Apparatus for ultrasound elastography, the apparatus comprising: an external vibrator comprising a patient-contacting board dimensioned to support at least a portion of the back of a patient and at least one exciter coupled to drive vibration of the patient-contacting board, the at least one exciter comprising a mass that is movable relative to the patient-contacting board to apply inertial forces to the patient-contacting board, whereby inertial forces from the at least one exciter cause motions of the patient-contacting board that generate shear waves in a patient lying on the patient-contacting board, preferably wherein at least one exciter is a variable-frequency exciter operable to excite vibration of the patient-contacting board having frequencies in the range of 45 to 70 Hz; wherein the patient-contacting board is placed on a bed mattress that supports the patient-contacting board while allowing the motions of the patient-contacting board; an ultrasound imaging system comprising an ultrasound transducer, driving circuits operative to drive the ultrasound transducer to transmit ultrasound pulses and to receive ultrasound echo signals; and a controller configured to process the ultrasound echo signals to detect and measure motions corresponding to shear waves generated by the external vibrator; wherein the controller is preferably configured to generate colour elastography images which indicate areas in which imaged tissues have different mechanical properties by different colours and the apparatus comprises a display connected to display the elastography images; characterised in that the controller is configured to process the echo signals to identify shear wave nodes located in a volume of interest and to control one or more operating parameters of the external vibrator in a manner that causes the nodes to move or disappear wherein the one or more operating parameters of the external vibrator comprise one or more of: a frequency of operation of one or more exciters of the at least one exciter, a relative phase of operation of first and second exciters of the at least one exciter, an amplitude of operation of one or more exciters of the at least one exciter, and a direction of vibration of one or more exciters of the at least one exciter; wherein the controller is configured to identify the shear wave nodes by processing the ultrasound echo signals to identify node volumes having a predetermined size and shape within the region of interest in which a maximum tissue motion is below a threshold. an external vibrator comprising a patient-contacting board dimensioned to support at least a portion of the back of a patient and at least one exciter coupled to drive vibration of the patient-contacting board, the at least one exciter comprising a mass that is movable relative to the patient-contacting board to apply inertial forces to the patient-contacting board, whereby inertial forces from the at least one exciter cause motions of the patient-contacting board that generate shear waves in a patient lying on the patient-contacting board, preferably wherein at least one exciter is a variable-frequency exciter operable to excite vibration of the patient-contacting board having frequencies in the range of 45 to 70 Hz; wherein the patient-contacting board is placed on a bed mattress that supports the patient-contacting board while allowing the motions of the patient-contacting board; an ultrasound imaging system comprising an ultrasound transducer, driving circuits operative to drive the ultrasound transducer to transmit ultrasound pulses and to receive ultrasound echo signals; and a controller configured to process the ultrasound echo signals to detect and measure motions corresponding to shear waves generated by the external vibrator; wherein the controller is preferably configured to generate colour elastography images which indicate areas in which imaged tissues have different mechanical properties by different colours and the apparatus comprises a display connected to display the elastography images; characterised in that the controller is configured to process the echo signals to identify shear wave nodes located in a volume of interest and to control one or more operating parameters of the external vibrator in a manner that causes the nodes to move or disappear wherein the one or more operating parameters of the external vibrator comprise one or more of: a frequency of operation of one or more exciters of the at least one exciter, a relative phase of operation of first and second exciters of the at least one exciter, an amplitude of operation of one or more exciters of the at least one exciter, and a direction of vibration of one or more exciters of the at least one exciter; wherein the controller is configured to identify the shear wave nodes by processing the ultrasound echo signals to identify node volumes having a predetermined size and shape within the region of interest in which a maximum tissue motion is below a threshold.