. A three-step protocol was performed including a dorsal approach with the limb extended and the linear transducer (step I), a plantar approach with the limb flexed and the linear transducer (step II), and a plantar approach with the limb flexed and the microconvex transducer (step III). After the US examination, the four limbs were frozen and sectioned, two in a transverse and two in a sagittal plane. Bony structures on the US images were matched with the corresponding anatomic sections. The distal tibia and both trochlear ridges of the talus were easily recognized on the US images using the proposed protocol. When combining the dorsal and plantar approaches, it was possible to visualize up to 75% of the trochlear ridges of the talus in the dog.

The use of Speckle Reduction Imaging (SRI) Ultrasound in the characterization of carotid artery plaques

BACKGROUND AND PURPOSE

Speckle Reduction Imaging is a new algorithm that improves the image quality of B-mode scanning by reducing the reverberation artifacts. In the present study the value of this method for the characterization of atherosclerotic plaques in the internal carotid artery was investigated.

METHODS

Two hundred and twenty two patients (161 men, 61 women; mean age 73 years) referred for carotid ultrasound evaluation were included in the study. Patients with plaques of the internal carotid artery as identified by conventional B-mode scanning were investigated also with the addition of Speckle Reduction Imaging (SRI) with the use of a 4-11-MHz wide band linear transducer. Plaque morphology was rated according to a standardized protocol by two independent observers.

RESULTS

For the determination of plaque echogenicity, the reproducibility of SRI (kappa=0.83) was higher than that of conventional B-mode ultrasound (kappa=0.68). The interobserver agreement for plaque surface characterization was also higher for SRI (kappa=0.8) than for conventional B-mode (kappa=0.61). At the evaluation of the image quality through a semiquantitative analysis, SRI was rated superior in the plaque texture resolution, plaque borders determination, vessel wall demarcation and fibrous cap depiction. In addition, the level of "speckle" was reduced with the use of SRI.

CONCLUSIONS

SRI is a technique that shows good general agreement with high-resolution B-mode and can be used for the characterization of atherosclerotic plaques in the carotid artery. Furthermore, because this advanced technique allows reduction of ultrasound artifacts, it improves the image quality allowing more precise visualization of plaque morphological details.

Ultrasound image compounding based on motion compensation

The quality of ultrasonic images is usually influenced by speckle noises. To reduce the speckle noise, compounding techniques have been widely applied to improve image quality through signal averaging. In this paper, a 2-D ultrasonic motion compounding system is proposed that uses a hierarchical adaptive feature weighted motion estimation method to realign the frames before compounding. Each frame is first registered to the reference frame and then compounded to reduce the speckle noises. Several compounding strategies are then used to remove the unwanted degradation. Performance has been tested on both synthetic and in vivo clinical ultrasonic images. The experimental results show that the motion can be assessed accurately and the compounded images achieve good signal-to-noise ratio for improving the quality of ultrasonic images.

Ultrasonic attenuation estimation in small plaque samples using a power difference method

Many studies have shown that atherosclerosis changes the ultrasonic attenuation properties of the vessel wall and plaque. Accurate estimation of the attenuation coefficient slope could therefore provide an early indication of atherosclerosis and the differentiation between low, mild and highly-attenuating plaque within the vessel. However, the traditional reference phantom method that fits the power spectrum in a region of interest fails to accurately estimate the attenuation coefficient for small irregular shaped ex-vivo plaque specimens. This discrepancy was primarily due to partial volume effects and the unknown backscatter coefficient of the plaque sample. We have developed a method based on the reference-phantom method that utilizes the difference in the acoustic power above and below the sample to accurately compute values of the attenuation coefficient ex vivo. Our results demonstrate that this approach overcomes the two drawbacks mentioned earlier and provides accurate estimates of the attenuation coefficient slope for small excised tissue samples.

Sonographic examination of the carotid arteries

Ultrasonography (US) of the carotid arteries is a common imaging study performed for diagnosis of carotid artery disease. In the United States, carotid US may be the only diagnostic imaging modality performed before carotid endarterectomy. Therefore, the information obtained with carotid US must be reliable and reproducible. Technical parameters that can affect the accuracy of carotid US results include the Doppler angle, sample volume box, color Doppler sampling window, color velocity scale, and color gain. Important factors in diagnosis of atherosclerotic disease of the extracranial carotid arteries are the intima-media thickness, plaque morphology, criteria for grading stenosis, limiting factors such as the presence of dissection or cardiac abnormalities, distinction between near occlusion and total occlusion, and the presence of a subclavian steal. Challenges to the consistency of carotid US results may include lack of a standard protocol, poor Doppler technique, inexperience in interpretation of hemodynamic changes reflected in the Doppler waveform, artifacts, and physical challenges. Hindrances in the classification of problematic carotid artery stenoses may be overcome by following a standard protocol and optimizing scanning techniques and Doppler settings.

