A system for providing an object in a body, wherein the object is a radiation source for radiation therapy and the system comprises: an ultrasound probe for monitoring an area of the body in which the object is to be provided, an instrument for providing the object in the body, the instrument comprising one or more ultrasound transducers (24, 26) which are arranged such that a reception characteristic for receiving an ultrasound transmission from the ultrasound probe and/or a transmission characteristic for transmitting an ultrasound transmission to the ultrasound probe changes with a presence of the object to be provided in a portion of the instrument and/or a passing of the object to be provided into and/or from the portion of the instrument, a processor arranged to determine, based on a signal output of the one or more ultrasound transducers (24, 26) of the instrument for providing the object in the body which are arranged such that a reception characteristic for receiving the ultrasound transmission from the ultrasound probe for monitoring an area of the body, in which the object is to be provided, changes with a presence of the object to be provided in a portion of the instrument and/or a passing of the object to be provided into and/or from the portion of the instrument, and/or based on a signal output of the ultrasound probe for monitoring an area of the body in which the object is to be provided, the signal output being in response to an ultrasound transmission from one or more ultrasound transducers (24, 26) of the instrument for providing the object in the body which are arranged such that a transmission characteristic for transmitting an ultrasound transmission to the ultrasound probe changes with a presence of the object to be provided in a portion of the instrument and/or a passing of the object to be provided into and/or from the portion of the instrument, whether or not an object to be provided by the instrument is present in the portion of the instrument and/or passing into and/or from the portion of the instrument. an ultrasound probe for monitoring an area of the body in which the object is to be provided, an instrument for providing the object in the body, the instrument comprising one or more ultrasound transducers (24, 26) which are arranged such that a reception characteristic for receiving an ultrasound transmission from the ultrasound probe and/or a transmission characteristic for transmitting an ultrasound transmission to the ultrasound probe changes with a presence of the object to be provided in a portion of the instrument and/or a passing of the object to be provided into and/or from the portion of the instrument, a processor arranged to determine, based on a signal output of the one or more ultrasound transducers (24, 26) of the instrument for providing the object in the body which are arranged such that a reception characteristic for receiving the ultrasound transmission from the ultrasound probe for monitoring an area of the body, in which the object is to be provided, changes with a presence of the object to be provided in a portion of the instrument and/or a passing of the object to be provided into and/or from the portion of the instrument, and/or based on a signal output of the ultrasound probe for monitoring an area of the body in which the object is to be provided, the signal output being in response to an ultrasound transmission from one or more ultrasound transducers (24, 26) of the instrument for providing the object in the body which are arranged such that a transmission characteristic for transmitting an ultrasound transmission to the ultrasound probe changes with a presence of the object to be provided in a portion of the instrument and/or a passing of the object to be provided into and/or from the portion of the instrument, whether or not an object to be provided by the instrument is present in the portion of the instrument and/or passing into and/or from the portion of the instrument.