Comparison of fundamental sonography, tissue-harmonic sonography, fundamental compound sonography, and tissue-harmonic compound sonography for focal hepatic lesions

Image qualities of fundamental, tissue-harmonic, fundamental compound, and tissue-harmonic compound sonography for evaluating focal hepatic lesions were compared. Two radiologists, blinded to the type of techniques and to the final diagnosis, independently evaluated 384 images of 96 hepatic lesions: hemangiomas (n=35), hepatic cystic lesions (n=28), cirrhosis-related nodules (n=22), focal nodular hyperplasia (n=1), and metastases (n=10). All images were graded in terms of lesion conspicuity, margin sharpness, and overall image quality using a 4- or 5-point scale. In the cases of cystic lesions, posterior acoustic enhancement and internal artifacts were also analyzed. A Friedman test was used for multiple statistical comparisons of the four techniques for all parameters. Compound imaging was significantly superior to fundamental imaging regarding lesion conspicuity, margin sharpness, and overall quality (P<0.05). For posterior enhancement and internal artifacts within the cyst, harmonic ultrasonography (US) was significantly better than fundamental US (P<0.05). For evaluating focal hepatic lesions on US, compound imaging provided better lesion conspicuity, better margin sharpness, and better overall image quality than fundamental imaging did. Tissue harmonic imaging also provided better posterior enhancement and fewer internal artifacts of the cyst than fundamental imaging.

Repeated visual and computer-assisted carotid plaque characterization in a longitudinal population-based ultrasound study: the Tromsø study

In a longitudinal population-based ultrasound survey, we evaluated the reproducibility of carotid plaque detection, off-line vs. online visual classification of plaque echogenicity and computer-assisted plaque echogenicity (grey-scale median, GSM) classification and plaque area measurements. The number of paired observations in the reproducibility analyses was 107 in the baseline study and 83 in the follow-up study. In addition, 198 and 222 images were selected from the baseline and the follow-up study for GSM- and plaque-area analyses. The total number of plaque images (11,160) was used to obtain comparative reference values. Despite good agreement in the reproducibility study (kappa values ranging from 0.52 to 0.57), there was a substantial drift in online visual classification of plaque echogenicity during the survey period. Inter- and intraobserver agreement on computer-assisted GSM classification was substantial, with kappa values (95% CI) of 0.77 (0.73 to 0.80) and 0.79 (0.75 to 0.84), respectively. A systematic bias in plaque area measurements was observed. Visual online classification may introduce systematic measurement errors that are not intercepted in a reproducibility study of restricted duration. Computer-assisted off-line classification had better reproducibility. However, the method is influenced by measurement errors, both in the outlining of the plaque and in the standardization procedure.

Two-dimensional spatial compounding with warping

Spatial compounding aims to improve image quality through signal averaging, but speed-of-sound (SoS) and refraction errors can misalign the component frames and blur the compound image. A 2-D compounding system is demonstrated that uses a nonrigid registration (warping) to realign the frames before compounding. Block-based estimates of local misalignments are interpolated smoothly to compute the warp vectors. Simulations and a specialized phantom, both with a 9% SoS distortion, were created, and compound images with and without warping were compared to the conventional image. Image sharpness was compared by measuring the diameter of point targets and directional edge sharpness. The average registration accuracy was 0.06 to 0.07 mm (approximately one pixel). The diameter of point targets increased only 2% with warping vs. 32% without warping and directional edge sharpness dropped 3.7% vs. 20.0%. Furthermore, most of the speckle reduction due to compounding is retained when warping is used. The tests on simulated and phantom data demonstrate that the method is capable of making a small, but significant, improvement to image quality. The examinations in vitro and in vivo show the correct operation of the method with real tissue features. Further clinical studies should be performed to compare spatial compounding with and without warping to see which applications would benefit from the small improvement.

Characterization of carotid artery plaques using real-time compound B-mode ultrasound

BACKGROUND AND PURPOSE

Real-time compound ultrasound imaging is a new technique for improving the image quality of B-mode scanning. We investigated the value of this method for the characterization of atherosclerotic plaques in the internal carotid artery.

METHODS

Thirty-two patients (22 men, 10 women; mean age, 75 years) with plaques of the internal carotid artery as identified by high-resolution B-mode scanning were investigated with real-time compound ultrasound imaging with the use of a 5- to 12-MHz dynamic range linear transducer on a duplex scanner. Two independent observers rated plaque morphology according to a standardized protocol.

RESULTS

The majority of plaques was classified as predominantly echogenic and as plaques of irregular surface, whereas ulcerated plaques were rarely observed. The interobserver agreement for plaque surface characterization was good for both compound ultrasound (kappa=0.72) and conventional B-mode (kappa=0.65). For the determination of plaque echogenicity, the reproducibility of compound ultrasound (kappa(w)=0.83) was even higher than that of conventional B-mode ultrasound (kappa(w)=0.74). According to a semiquantitative analysis, real-time compound ultrasound was rated superior in the categories plaque texture resolution, plaque surface definition, and vessel wall demarcation. Furthermore, there was a significant reduction of acoustic shadowing and reverberations.

CONCLUSIONS

Real-time compound ultrasound is a suitable technique for the characterization of atherosclerotic plaques, showing good general agreement with high-resolution B-mode imaging. This advanced technique allows reduction of ultrasound artifacts and improves the assessment of plaque texture and surface for enhanced evaluation of carotid plaque morphology.

Comparison of conventional sonography, real-time compound sonography, tissue harmonic sonography, and tissue harmonic compound sonography of abdominal and pelvic lesions

OBJECTIVE

The purpose of this study was to compare conventional sonography, real-time spatial compound sonography, tissue harmonic sonography, and tissue harmonic sonography merged with compound sonography for overall image quality, lesion conspicuity, and elimination of artifacts. SUBJECTS AND METHODS. In this study, 150 lesions in 122 randomly selected patients with various abdominal and pelvic lesions were evaluated. For each lesion, sonograms were obtained with four techniques: conventional sonography, real-time spatial compound sonography, tissue harmonic sonography, and tissue harmonic compound sonography. All images were reviewed and graded independently by two observers for overall image quality, lesion conspicuity, and elimination of artifacts.

RESULTS

Statistical analysis showed that for overall image quality, lesion conspicuity, and elimination of artifacts, tissue harmonic compound sonography was significantly superior to all of the other techniques; real-time spatial compound sonography was better than tissue harmonic sonography; and conventional sonography was the least valuable of all (p < 0.001). When data were analyzed separately according to lesion types, tissue harmonic compound sonography was significantly superior for revealing stone diseases, liver cysts, gallbladder polyps, and uterine myomas. For the remainder of lesion groups, spatial compounding was superior to tissue harmonic sonography for all aspects of evaluation, and conventional sonography was the least valuable (p < 0.05).

CONCLUSION

In abdominal and pelvic scanning, tissue harmonic compound sonography provides the best overall image quality, best lesion conspicuity, and least artifacts of all the evaluated imaging modes. Spatial compound sonography is better than tissue harmonic sonography for the evaluation of lesions in general, despite some differences among lesion groups.

Improvement of sonographic needle visibility in cirrhotic livers during transjugular intrahepatic portosystemic stent-shunt procedures with use of real-time compound imaging

The visibility of a transjugular intrahepatic portosystemic stent-shunt (TIPS) puncture needle was assessed with use of real-time compound imaging and single-line ultrasonography (US). In vitro and in vivo, needle-tissue contrast was calculated. With an increasing angle of incidence, the decrease in needle visibility was more pronounced for single-line US than for compound imaging. At angles of incidence of more than 30 degrees, the needle was barely visible with either technique. Subjective evaluation during TIPS procedures in 10 patients proved that the needle tip was significantly better seen with compound imaging. Therefore, real-time compound imaging significantly improved contrast and subjective visibility of the needle.

A method to obtain reference images for evaluation of ultrasonic tissue characterization techniques

A general problem when evaluating ultrasonic methods for tissue characterization is that "a golden standard" is seldom known. This paper describes a manual method to obtain a reference image, with the same geometry as the ultrasound image, indicating spatial location of the different tissue types present in the biological tissue scanned in vitro. A 30 x 10 x 2 mm3 piece of formalin fixed porcine tissue was molded into an agar block, which on the top surface, contained a set of fiducial markers, spaced 2.5 mm. The block was submerged into 20 degrees C water and a set of parallel 7.5 MHz spatial compound ultrasound images of tissue and fiducial markers were recorded each 0.5 mm. Guided by the fiducial markers, the agar block was subsequently cut into slices 2.5 mm thick, photographed and finally analyzed histologically identifying these tissues: collagen rich, collagen poor, micro vessels and muscle fibres. Due to: (1) the cutting procedure, (2) the finite size of the ultrasound beam and (3) the spatial variation in propagation velocity, the macroscopic photographs did not align completely with the ultrasound images. Likewise, the histological image is a geometrically distorted version of the macroscopic photograph, due to the histological preparation process. The histological information was "mapped back" into the format of the ultrasound images the following way: On the macroscopic images, outlines were drawn manually which defined the border of the tissue. These outlines were superimposed on the corresponding ultrasound images (identified via the fiducial markers) and modified to encompass what appeared to be tissue regions on the ultrasound images and subsequently re-applied to the macroscopic image. This modified macroscopic outline was used as guideline when drawing outlines identifying regions of the various tissue types. Specifically, the macroscopic image revealed the borders between the different tissues, while the histological image identified the four tissue types. A set of 12 reference images based on modified macroscopic outlines was created. The overlap between the ultrasound images and the macroscopic images--which are the geometrical basis for the final reference images--was between 77% and 93%. A set of 12 reference images spaced 2.5 mm, identifying spatial location of four different tissue types in porcine muscle has been created. With the reference images, it is possible to quantitatively compare different ultrasound based tissue classification techniques.